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!

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.