Featured VT: 6LR8

I had a few of these come my way: the 6LR8. This is a triode/pentode originated for use as a combination plate coupled multivibrator and vertical deflection amp. For a vertical deflection type, the ratings come in pretty stiff: P_D= 14W for the pentode power final. I drew up some loadlines, and these do indeed look promising: The h₃ estimate is quite low, and it doesn't seem to be too far into Class AB₂. If there are any problems with higher order harmonics, and I wouldn't be expecting these anyway since vertical deflection duty values good linearity in the finals. Any residual pentode nastiness can easily be tamed by means of local NFB. A PP pair could easily do 30W into a quite reasonable 6K (P-2-P) load, and stock Edcors come quite close to that anyway. Conveniently, the same envelope includes a small signal, high-μ triode. Its characteristics look linear enough for audio amp duty, and you could use them to form an LTP driver, or use them as grid driver cathode followers. Though I'd prefer the latter, as there are better small signal audio triodes for front end/phase splitter duty. The V_HK ratings also high enough so that cathode follower duty isn't going to be a problem in that regard. The possible drawbacks are that Novar (large nine pin) base, and the rather stiff heater current requirement. Those pins are awfully thin to be pumping 1.5A through them. You also have to be careful when selecting sockets. There are all too many cheaply made Novar bases with the holes out of alignment. If you have to force the pins into the socket, that's just begging for a failure of the glass seals. Letting out the vacuum, especially when powered up, will certainly have some pyrotechnic displays, and possibly catastrophic failures elsewhere. The other good thing about the type is that it is also available in some odd heater voltages originally intended for operation as a series string across the AC mains. 21LR8 -- 21V/0.45A 31LR8 -- 31.5V/0.3A These types are definitely cheaper, and probably more available due to reduced demand. You could either wind your own heater xfmrs, or have custom jobs (Edcor) make them up for you. As with all these TV deflection pents, the screen grid ratings are low enough to make pseudotriode and UL operation impractical. Another definite audio "sleeper" that didn't have audio final use mentioned in the spec sheet.

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.

The Weak Link

The major problem with hollow state is that the vacuum tube is a high voltage, low current -- and therefore Hi-Z -- device. Electromechanical speakers are just the opposite: requiring high currents at relatively low voltages. The usual solution to this problem is the impedance matching transformer. Unfortunately, no transformer that has a ferromagnetic core is a linear device. Air core transformers that can operate at audio frequencies are not exactly efficient, and what you gain in core linearity you promptly lose in the enormous stray capacitance that the huge number of turns such a thing would require.

So the ideal situation would be to get rid of that transformer completely. There are a couple of factors that go against this: the Hi-Z nature of the device itself, and the fact that there is no such thing as a "P-Channel" VT. Given that, you are limited to some sort of "totem pole" output stage. Here, the problem is the lack of symmetry: one half will be acting as a grounded cathode amp with its characteristic voltage gain, while the other half will be, more or less, working as a cathode follower with its less than unity voltage gain. There have been various attempts to deal with this problem such as asymmetrical drive -- the SEPP output, or the Futterman variations that include feedback to either make the cathode follower half behave more like the grounded cathode half, or the inverted Futterman that applies NFB to the grounded cathode half to make it look more like a cathode follower.

There is another topology that at least takes care of the balance problem: the Wiggins Circlotron design, this being a bridged output stage. Even though originated for use with an OPT for the purpose of eliminating Class AB switching transients from the primary, it also makes for a very good OTL design as well.

The other problem remains: the low current nature of the VT itself. You really don't have much choice here, so far as possible types are concerned.

There have been VTs that were intended to give some serious output currents, these being the horizontal deflection finals. Even though the screen voltage specs limit the screen voltage to rather low levels, these can still pull some serious current at lowish V_PK's. Still, you are going to have to parallel up individual units in order to get the necessary output currents. The 6BQ6GTB can easily pull some 350mA of peak current when the plate and screen both operate at 150V_DC, if you go a bit into Class AB₂. For 30W of output into 8R, it will still require 16 6BQ6GTBs (8 per phase). That's a bit too much.

Other possible types would include the 6C33C (Series pass regulator used on the MiG fighter). There have been a lot of problems concerning this type, as its reliability is questionable, even with the special "burn-in" trick. Also, the pins are way too thin for the heater current requirement. MiG technicians replace not just the 6C33Cs, but also the socket as part of routine maintenance. I would avoid it.

The most likely type is the 6AS7 (and its relatives: 6080, industrial type, or the 6082, 26.5V heater version) -- another type originated as a series pass regulator tube. As such, it has the high current capability with reasonable V_PK's, and it is also a dual triode, so that you require half the number of bottles. Though its plate characteristics aren't spectacular, they aren't half bad either. It's not likely you'd want to use this in a conventional output circuit as there are better power triodes for that. Still, the 6AS7 can pull significant currents without taking the control grid positive. Eight 6AS7s (16 sections, 8 per phase) can get you 30W of output without serious spec busting. The type is also well known for having a high g_m and a very low r_p. The 6AS7 has been used to drive VT Class B audio output stages (AM plate modulators) for a very long time now.

Does getting rid of the OPT really represent an improvement in overall linearity? What you might gain in one area, you might lose back in another: operating the VTs into a very steep loadline (eight sections in parallel to drive 8 ohms is just 64 ohms per section). You might be just as well off in substituting MOSFETs for the hollow state output. There have been some complaints that paralleling finals leads to sonic degradation, since the individual VTs won't all be operating at precisely the same currents with the same exact characteristics. Whether or not this makes a difference is a whole 'nother story. At least the very low (for hollow state) impedance of the 6AS7 does help to mitigate this effect. This is another one of those areas where you just might have to try it to see and make up your own mind if it's worth it.

As for trying this, it is definitely something to take a shot at. It is especially helpful in that the 6AS7 is a current production VT, and at least in this case, the new production is superior to NOS offerings: better matching between sections, and a more robust design that will stand up to the slight spec-busting that can reduce the number of triode sections per phase. The 0.125A plate/cathode current rating is for DC regulator use. That peak current can be increased to 0.375A in audio final use. Music and voice programs won't be spending very much time at the maximum peak current. (It would be a different story for DC use or continuous max P_RMS applications. (If you were doing that, then consider the 0.125A spec to be a DC/RMS value.)

The only other requirements are DC offset detection/correction since you definitely want to keep DC out of speaker voice coils, fault detection in case one 6AS7 (or one section of a 6AS7) develops some fault that causes it to pull more current, and HV delay. Being that the intended purpose was series pass duty, the heater-to-cathode insulation is a good deal thicker to stand up to higher than normal voltages. This also slows down greatly the cathode warm-up, and the specs call for preheating to make sure the cathodes are at operating temp before hitting 'em with the HV.

The specs also strongly discourage the use of fixed bias. The problem of using the much more desirable fixed bias can be solved by mixed fixed/cathode bias, and over current detection. A 6AS7, OTL, Circlotron design is definitely "in the works".

The Numbers Game

In most of the electronics field, the characterization of
performance is pretty straight forward. If a video amplifier has
inadequate bandwidth that causes the loss of fine detail, and excessive phase nonlinearity that smears images across the screen, or puts out all the wrong colours, no one who isn't blind as a bat will disagree that it is a very poor design. If a servo doesn't track accurately, no one will argue about the finer points of that kind of distortion.

The audio field is something else entirely. Eyes don't lie, but ears deceive. Characterizing the performance of audio electronics is complicated by the very fickleness of the end user. What sounds just great to one will be judged inferior by another. Different people hear different things, and there may very well be those (the "golden ears") who hear things that the rest of us can not hear at all. Throw in musical preferences, and it gets even more complex. It's highly doubtful that the head-banger and the Classical aficionado will agree that the same amp is superior to other offerings.

The marketing department needs a selling point, and that has long been THD. The Williamson of 1948 made THD fashionable, and started the fallacy that gNFB was the cure for all ills. Between 1948 and 1956, the Williamson design was all but universal. The commercial "Big Box" manufacturers went for more power and ever greater feedback factors to drive those THD numbers ever lower. By 1960, Big Box products had excessive bandwidth (necessary for stability) and enormous feedback factors. UL EL34s in Class AB had become quite fashionable. The demand for more feedback led designers to abandon the lower distortion, lower gain types (6C5, 6J5, 6SN7, 6CG7) for high-u triodes and small signal pentodes. These high gain devices are less linear, adding more open loop distortion that was then corrected by high gNFB. Solid state was a natural progression since transistors have almost unlimited gain available, getting rid of the OPT removed an obstacle to driving up gNFB, and, of course, the gross nonlinearity of transistors demanded even more correction. This naturally led to a certain sloppiness in the open loop design. Why worry about poor open loop performance, add enough gNFB and you can sweep your mistakes under the carpet where no one will ever find them. Unfortunately, you also sweep away much of the musical detail.

The end result: sound-alike amps, with that lifeless "Big Box" sound.

The supreme irony of this is that there is almost no correlation between the THD figure and listener satisfaction. Unless something measures really badly can it be said that it will sound really bad. If THD had everything to do with it, we would have attained audio perfection a long time ago. Solid state amps have THD figures of 0.001% or even better, yet when was the last time you ever heard a manufacturer of VT equipment brag that their amps had that great transistor sound?

There are further complications that you just don't see in other areas of electronic design. An RF amp works on a very narrow band of frequencies into a well characterized load that's almost pure resistance: either a transmission line or antenna. If there are any reactive components, you can tune them out. Whatever device distortion is produced is of no consequence since one or more tuned LC circuits and/or bandpass filters will reject any harmonic distortion. This does not apply to audio. Even though we draw nice, straight loadlines representing a resistive load impedance, speakers are anything but. A spec that says a speaker is an "8 ohm speaker" refers to the DC resistance of the voice coil winding. In operation, the device is an AC synchronous motor. The only difference is that the voice coil moves back and forth instead of
going around. Any time you have coils moving in a magnetic field, you have a voltage induced in the coil. That induced "back EMF" adds to the incident voltage to produce an impedance that varies in both magnitude and phase. In practice, your loadline opens up into an ellipse (for one frequency only). This is bad news for your output devices, which like to see a constant load. Then there's the question of OPTs. A transformer designed to operate at a frequency of 20Hz is going to be very different from one that's designed to operate at 20KHz. And yet, OPTs are expected to do both, everything in between, all at once. By all rights, it shouldn't work at all, let alone as well as it does.

Distortion is not created equal, nor is distortion necessarily undesirable. If that were the case, then no one would ever use a tone control or graphic equalizer. Since these suppress or enhance various frequency components, what comes out is no longer the same as what went in. In other words, the output is distorted. The undesirable, but inevitable, distortions are not created equal either. You can see this with an oscilloscope. If you add 10% of second harmonic to a pure sine wave, the difference is hardly noticeable at all. Even if you know what to look for, you'll probably miss it. It's not until you get to about 20% h₂ that the disturbance becomes obvious (flattening of the peaks on one side only, all even harmonics produce asymmetric waveforms). Just 10% of the third harmonic will make for clearly distorted waveforms (nearly triangular waves). Smaller amounts of higher order harmonics make for even more severely distorted sine waves. This means that you can tolerate more lower order harmonics than high order. If the THD is several percent, but it is all h₂, you probably won't notice it at all. It is this type of distortion that low-u triodes tend to produce. On the other hand, even very small amounts of high order harmonics can be all but unlistenable. The cross over distortion produced by underbiased transistor amps -- or Class B VT amps -- is a good example of this, and it does sound quite nasty, even in small quantities. It is for this reason that Class B is relegated to RF amplification, or audio amplification where power and efficiency are more critical than fidelity.

As for what the various distortions sound like, well, that depends. Unless the distortion is extremely bad, it isn't really all that obvious. Some harmonic distortion doesn't sound any different than background static, or hissy "esses", or background crackles. Often, there really isn't any one thing one can identify, other than listening becomes fatiguing after awhile. The least harmful is h₂.
Since the second harmonic is musically correlated, being one octave higher, it tends to make for a "richer" sound. Excessive amounts tend towards a "darker" sound, as has been the complaint against the very nonlinear 12AV7. From a design point of view, this would not look like too much of a problem, since even order harmonics are nulled with balanced circuitry. As for h₃, this, too, is musically correlated, being nearly an octave-and-a-half. The effect of h₃ is to lend a sense of "detail". More h₃ lends "edge" or "brightness" to the sound. For some types of music, this can be desirable. As for higher order harmonics, these aren't musically correlated, and sound very dissonant and unpleasant. The higher order the harmonic, the less of it you can tolerate.

THD numbers really tell you very little. The spec sheet for the 6L6, for example, gives a THD figure of just 1.8%, before any NFB is ever applied. That is really quite outstanding. The lowest THD figure for the 6V6 -- another audio type -- is 3.5%. However, operating open loop without any NFB, 6L6s definitely sound worse, with lots of "pentode nastiness" that quickly wears the listener out. With some program material, the effect is as annoying as fingernails on a blackboard. O. Schade, the developer of the 6L6, recognized this problem back in 1936. O. Schade recommended that local NFB be applied to correct for the problem of higher order harmonics. Even though the 6V6 measures worse by a significant margin, it produces mainly h₃. This doesn't mean that you won't need NFB, however, running open loop, produces mainly an overly "aggressive", "edgy", or "bright" sound. The type doesn't need the added assistance of local NFB in order to sound right. Fifty or so years ago, Norman Crowhurst proposed a weighting system of distortion measurement that would account for this: undervaluing the less destructive low order harmonics, and emphasizing the higher order harmonics. Simple THD measurements don't do this. Of course, the industry wanted nothing to do with it.

There is another type of distortion that's much worse than harmonic distortion, and that's IMD. IMD arises from the same source as harmonic distortion: active device nonlinearity. IMD occurs whenever a lower frequency mixes and modulates a higher frequency. It's really the same as AM with a very small modulation index. All IMD is quite bad since its frequency components are rarely, if ever, musically correlated. This is the reason to stay away from nonlinear devices and nonlinear operating points. IMD is also the reason why poorly filtered DC sounds worse than AC for filament heating. With AC, you have just the one frequency to deal with. DC, unless well filtered, adds quite a few harmonics of the line frequency. Even if the level doesn't seem all that high, there is sufficient to mix with signal frequencies via intermodulation to degrade sonic performance.

The simplistic answer is to add as much NFB as you can manage while maintaining a reasonable level of stability. Unfortunately, it's not that easy. Over fifty years ago, Norman Crowhurst looked into the problem, and discovered that the main effect of NFB is the reduction of h₂ and h₃. It doesn't do much, if anything, for removing low levels of higher order harmonics. NFB can make the situation even worse by filling the noise floor with lots of low level, high order harmonics. This can, and does, ruin any pretense to sound stage. In much the same way as NFB, the unavoidable quantizing noise that accompanies analog to digital conversion also adds lots of uncorrelated harmonic noise to the noise floor. It has been proposed that this quantizing noise is the reason that some find the CD sound to be inferior to vinyl and tape. This is also a problem with certain audio file compressions, such as mp3. This is a "lossy" compression, in that some of the bits are thrown away. The first consideration in which bits those should be were the ones encoding near noise floor audio. These mp3 audio files just don't sound quite right because of that missing detail (And to think some people actually expect you to pay for mp3's!) What's going on at the nearly subaudible level seems to be a good deal more important than once suspected.

The main lesson is to design for the best open loop performance, and add just as much NFB as needed to improve the overall sonics. Of course, it is a trade-off: sound stage for better bass and less of the excessive harshness that often comes with open loop operation.