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.