Featured VT: 6CB6

 

The 6CB6 was originally designed for high frequency work, especially as an IF amp for TV sets operating at 45MHz. You can see here that the plate consists of parallel planes on opposite sides of a wide, flat cathode. The plates are spaced far from the grids in order to minimize internal device capacitance. The type has a fairly high g_m, though not as high as some other types used for high frequency and/or wideband work. It also features an unusually high V_HK spec that can make it useful for other purposes, such as DC error amps, or DC coupled LTP's. As for audio performance, this loadline looks quite good.

 

Who Says Pents Have to Make High Order Harmonics

 

This particular loadline shows that h₃ and h₄ are equal to 0: this produces h₂ only. If you don't require the full output swing, the distortion decreases proportionately. If used as an LTP with active tail loading, this would be a clean gain stage indeed. The excellent audio loadlines have lead to the use of the 6CB6 in both professional (one example is given in the RCA Receiving Tube Manual) and DiY designs. It isn't exactly an audio "sleeper", but not audiophool expensive either. It also comes in some odd heater voltages: the 3CB6 (3.15V/0.6A) and the 4CB6 (4.2V/0.45A) the latter types intended for use with TV sets that didn't include a PTX and daisy chained the heaters across the AC mains. This should keep them available.

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 50C5

 

 

If you were there back in the "good ol' days", you saw 'em in almost every cheap AM radio, some really bad FM rigs, those cheap record players you gave the kids so's they'd keep their mitts off the really good stereo or Hi-Fi, home intercoms/PAs, low power guitar "practice" amps.

Those memories are likely to be less than fond ones. Everything the 50C5 appeared in sounded really bad. It wasn't just due to the single-ended pentode topology (which is bad enough) the non-use of any sort of corrective NFB, but the use of really hideous OPTs. These were always tiny affairs, usually riveted right to the speaker's basket, and with a low end frequency of maybe 200Hz (this being done deliberately to attenuate the 60Hz hum that universally plagued these thingies). The spec sheet doesn't even mention push-pull, and it's pretty obvious from the plate characteristics that Class A₁ was what was intended from the get-go.

There is another side to the 50C5, and one the manufacturers definitely intended. Unlike most pentodes, the spec sheet includes plate characteristics for both 110V_DC and 90V_DC on the screens. The plate characteristics for V_2K= 90V_DC are a good deal more linear. It is also a convenient voltage as this is what the VR-90 voltage regulator tube produces.

50C5 PP Loadline

 

With some slight spec-busting, it is indeed possible to find a really decent PP, Class A₁ loadline. Here, you're getting almost seven watts of output, and with a very small H₃ estimate. Since the 50C5 was intended to stay in Class A₁, that brings up another possibility: use the PP finals as an LTP phase splitter. A 60mA CCS in the tail will sacrifice some power, as you'll get just a bit over three watts if you do this. Restricting the output swing will also improve on distortion performance.

This could be a good solution to a low power requirement, such as computer speaker/amps, or a small stereo amp., especially if you have efficient speeks and don't need a whole lot of power anyway. Of course, if you did this, you would need premium OPTs, and definitely not any OPT designed back in "the day" for 50C5s. Those OPTs were universally hideous. That probably means a custom job (you can get these from Electra Print for a reasonable price).

This is another type that's widely available due to the sheer numbers of them made back in "the day", they don't have that "audiophool premium", and they're not terribly expensive. There is just one word of caution: UNDER NO CIRCUMSTANCES ATTEMPT TO RUN ANY PROJECT DIRECTLY FROM THE AC MAINS. Yeah, it was done often back then (AC/DC operation meant no PTX). That is likely to ruin equipment due to "hot" chassis, cross-connected AC mains that meant short circuit, and could even get you killed. It was a really bad idea then, and it's still a really bad idea today. You can light up your 50C5s with a cheap "doorbell" transformer: connect its 12V secondary to a 5.0V secondary on your PTX, and you will get 50V from the backwards connected doorbell transformer.

50C5-oids

50B5 -- This type has the same characteristics, but there was a problem with it since it used the 7BZ pinout (same as the 6AQ5) that put the plate connection right next to a heater connection. The 50C5's 7CV pinout puts the plate connection on the opposite side. This was done due to flash-over problems. When the filaments are series connected across the AC mains, the cold heaters don't drop the voltage.

25C5 -- 25V/0.3A heater version, otherwise the same characteristics

17C5 -- 16.8V/0.45A heater

12C5 -- 12.6V/0.6A heater

6CU5 -- 6.3V/1.2A heater (This is the 6.3V version despite the extra letter. 6C5 was already taken: an Octal, singleton triode.)

The 35C5 definitely is not a 35V "50C5", as there is no such thing. It's a different type with different characteristics, even though it was often used for the same purposes as the 50C5. It doesn't have quite the plate dissipation spec, and the characteristics look less linear.

Featured VT: The 6X8

 

This is one of those odd types that were designed for a very specific purpose: a local oscillator/mixer for both VHF TV sets and FM radios. As you can see from the pinout, this type uses a common cathode for the triode and pentode. Also, it is not built the way you would expect: one triode and one pentode with the cathodes internally tied together. The 6X8 has but one flat cathode. The two types share it, with the control grids mounted on opposite sides of the single cathode.

 

 

You may be wondering: what the hell is it good for? In audio applications, not a great deal. Except for one thing, that is. The 6X8 can be used as an unbalanced differential amp for active screen regulators, and other series pass regulators. The pentode half is capable of some considerable gain, the V_HK rating isn't half bad, coming in at 100V_DC (both polarities) and could be used without heater lift if using VR-75 or VR-90 types as voltage references.

 

 

In this schemo, the triode half forms the non-inverting input, with the "signal" consisting of the voltage developed across the VR tube. The pentode forms the NFB summing node and error correction amp. This regulator topology has the advantages over the more conventional designs since there is no load on the VR tube. The noise can be reduced by a simple RC lowpass filter. This prevents the possibility of inadvertently making as oscillator by connecting a bypass capacitor directly across the VR tube. (Not like Zeners, since the VR tube depends on a glow discharge that has negative resistance.)

I have used this actual design, and it works nearly as well as its solid state counterpart. Varying the DC rail voltage by some 40V caused a 0.8V drop in the output from the regulator. When using pentodes as power finals, sonic performance is definitely improved by good screen voltage regulation, assuming you're not using Ultralinear, or pseudotrioding the pents. This is where commercial designs cut corners: very few commercial (or even industrial) designs did anything more than use a voltage divider to supply the screens. Most relied on a single series dropping resistor. YUCK! That's the advantage to DiY: we don't have to answer to the pencil-pushers in the Accounting Dept.

You can also use the 6X8 as a "quick 'n' dirty" floating paraphase, but you wouldn't want to use that anyway if you're looking for sonic excellence. It could save some Miller capacitance since the triode half, operating as essentially a cathode follower, has no Miller capacitance. However, would that save enough to make a difference? That's what pentodes were invented for in the first place. The exception would be a guitar amp, and the 6X8 just might find some good use there.

As with any high gain pentode circuit, care must be taken with the construction if instabilities are to be avoided. This includes grid and screen stoppers. If these aren't needed, no harm done, but if there are instabilities, they are definitely necessary. Keep all connecting leads short as possible, as for an RF project, use sockets that include a central pin, and connect it to the ground plane for electrostatic shielding. The screen bypass capacitor can likewise be installed across the bottom of the socket, between the plate and grid connections, for additional electrostatic shielding.

Getting more use from "useless" Tee-Vee T00bz. They're cheap, they're out there, light 'em up!