Featrued VT: The 12BY7A

 

Featured VT: The 6AU6

 

6AU6

The 6AU6 isn't glamorous, and the wrap-around plate doesn't make it especially interesting to look at. Not the least bit glamorous, just a reliable work horse that does just about anything well: CCS duty, voltage regulator error amp service, small signal audio and RF amp. It has a generous V_HK= 100V (heater positive) / 200V (heater negative) rating that suits it for both error amp and CCS duties.

As a small signal amp, the linearity is very good, especially if you can get the passive plate load up, and run it at small plate currents, consistent with drive capability of the load.

The 6AU6 is a miniature sharp cutoff pentode primarily designed for use as a high gain radio or intermediate frequency amplifier. Its low grid-plate capacitance and high transconductance make it especially suited for high frequency, wide-band applications. -- GE Spec sheet

Given that the spec sheet doesn't mention audio, the included plate characteristic isn't especially helpful. For doing audio design work, the included plate and screen transfer curves are more useful. From these, one may pick off bias points, screen voltages, and plate current swings to calculate likely low distortion Q-Points. As this would most likely appear in a low level stage, linearity isn't hard to come by for small plate voltage swings. Though these curves were taken with V_PK= 250V_DC, this presents little problem as the plate characteristic is quite flat for voltages above 80V. Operating at lower voltages might require some adjustments to the screen voltage and/or bias point.

As with any other pentode, there are the usual considerations: microphony since there are three grids inside the bottle, as opposed to one, therefore, more parts that can jiggle. There is also the possibility of increased noise. This is especially vexatious at audio frequencies as such things as cathode flicker noise and screen partition noise decrease with increasing frequency.

There are two other ways to use the 6AU6: as a pseudotriode. This can be done in the usual manner: paralleling the plate and screen. The spec sheet even includes the pseudotriode plate characteristic. This gives a μ= 36, which falls nicely between the gains of types like the 6SN7-oids, and high gain types such as the 12AT7 or 12AX7. Convenient when you need gains that fall between those two types.

Triode Plate Characteristic

Whether to connect the suppressor to the plate or cathode is another gray area: some say better sonic performance is had one way or the other. There is no consensus for this. Changing the suppressor connection doesn't alter the plate characteristic all that much in any case.

The other way to make a pseudotriode is to connect the plate to ground, and use the screen as the triode plate. If the screen spec isn't busted by this, then the plate becomes an internal shield. This would prove useful in electrically noisy environments. It changes the actual characteristic very little since it's the screen, not the plate, which determines the actual operation. In any pentode, due to the isolation of the plate, the plate serves mainly as a pseudo anode that simply collects the electrons at the end of their journey.

Don't overlook the 6AU6 if you decide your design could use a pentode.

Featured VT: The 7193

 

I picked up a half dozen of these at the Dayton Hamvention recently. This is an "industrial" type since it has a numerical type designation, as opposed to the alpha-numeric identifiers more commonly known as "consumer" types. It's a singleton, small signal triode. The configuration is decidedly odd. The octal base comes complete with all eight pins in place, though only three of them are connected to anything: two heater connections, and the cathode connection. It seems that this was designed as an avionics type, and the full compliment of octal pins included for extra mechanical stability.

The plain white boxes they come in:

 

The two top cap connections are by design, as the type originated in the early 1940s, as VHF tech was coming into its own. The top cap connections being to allow for short, low impedance, connections, as a VHF oscillator. Other than that, electronically it is very similar to the 6J5: an excellent small signal audio triode that is basically one section of a 6SN7 dual. It can be used as a direct substitute in any design that requires the 6J5, and they are dirt cheap and available. That's a good thing in case 6J5s become difficult to acquire and/or audiophool expensive. Since I use 6J5s, this was a good acquisition. Some like 'em for their funky appearance and/or as a substitute for the more expensive and harder to acquire glass versions of the 6J5. (There's a whole bunch of audiophoolery concerning the undesirability of metal types.)

If using the 7193, care must be taken with those top cap connections, as transposing the plate and grid connections will result in poofage. The 6J5, unlike the later, hardened, 6SN7GTA-B types intended for TV vertical deflection duty, was never intended to run into significant grid current. A hundred volts or so between the grid and cathode will be a disaster. The connections are clearly visible, so getting them right isn't a problem if you know what you're doing. This could pose problems if doing "consumer" designs for end users who don't know what's inside the glass and couldn't care less.

Otherwise, use 'em like any other type with top cap connections, as the caps fit connectors designed for the small top caps such as the ones fitted on 6BQ6's and other power finals.

Featured VT: The 10JA5

 

This is a vertical deflection pentode for color TVs. It has the 12 pin "Compactron" base, all glass construction, and does not include a top cap connection. The rated P_D= 19W. With a modest voltage, a PP pair can give 34W, and with an estimated h₃= 3.57%. This would put it in the same category as the 6L6-oids, so far as operating voltages and power output. As for how this actually sounds, that depends on the harmonic "personality" of the distortion. If it's mainly h₃, then it won't require any extra help from local NFB: just add enough gNFB to take the "edge" off, and, of course, to improve speaker damping. As with the horizontal deflection types (and the 6L6-oids) the screen voltage rating is limited. This makes UL difficult to implement without OPTs with special tertiary windings. Should local NFB be necessary, it can always be implemented as cathode feedback, or parallel feedback. A stiff (preferably active regulation) Lo-Z screen supply is best for sonic performance. Being that this is a vertical deflection type intended for developing max RMS power, the ratings are on the conservative side. With a bit of spec busting, you can run it hotter than rated, as audio final use is far less demanding. This will get you closer to Class A, and reduces x-over distortion.

It does, however, like a heavier load: 5K44 (P-2-P). The heavier cathode gives the 10JA5 a higher current capability than usual for audio finals. This can help the sonics, especially bass-heavy material. If there's a drawback, it's the odd heater voltage: 10.5V. This being due to the use of series strings operating directly from the AC mains for heater power. It will require an extra heater PTX for that reason.

The 10JA5 looks to be another audio "sleeper", in that audio final use isn't mentioned in the spec sheet. Of course, this isn't a type you'd use in Class A anyway. As with most of these TV tubes, the most linear portion of the plate characteristic is well within red plate territory.

For a 30+ WPC stereo, it's one to consider.

Load Line

Featured VT: The 6BQ7

This is a small signal dual triode in the nine pin mini format. From the spec sheet:

The 6BQ7A is a miniature, medium-μ, twin triode primarily designed for use as a cascode radio frequency amplifier in very high frequency TV tuners. In this application, its performance is characterized by low noise figures and high gain.

From the piccie, it's obvious that this is indeed a high frequency triode. It has the additional advantage of a high gm, made possible by the comparatively enormous cathodes. These are much larger than those of other small signal types like the 12AU7, 6FQ7, 6SN7, 6C4. The 6BQ7 also has a μ-Factor that falls nicely between types like the 12AU7 or 6FQ7, and the 12AT7.

The spec sheet even includes a composite plate characteristic for cascode operation. What you don't find is any mention of any sort of audio applications. The RCA Receiving Tube Manual does make a concession to this sort of usage, including the 6BQ7 in its example designs for RC coupled voltage amps.

There is something squirrelly about this plate characteristic. Is that an undocumented variable-μ characteristic? This makes it difficult to find a good, low distortion, audio loadline, but it is not impossible. The key is to get the V_PK up, and keep the voltage swing well away from the bottom of those curves. This will require either an unusually high rail voltage with passive plate loading, or active plate loading, then good audio performance may be obtained. The 6BQ7 is a good deal more linear than its VHF cascode cousins, types like the 6BK7B, which is hopeless as an audio amp. Whether it's as linear as the more common audio small signal triodes is another matter entirely.

So what good is it? The one thing that brought this type into consideration was the design of an audio cascode subsystem. In solid state practice, transistor cascodes are not that unusual. In hollow state practice, I came up with very little information. Nearly all references to hollow state cascodes were about VHF amps. For audio, it was all for guitar amps, and the emphasis was on voltage gain, not sonic performance. Of course, you like distortion in a guitar amp, as this makes for "tone". Still, there is the solid state practice, since cascoded transistors operate more linearly than do singleton transistors. Was it just that hollow state audio cascodes were "weird", and therefore not used so much?

The usual audio suspects didn't work out so swell. Cascoded 6SN7s didn't have enough gain, and 12AT7s showed a g_m rolloff with decreasing current that likewise made them unsuitable for this design. The RCA Manual mentioned the 6BQ7 as a cascode.

This was a case of try it and hear. I designed and built an LTP phase splitter made from two cascoded 6BQ7s. Even though this costs you half your voltage gain, there was sufficient to eliminate an additional gain stage while maintaining sensitivity even when gNFB is included. As to performance, it was quite excellent, producing excellent phase-to-phase balance with an active tail load, and undetectable harmonic distortion. Since the LTP phase splitter is also a differential, it provides a ready gNFB summing node. As an audio cascode LTP splitter/differential amp, the performance is excellent. A singleton 6BQ7 can also serve quite nicely as a triode LTP, especially if you include active tail loading.

The fat cathodes glow nicely as well.

Cascode LTP Example

You do have to watch out when using the 6BQ7A. Pay attention to how the heaters are connected. There are two ways this was done: an internal series connection wherein the heaters are wound with one continuous filament, and those where each heater is individually connected to the heater support pins. The series heater versions tend to be microphonic, and they ring like bells. The parallel heater versions aren't microphonic. That goes with the territory: series heaters are cheaper, and less attention to detail was paid. I suppose they figured that ringing at audio frequencies was not going to be a problem at VHF. The series heater versions are also more susceptible to filament burn-out. That unguarded length of heater filament is the first to heat up. It can flare brightly for a second or two before the rest of the filament has a chance to heat and develop enough resistance to limit the current. Cheap tubes and cheaply made tubes aren't good tubes. Going to ham meets to acquire a stock is well worth it, as you can examine before buying.

6BQ7-oids:

There are two other types with different heater voltages:

4BQ7A: 4.2V / 0.6A
5BQ7A: 5.6V / 0.45A

These types were designed for use in TV sets that didn't have a PTX, and used a series heater string to light the VTs. The currents were quite "standard" for series heater operation. Types with odd heater voltages might be more available and/or less expensive. If using with 6.3V heater supplies, just add enough series resistance, preferably split equally between both legs to maintain balance, to drop the voltage to the rated heater voltage.

One last consideration is that the type includes an internal baffle shield between triode sections. This shield needs to remain negative to any cathode inside, or it could start to function as another plate. That could possibly throw off the bias, or lead to instabilities. It may be a problem with some LTP applications if the cathodes go negative.

The 6BQ7 isn't an easy VT to use in audio work, but when you need more gain than a 6FQ7 can provide, but not so much as a 12AX7 or 12AT7, then it definitely meets the design criterion. The cascode LTP splitter/gain stage is pretty much my universal, "go to" front end design. It provides the gain that you would get from a cascade of a 6SL7 LTP DC coupled to a 6SN7 differential stage, and with a greatly reduced input capacitance that would otherwise interact poorly with a high resistance volume control or long cable runs.

Featured VT: The 6J5

 

This type is a singleton triode with an octal base. As nine pin mini types proliferated during the early 1950s, few singleton triodes were made, and the few that you do find are almost always VHF amp types intended for 400MHz+, running as grounded grid amps. Most triodes were made in pairs, such as the 12A*7 series, or the 6FQ7, 6DJ8, etc. If you need a singleton triode in the seven or nine pin mini format, you will either have to go with a type like the 6C4 (a type that is not recommended as an audio triode, but rather a low power, Class C RF driver/final) or make a pseudotriode from a small signal pentode. (The 6AU6 works nicely for this, and it has a μ-factor that fits nicely between the medium-μ triodes like the 12AU7 and the high-μ types like the 12AT7 or 12AX7).

The 6J5 appeared with the Octal base, and metal envelope. There is also a glass Octal as well, but is much harder to find, and more costly to acquire. Regardless of packaging, the type is a small signal, medium-μ, singleton triode. It is also basically half of a 6SN7. As such, it can be used for some of the same things: small signal voltage amps, QRP finals, cathode followers, and cathodyne phase splitters, and oscillators. The characteristics are also quite linear, making this type a good one for audio amplification where moderate levels of gain are required.

These are nice, linear plate curves indeed. It is easy find loadlines that exhibit very little harmonic distortion. The type has the linearity for small signal amplification, and the plate dissipation that allows for use as a cathode follower for driving moderate currents into difficult loads. The latter does include the grids of PP finals driven from a cathodyne phase splitter.

There is a bit of audiophoolery regarding the type. Some dislike the metal envelope, and have made accusations of all sorts against it. Yes, the metal cans do cost you a few glowey bottle kewlness points, and the paint offers little protection in circuits that use negative rail feed to the cathode, instead of/in addition to the positive DC rail. In such cases, it would be safer to opt for the all-glass version. Otherwise, I don't know what difference it could make, and the auto-shielding of the metal envelope is an advantage for the typically low level signals of small signal work. Unless you can specify a specific need, like the extra insulation or something. Otherwise, it's an unnecessary audiophool premium.

That does bring up one extra consideration: the cathode must never become more negative than the envelope. If it does, then the metal can can work like a second plate, attracting stray electrons. This can lead to, at the very least, increased cathode current and excessive bias. Various parasitic instabilities are also a possibility. The safest thing to do with the metal can is to connect its pin to the cathode pin. It's never a problem when using the type in the most common manner: cathode bias resistor, the plate supplied from the positive, DC rail only.

The other bit of audio mythology is that the 6J5 needs to pull at least 10mA to sound good. This just isn't true, and you seldom need that much plate current unless you're designing some sort of QRP application with it. The 6J5 was designed for low power applications, and the 6SN7 was often used in B & W TV sets as a combination plate coupled multivibrator/power amp for vertical deflection duty. The 6SN7GTB is a "hardened" version made as TV screens grew larger, and the demands on the deflection systems greater.

Here you see the loadline for low current operation. The estimated H₂ is virtually non-existent, and in practice this design worked as promised. There was no measurable distortion in evidence, the Twin-T test showed nothing above the noise floor, and subtractive testing showed nothing more than a pure sine wave arising from phase shift. No distortion of any importance.

The main draw back to such operation is that the low plate current drives up the plate resistance. This compromises high frequency performance, with the high frequency -3.0db point coming in at 45KHz. It is trivially easy to get bandwidths a decade larger with any small signal transistor operating at similar voltage gains. Whether or not that makes a difference depends on the application. It definitely isn't how you'd want to operate a 6J5 in a wideband,"DC to Daylight" application.

For the design where the 6J5 was used as a "current trickler", it worked very well indeed. If you need just such a VT, or you have an especially low voltage application, then you don't need to exclude the 6J5 (or the other 6SN7-oids) from consideration.

Featured VTs: The 6BQ6

 

The 6BQ6 as an audio final came to my attention as a chance result of finding an old article from a Portuguese (Brazilian? -- it was written in Portuguese) ham magazine. This described a simple AM plate modulator that claimed an output of 30W (fixed bias) or 25W (cathode bias) that used a push-pull pair of these VTs. You would expect to see 6L6s or 807s used in this particular application. Why 6BQ6s, and what were they?

The 6BQ6 is a large signal beam former. It has no audio pedigree whatsoever, and the spec sheet makes no mention of its use as an audio final. During the 1950s, screen sizes and deflection angles increased, giving a larger viewing area, and a shorter CRT for more compact TV sets. This development meant that the usual audio finals and RF types became increasingly unsatisfactory for horizontal deflection duty. New types more suited to the task were developed, and one such type was the 6BQ6, in several different iterations for large screen, B & W TV sets.

The 6BQ6, as with all HD types, is capable of pulling big currents through the horizontal deflection coils. This was accomplished by cathodes much larger than those found in comparable audio and RF finals, such as the 6L6-oids. Compare heater voltages and currents:

6V6: 6.3V / 0.45A (2.835W)
6L6: 6.3V / 0.9A (5.67W)
6BQ6: 6.3V / 1.2A (7.56W)

This makes possible a maximum cathode current spec of 400mA. In terms of solid state, that isn't very spectacular, but for a nominally high voltage, low current device, it's pretty good. Other design features include a low voltage capability, made possible by the close proximity of screen grids to the cathode, and that's what you want for drawing big currents through deflection coils, and for operation from transformerless power supplies that derive the working DC directly from the AC mains. The latter consideration has spawned versions of this tube with odd heater voltages for operation in series heater strings.

As an audio final, the high current and low voltage capabilities make for lower load impedances, and therefore, for easier to design OPTs. The lack of any audio final usage requires one to draw up loadlines to find suitable operating points. Here is one such audio loadline:

As you can see, the load is just 1K1 per phase. This is a Class AB loadline, so you would need an OPT with a primary that can match 4K4 (P-2-P) to your speaker load. It's is a good deal easier to manufacture an OPT with excellent high and low frequency performance than it would be for the 10K (P-2-P) load a PP pair of 6V6s need. It's also a convenient value since there are off-the-shelf OPTs that were designed for Class A, PP, 6L6s that match this value. The only drawback to using one of these OPTs is that they are a bit under powered for the 6BQ6.

As for why no audio uses are specced, the plate characteristic tells all. It is not possible to use the 6BQ6 in Class A. The most linear part of that characteristic lies well within red plate territory. You are committed to Class AB, push-pull only. That isn't necessarily a bad thing, though.

If you stick with the usual convention for determining the static plate bias current, you are deep into Class AB. The closer you come to Class B conditions, the more cross-over distortion you have. The estimated H₃= 5.0% is barely acceptable. There is something you can do about this. The 6BQ6 was specced very conservatively, since its main purpose is to output max RMS power for hours a day, all the while maintaining a reasonable service life. Audio amplification is a good deal less demanding. If you increase the bias, and bust the plate dissipation spec, the performance improves greatly.

Doubling the bias current to 50mA per plate, makes for 17.5W of static dissipation: exceeding the spec by 6.5W (the 6BQ6 is rated close to the 6V6). This is of no consequence, as you don't get any color on the plates. The RCA gray plates can handle up to 70mA, with just a trace of color. The Sylvania black plates aren't showing any color even at this extreme 24.5W of static plate dissipation. 50mA of bias won't harm this type, even if it's outside of the spec. The increased bias current, and the much lower H₃ estimate, does make a noticeable -- and improved -- sonic difference in practice.

As to the sonic performance of the type that had no audio pedigree, in a word: excellent. The 6BQ6 easily matches the performance of the legendary 6V6, and it pumps out more than twice the power. Running open loop, the 6BQ6 doesn't have any of that expected "pentode nastiness" until it's nearly at the point of clipping. There's just an "edginess" or "aggressiveness" to the sound. That would account for why the Portuguese (Brazilian?) hams elected to use it in their plate modulator project (it was an open loop design). 6L6s tend to sound much worse open loop.

The Twin-T test also bears this out. After nulling the fundamental, the residual is almost a perfect sine wave at three times the frequency. There is little higher order harmonic components. Though high fidelity isn't legal on the ham bands, the 6BQ6 plate modulator would indeed sound better. For excellent sonic performance, all that's required is enough gNFB to take off the edge. The 6BQ6, like the 6V6, doesn't require the additional help of local NFB. That is a good thing in that the low screen voltage spec pretty much eliminates Ultralinear. Operate them in full pentode mode with a regulated screen supply. As with any pent that can't run the plate and screen at the same potential, the best sonic performance results when the screen voltage is tightly regulated, and supplied from a source with a very low impedance, as is the case with an active regulator.

The 6BQ6 also seems to be a good deal more stable in operation. It doesn't produce the snivets that are seen with other types, even if you don't include plate and screen stoppers. Even if not strictly necessary, it does no harm to include these anyway. Screen stoppers can be 680Ω/0.5W C-comp resistors. Plate stoppers can be made from ten turns of #18 wire, space wound 7/16ths inch ID, with 100Ω/2W (or four 470Ω/0.5W C-comps in parallel) de-Qing resistors mounted inside the coil. That should take care of any possible instabilities, or tendencies to make RF, due to lay-out issues.

6BQ6-oids:

This type appeared early on in TV development. The first iterations were in ST glass. As TV screens got bigger, new versions appeared. The 6BQ6GTA has the small profile, tubular glass envelope. This type has smaller cathodes, and is not quite so robust as later iterations. As such, it will red plate badly if used at the Q-Points mentioned previously.

The 6BQ6GTB has the same small diameter envelope, but does work as described above. The 6BQ6GA has a larger bottle, but otherwise biases identically to the 6BQ6GTB. As for sonic performance, the 6BQ6GA may have a slight sonic edge, but I'm not 100% certain of that. Either version sounds great, and will stand up to the spec busting for long service life in audio amplification applications, which is what we're interested in here. Of course, audio amplification is not the same as horizontal deflection duty, or "brick on the key" RF modes (FM, Packet) either.

The 6BQ6 really is a tougher, higher powered, 6V6. The main difference is the greatly reduced screen voltage spec that precludes Ultralinear, and makes pseudotrioding problematic. As a pseudotriode, since the screen voltage never exceeds the plate voltage, you may be able to get away with running at higher than specced screen voltages without either poofing the screens or plate current run-away. That remains to be seen, if you're so inclined to experiment with it.

The advent of TV sets running transformerless power supplies has also led to the development of *BQ6s that have some odd heater voltages:

12BQ6: 12.6V / 0.6A
25BQ6: 25V / 0.3A

These types were intended for use in series strings, as there was no PTX to provide heater power. That such types exist is beneficial in case the 6.3V version should become harder to acquire, and/or the price becomes excessive. Five years ago, you could get 6BQ6s for under a dollar a pop. Due to inflation, and probably the publication of designs that use the type, the prices have been going up. Still, it's not like audiophool expensive, even for quality that's dubious, of finals that do have that audio pedigree. Availability should not be a problem, since the 6BQ6 was widely used in a great many brands of wide screen, B & W TV sets.

6AV5 and Other HD VTs:

This type is very close to the 6BQ6 in terms of specs and sonic performance. The main difference is that the 6AV5 lacks a top cap connection to the plate. This would be a plus for designers who're leery of plate top caps in audio amps that potentially expose end users to dangerous voltages.

If you can wind your own OPTs, or don't mind paying extra to have custom OPTs wound, other possibilities present themselves. As B & W screens increased in size, new types that could handle the increased current demand were developed. This would include types such as the 6CB5 and 6DQ5 -- both capable of even higher output power.

With the advent of color TV, the demands put upon horizontal deflection subsystems grew even greater: color TV CRTs operate at much higher voltages, require more cathode current, and also had the large screens and deflection angles. To meet these demands, some truly awesome power tubes were developed for increased current sourcing, and power handling.

This beast is the 36LW6: Pd= 40W, Imax= 1.75A

It was one of the last generation of Octal HD finals before the advent of the Duodecar (12 pins) and Novar (9 pins) all glass tubes that began to appear during the early 1960s. Part of this development was to allow for putting more tubes in the same envelope. You had a single bottle that might include two RF pentodes for TV IF amps, or combination power triodes and small signal triodes for vertical deflection duty, or triode/pentodes for sync sep/horizontal deflection time bases. FM demodulator/audio voltage amps and power pentodes were likewise included in the same bottle. It was common to put the HD final and the damper diode in the same bottle.

The 6.3V version has become extremely difficult to find these days since a good many were diverted to ham use as RF finals. Even worse was the diversion to illegal CB linears during the mid-1970s CB craze. These illegal rigs were all too frequently badly designed, and poofed the finals too quickly. However, there are quite a few still available if you go with the odd heater voltages, and the prices aren't exorbitant either. You'll have to either wind your own heater PTXs, or have them custom wound. The characteristics remain the same. This type was also designed for operation from a DC rail derived from a transformerless voltage doubler. The spec sheet mentions that it needs just 280V_DC to work, and the published plate characteristic doesn't extend beyond that particular voltage. That would account for the variety of heater voltages. This is a definite benefit for audio design: low load resistance, and the avoidance of high plate voltages.

This looks promising indeed. That's nearly 95W of audio power from a single pair of PP finals, running conservatively, and with a Vpp= 300V_DC. It is helpful that the grid voltage swing isn't out of line with the more common audio power pents, as this allows for lower distortion from the front end. A good many power finals for RF use in particular would need to run at twice -- or more -- the DC plate voltage and likely Class AB₂ operation. Getting that much audio power would require PP-parallel operation of the more usual audio power finals running Class AB₁.

The estimated H₃ for this loadline comes in at about 0.2%. That bodes well for sonic performance. Given the low screen voltages, there is another possible way to use this: screen drive. Use a MOSFET source follower to drive the screen grid, with the control grid bypassed to AC ground, and used only to set a Q-Point current. This becomes a type of Class AB₂ operation that can boost the output power while maintaining a reasonable plate dissipation. It is also helpful in that the screen current remains low until the V_pk is taken to very low levels indeed.

I'm definitely going to have to give this a go some day.

Featured Vacuum Tube: The 807

Featured Vaccum Tube: The 807

The 807

The first of the beam formers is also one of the most enduring types: the 6L6. Developed by RCA in the mid-1930s, this type was originally intended for use as an audio final. It included other, then new, features besides the elimination of an actual, physical suppressor grid required to smooth out the screen grid "kinks". This included the now standard Octal base (up to eight pins possible, and with a keyed base for proper socket alignment) and a metal envelope. The latter was made in one of two ways: a glass envelope VT slipped into a metal shield can, or using the shield can as the envelope, with a glass base to bring out the connections. Other improvements was to give the control grid and screen grid the same pitch and wire diameter. By overlaying these two grids, the negative control grid serves to "shadow" the screen, thereby reducing the useless screen current for improved overall efficiency.

Though the metal envelope provided excellent shielding, these types ran very hot. Even the small signal metal envelope tubes get unusually warm. As a result, the Octal base was adapted to the glass envelope. Thus, the 6L6G type appeared. The first had the "ST" profile. The 6L6GT with a tubular glass envelope appeared by the early 1950s. The 6L6 (in its various iterations) proved to be quite successful, and is still in production today.

By the early 1940s, the 6L6 was adapted to the "ST" glass envelope, the then-standard five pin base, and with a top cap connection to the plate. This RF version became the 807. The main drawbacks to the 6L6 are that the highest voltage pin is right next to the lowest. For the usual audio operating conditions, this was of no consequence. It did, however, limit the available power when operating as a Class C, RF final. The top cap is also highly convenient, as this allows the output circuitry to be shielded from the input by the chassis itself. It also allows for plate voltages that would cause flash-overs between the plate and heater pins.

As for the characterizations of the type, the spec sheets cover all operations from Class A₁ SE, through Class C. Audio final applications are explained for everything from Hi-Fi through PA and AM modulator duties. Having the AM plate modulator capable of using the same finals as the RF deck was a convenience. For PA and AM modulation, efficiency and power output are more important than sonic performance. The data for efficiency includes Class AB₂ and Class B (actually a very deep Class AB₂). Fidelity by Class A₁ or Class AB₁, push-pull. As with any pentode, the SE performance isn't so good as a triode final.

Even though this type boasts some outstanding THD figures (THD= 1.8% for push-pull operation) it has gotten a reputation as being a guitar amp final. Perhaps because that's where most are used these days? The 807 does have a tendency to make lots of nasty, high order harmonic distortion. This shows up when performing the "Twin-T" test. The residual after nulling out the fundamental is quite distorted, resembling a sawtooth wave at three times the frequency. This waveform must contain a lot of harmonics, H₅ and above.

While running open loop, 807s do sound nasty: lots of listener fatigue. How nasty depends on the program material, but is always there. This particular deficiency was noted by the type's inventor: O. H. Schade. He recommended local NFB to tame that harmonic nastiness. This is frequently accomplished by "Ultralinear" operation. This isn't possible since the 807 has a very low screen voltage limit. Unlike many audio pentodes, the screen can't be run at the same DC voltage as the plate unless you're going to sacrifice a lot of output. That leaves either cathode or parallel NFB. Schade recommended feeding 10% of the AC plate voltage back to the control grid, anode follower style.

When this is implemented, the 807's sonic performance improves. All that remains is to take off some of the "edge" with some additional gNFB (~7.0db_v does nicely if the rest of the open loop design is well implemented). The nastiness can be tamed, and the type capable of excellent sonic performance. With cathode follower grid drivers, the 807 can easily produce some 30W of audio output, though specced at 26.5W. If you're willing to go Class AB₂, you can get 80W or more, which requires increasing the plate voltage to 600V_DC, and is not recommended with 6L6s. This wasn't done very often back in "the day", due to the problems inherent with providing suitable grid current from a source with the lowest possible output impedance. That usually meant interstage transformer coupling, and that's not compatible with Hi-Fi since it becomes very difficult to include gNFB, due to the pecular phase performance of any xfmr with ferromagnetic cores. These days, a MOSFET source follower can easily drive the grids positive with very low output impedances.

Other Considerations

This type definitely likes to make RF. The spec sheet recommends the use of plate stoppers, either 47Ω or 100Ω, C-comp resistors, or a suppressor coil. The latter can be ten spaced turns with an ID= 7/16 inch to provide about 1.0μH of inductance. This is low enough to be of no consequence at even the highest audio frequencies. The coil can be "de-Q'd" by paralleling it with a 100Ω/2W C-comp resistor (or four 470Ω/0.5W C-comps in parallel. Mount the resistor(s) inside the coil. The plate stopper needs to be mounted right at the top cap connector, as is the case with any stopper resistors or coils.

The 807 also likes to make "snivets" when operated in Class AB. When one final goes into plate current cutoff, this triggers a damped Barkhausen oscillation. The frequency being determined by the leakage inductance and stray capacitance of the OPT (usually 60KHz+). The fix here is to include screen suppressors. 1K5/0.5W, C-comp resistors should be soldered to the screen pins with the least lead length. These stoppers will prevent the snivets. (The other possible cure would be a slight positive bias on the beam formers, but that's not possible since these are internally connected to the cathode, and not brought out to a pin.)

If the design work is done correctly, and attention paid to details, there is no reason not to use 807s for Hi-Fi work.

One other application that is seen quite frequently is the use of the 807 in pseudotriode mode as a series pass voltage regulator.

Other 6L6-oids: 1625 and 6BG6

The 6BG6 TV HD Final

The original type proved so useful that it spawned other spin-off types, of which I've discussed one. The 1625 is the 12.6V heater version of the 807. It has the same shape and size, but uses an uncommon seven pin base, presumably to keep people from sticking the wrong VT in the wrong hole.

The 807 also proved useful as a horizontal deflection PA for B & W TV sets (before screens got too big, that is). It was desirable to have an Octal version, and so the 6BG6 is the Octal "807". Internally, and electronically, the types are the same. You could consider the 6BG6 to be an Octal 807, or a 6L6 with a top cap. As with the 807, the top cap keeps the high voltage well away from any low voltage points to prevent flash-over. The 1625, 807, and 6BG6 will all work in any circuit designed for the 6L6.

One thing that needs clarification is the oft stated notion that the 1624 is a "DH" 807. It isn't, as the 1624 was designed for a very specific application: mobile transmitters. Being a DH type, the filament could be turned off during receive and/or monitoring to save battery power. It's an RF type from the get-go, and was intended for Class AB, B, or C. At V_gk= 0, the plate current is quite low, under 100mA, and it won't produce any more than ~10W as a Class AB₁, push-pull amp. You can get a bit more power at half the voltage from a pair of 6V6s or 6AQ5s. The 1624 must be operated as a Class AB₂ amp in order to get the most out of it. In that case, it gives just a bit less than what a pair of push-pull 807s will provide. If there is a similarity, it's that the 1624 also boasts of some excellent THD performance if driven from a sufficiently Lo-Z grid driver.

SRPP Demystified

SRPP

There is considerable misunderstanding and mystification regarding this particular circuit, including even what it's called. "SRPP" seems to mean: Shunt Regulated Push Pull, although you see different words assigned to the acronym.

As for what, exactly, this thingy is, it should be immediately recognizable to anyone with a solid state design background: it's an active pull-up/active pull-down circuit. (It greatly resembles the output stage of the TTL family.) The main difference is that it is quasi-complimentary by necessity since there is no such thing as a "P-Channel" VT. This means it is, indeed, push-pull by definition. The upper triode acts to source current to the load, and the lower triode sinks current from the load. As with any push-pull topology, it reduces distortion by nulling even order harmonics.

The big point of departure from solid state is that the SRPP is balanced for three load conditions only: a dead short (makes both triodes into grounded cathode stages -- not very useful) an open circuit (or at least a very high load impedance -- if there is just one path for the current, equal currents must flow through both triodes) and the one impedance for which it was designed. For any other load conditions, the SRPP goes out of balance, and distortion rises, more or less, rapidly. This is OK, considering the purpose for which the SRPP was originated: a line driver. As a line driver, it operates into the characteristic impedance of the T-line.

As an audio circuit, this leaves a lot to be desired. If it works into Class A*₁ grids, then it's OK since that's nearly an open circuit. The trouble starts when the driven control grids are driven positive, and draw current. Under grid current conditions, the resulting impedance is neither constant nor linear. This is not the type of load an SRPP wants to see. Including such a driver will lead to poor clipping behaviour. Unfortunately, it is grid current conditions where you'd like to include active pull-up to not only supply that grid current, but supply it from a Lo-Z source to minimize distortion.

You also see SRPPs used as audio finals. This, too, is not the place for this since any speaker represents anything but a constant load. Speaker impedance varies not only in magnitude, but also phase angle. This will play hell with an SRPP, as it will be operating off its optimum load impedance almost all of the time. That will generate lots of avoidable distortion which will require that much more NFB to correct. This is not what you want in good open loop design.

As for how the SRPP develops voltage gain, consider the R_P. The far end of that plate load resistor has a cathode follower sitting on top of it. This means that the voltage across that resistor is much less than it would be if connected directly to the DC rail. The effective AC resistance is much higher than its DC resistance. This gives the lower triode more voltage gain that it would otherwise have. The next question becomes: can we make that resistor even larger to increase voltage gain? If you break the DC coupling, the answer is "yes". This gives us the variation called a "Mu stage", so called since its voltage gain can approach the amplification factor (μ) of the lower triode.

Mu Stage

The AC coupling allows for a larger R_P since it is no longer doing double duty as a cathode bias resistor. The mu stage no longer has any pretension for being a balanced topology. It is designed for large voltage gains. Is this a useful topology? It was back in "the day", however, it is obsolete and should not be used. Today, we have solid state devices which can operate as excellent CCSs. You will do much better loading a triode with a constant current, as this gives a horizontal loadline that both maximizes output swing, voltage gain, and minimizes harmonic distortion. Back in "the day" we didn't have the ICs, BJTs or MOSFETs that could have made for decent CCSs. Before then, your only other recourse was to use a pentode as a CCS. That would give you something quite mu stage-ish anyway.

If you need lots of gain, then use a solid state CCS. These old fashioned circuits serve no purpose these days, other than nostalgia appeal, or audiophool trendiness on the part of those who like "exotic" circuits for the exoticness.

Cathode Follower

 

AC Coupled CF

In this schemo, R_K is the normal cathode bias resistor. R_L represents the tail load in parallel with the load impedance. R_G is the control grid DC return. The CF gives excellent high frequency performance since Miller Effect is absent, and the C_GK sees very little current since the grid and cathode are always at nearly the same potential. This makes the C_GK effectively smaller than its static value. The main component of input capacitance will be the reverse transfer capacitance: C_GP. With small signal triodes, it is easy to present a Hi-Z, Lo-C load to the driving stage. This isn't just helpful at RF.

This is another circuit which has lately come under unjustified criticism within certain audiophile circles. Much of this is unjustified on the basis that the CF is a negative feedback circuit. This view that all NFB is all bad does have a basis in fact. It has been all too common to use NFB to cover up for poor open loop designs. If your open loop design is poor, just pour on the NFB to force "the numbers" to look good. Sure, you can sweep your mistakes under the carpet that way, but you will also sweep away much of the vitality of the music. However, this is a misuse and abuse of NFB. The blame properly belongs to these lazy designers who can't be bothered to correct their open loop designs.

Another big part of the problem lies with the nature of the cathode follower itself. Yes, it can present a low impedance source, but only to a high impedance load. There is a big difference between source impedance and load impedance. For example, a 6C4 small signal triode could be used to implement a cathode follower. If you implemented it thusly:

V_PP= 330V_DC
V_PKQ= 140V_DC
I_PKQ= 4.0mA
R_tail= 47K

r_p= 10.5K
g_m= 1.4mA/V

You could easily calculate a Z_o= ~647Ω Given that Z_o, you might think you could drive a set of 600Ω headphones with this cathode follower. However, you would be quite wrong about that. Your undistorted power output will be just under 10mW as you'll only be able to swing just 3.4V_p into that load. So what happened? When you connected a 600Ω load across the 47K tail, you killed most of your gain by making a nearly vertical loadline. Less open loop gain means less effective NFB. If it can not drive a 600Ω load connected to the plate it can not drive that load any better if you connect it to the cathode.

So what is it good for? This cathode follower would be ideal for isolating a grounded cathode gain stage from, let's say, a tone stack. The GC amp will have a very large Z_o. You could incorporate that source impedance into the resistances of the tone stack, to be sure. However, tube characteristics vary considerably with the manufacturer, models within brands, with age. The output impedance probably won't stay put, and if it varies, the poles 'n' zeros of your tone stack will change with it. Isolating the tone stack with a cathode follower representing an insignificant portion of the tone stack resistances will prevent this. It will also allow for more resaonable values of resistance and capacitance in the implementation.

Cathode followers can also be used as active pull-up circuits to drive the control grids of audio finals. Even if you stay with Class A*₁ operation, the input capacitances (C_GK + C_Miller + C_stray) are still going to require current to charge. If the current sourcing capability isn't there, then you will run into slew limiting at the higher audio frequencies. That sounds nasty. The CF can supply enough current to prevent this from happening, especially if you follow the "Rule of Five" from solid state practice: make the Q-Point current of your CF at least five times greater than your anticipated peak current.

Don't ever forget: the vacuum tube itself neither knows nor cares whether the load is connected between the plate and the positive rail, or if it's connected between the cathode and DC ground. It's always the same loadline, the same load resistance. You don't gain anything by trying to force the device into being something it will never be: a high current, low voltage device. A CF is not a magical power gain stage.

Attempting to use it otherwise will lead to degraded sonic performance. If you have a bad-sounding CF, blame the designer, not the topology.

As with any other audio subsystem, if the CF is designed properly, and used within its limitations, it is the most sonically transparent audio subsystem. If designed badly, it will sound bad. It's as simple as that.

Welcome to Dolphin Hollow State Labs

 

For audio amplification, the first active device remains the best active device: the triode vacuum tube. The triode is unique among the active devices: vacuum tube pentodes and transistors of all sorts. With these devices, the plate/collector/drain current is largely independent of the voltage across the device. This property makes them excellent approximations of an ideal current source.

With triodes, however, this is not the case. The plate current can vary with the plate-to-cathode voltage. This is the meaning of "amplification factor" --

μ= ΔV_pk / ΔV_gk (at Ip= constant)

For most triodes, the amplification factor will usually be between 10 to 100. For power triodes, this can be smaller (audio power finals, vertical deflection power amps, series pass regulators) or higher ("zero bias" RF finals). Amplification factor is largely meaningless for other active devices since it's so difficult to measure directly.

The triode, like every other active device, has an inherent degeneration when operated as a grounded cathode amplifier. This being the cathode resistance: r_k= ~1 / g_m. This resistance acts in precisely the same manner as if it were an unbypassed resistor soldered into the circuit. Just because you don't see it doesn't mean it's not there.

There is another feedback mechanism at work as well. When the signal pulls the V_gk less negative, the plate current increases, and with it, the voltage drop across the plate load. This results in a decreasing V_pk. As this voltage decreases, it tries to pull the plate current lower. Since V_gk and V_pk are pulling in the opposite direction, this is negative feedback by definition.

It is this additional source of NFB that serves to correct for harmonic distortion to a greater extent than you will see with other active devices. What harmonic distortion remains is mainly the second harmonic, h₂. Sonically, h₂ is the least detrimental. It is this h₂ that lends to the so-called "tube sound", described as "warm", "rich", "full", etc. More h₂ is described as "dark". No distortion would be the best, but the perfect amplifier, like the perfect lens for telescopes, has yet to be invented. The question becomes how to minimize those defects which you will never completely eliminate.

Of course, not all triodes are equally good sounding. As far as the effectiveness of plate current control by plate voltage, this is measured by the term: plate resistance: r_p. A good many high-μ triodes attain that large amplification factor by driving up the plate resistance. For the 12AX7, the spec sheet gives: r_p= 80K (nominal). This is comparable with the r_p's of small signal pentodes. Is it any wonder why the 'AX7 tends to sound like a small signal pentode?

Even if the r_p isn't excessive, some triodes just won't perform all that well for audio. This being caused by excessive variation in g_m with plate current variations. For all active devices, amplification tends to increase with increasing current. If the positive going half cycle of a sine wave receives more amplification than the negative half cycle, then the two half cycles hit different peak voltages. That's not a sine wave any more, and something has been added. Since this is asymmetrical distortion, it is even order, and mainly h₂. Types such as the 12AV7 tend to produce much more of this distortion than you'd like to see. (Although it just might be useful for audio effects.)

Nothing sounds better. It is for this reason that this "obsolete" device is still around over a century after its invention. This has been known at least since the early-1950s when the quest for a solid state device that could match the distortion performance of the VT triode began. So far, no one has found such a device.