Parallel output tubes/valves in push pull. Advantages/disadvantages?

[exeric]

The 6SN7GTA/B is equivalent to the older, lower plate dissipation version. The main differences are an enhanced plate dissipation rating, and the extension of the plate characteristic into positive Vgk territory. Most of the 6SN7s that you see today are the GTA version anyway. So far as I can tell, the 6SN7 wasn't originally a vertical deflection type, but was quickly adapted to digital and quasi-digital operations (during WW II, and essssss-loads of 6SN7s found their way into radar sets) and was the choice for vertical deflection duty after the war when TV started coming into its own during the late 1940s.



A power MOSFET can easily drive 240pF of load capacitance (22K1 @ 30KHz). The r(don) of power MOSFETs is extremely low, so this is NBD.

Thanks for the all the information. I'll have to study some more on the characteristics of MOSFETs to really understand this. It is a relief though.

I've been doing some sims and the idea of not using a front end tube amplifier is a washout. Only puts out about a watt or so even with an input transformer with a step up of 1:2. It was actually a pretty dumb idea of mine as more tubes only lowers the average dissipation per tube given the total amplification. So in that case 1 watt would just be divided by more tubes.

I still might use the idea in spite of all that for one simple reason - I can get away with it. My dac can be selected to put out 6volts rms ors 16 volts pp. One can find line input transformer with 1:2 ratios that are good from about 10h to 70khz +-1 db. Once one goes above 1:2 you really start degrading the response at the extremes. That gives me a peak watt level on each tubes sim waveform of about 7 watts. (Thats one of the two plates in each tube.) If I'm not mistaken, (and I could be) the effective wattage would be somewhere around .5 to .707 of that. So with that kind of input level with a 1:2 input transformer I'm getting to the limits of 6 tubes per channel. So it works for me and I'll enjoy not having coupling caps in the mix.

The thing I like about this idea is the simplicity for the power output. You only need one positive voltage, not counting filaments, and one negative supply for the Mosfets. There are always tradeoffs in any design. So while some may hate the idea of paralleled tubes one can get benefits in other areas. No cap in this case and simplified design requirements. So Ken, one just has to decide which things are important to you. Everyone, including me and you, will come down differently on how we weigh these things. It's best to just except that different people are different in these things.🙂

 

[kenpeter]

I think one could say, using your argument, that one should never use tube PP because no two tubes ever have consistent characteristics between the two sides. 🙄

Don't go there, I'd have to bust out with the ol' evil "Anti-Triode" rant.

Of course, you could have push pull triodes in parallel with Anti-triodes,
and make both sides come out exactly equal? I think MJK proposed that
quite some time ago. I don't think the same parallel "cheapening" of
unmatched triodes applies, as the cutoffs would be on different ends
of the curve.

I am not your father's Old Spice, nor his Oldsmobile. Get over it...
I'm not telling anyone what to do, only balancing weird questions
with even weirder (hopefully truthful) answers. Its what I do.

 

[kenpeter]

Aside from all that: I think parallel triodes is a fine thing for low noise pre-amp.
Paralleling triodes reduces all kind of noise except B+.

If you load with a true CCS (perhaps a Mu-Follower?). Keep the loadline flat,
you can avoid seeing how a "cheapened" Mu compresses at reduced current.

But that sort of avoidance is never gonna work for a driving the loudspeaker...