Has anyone tried a pair of 50C5s in push-pull?
Haven't actually dunnit myself, but you are right: 50C5s are easy to come by. As for designs, I've done some of the preliminaries already (see attached).
The keys to sonic goodness with this particular type is to run them at Vsgsg= 90Vdc (included in the spec sheets) and not the much more common practice of running the plates and screens at the same voltage. Also, like every other pent, regulate that screen supply. Active is best, but, since this may be a project where there is a premium on "cheap", a VR90 type can be used to regulate the screens.
You can also find better PP loadlines with just a bit of spec-busting, and operating from a higher voltage DC rail. That gives the extra headroom for a somewhat shallower, and more linear loadline.
Being that the 50C5 was made for Class A, that means you can ditch a phase splitter stage. Just run the PA as its own splitter. Active tail loading of a power differential stage gives excellent balance between phases, and enforces Class A operation (not a prob here, since the 50C5s will be operating Class A anyway).
I figure you could run the filaments in series with a resistor, right across the 120V AC line.
Expect just ESSSSSSSSSS-loads of hum if you do that. These high voltage heater types are more susceptable to that. Better to go with DC on the heaters. If you don't want to do that, a dirt-cheap, 12V "doorbell xfmr", run "backwards" from a 5.0V winding otherwise used to light up power diodes like 5U4s or 5Y3s will produce close to the 50V the 50C5 wants to see. It also gives you your line isolation so's you can include hum balancing at the 50C5 heaters.
The major sticking point in any 50C5 premium design is that the OPTs that were most commonly used for this type are pure:
An externally hosted image should be here but it was not working when we last tested it.You will probably need to have these custom wound (Edcor and Electra Print can both do that.)
Just to be totally clear to everyone reading this: DO NOT BUILD THE CIRCUIT IN POST #4.
Yeah, don't. 35C5s are not the same as 50C5s, and are a good deal inferior, both from a linearity PoV, and they don't have the Pd rating that the 50C5 has. It's a question of when, not if, that thing poofs a computer sound card, a CD/DVD player, etc. May be you'll even be around to tell us about it.
(If you need 50C5s with other heater ratings, your choices are:
25C5, 17C5, 12C5 -- there is no 35V version of this type.
The 6.3V heater version is the 6CU5.)
That design just plain SUX. Its phase splitter is as inferior as you could find, has no provisions for balancing that thing, will not exhibit AC harmonic distortion balance, is not even amplitude balanced between phases. It also probably doesn't have enough open loop gain to support the required gNFB. Back in 1959 it probably sounded OK, with the cheap tuners, and turn tables that it was intended for. Of course, that's no endorsement.
These days, a solid state amp will probably sound better.
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