6S41S P-P Amp Initial Query

[wrenchone]

I recently scored a few pieces of 6S4S with the intention of making a P-P amp, but recently some posts about the 6S41S got my attention. This is still a reasonably priced tube, with higher plate dissipation than the 6S4S. Not as high a plate capability as the 300B, but I can buy a whole boatload of 6S41S for the price of even the cheapest 300B. Anyway, I've seen some SE schematics for the 6S41S, but no P-P. Any leads? I'll probably end up doing my own thing, but it's still nice to see what others have done.

Suitable iron is another question. I have some Fisher 500B output transformers, but I kinda doubt that the impedance is all that suitable for a tube with this low of an rp. I was thinking of running lower voltage, higher current, with an 8 ohm load on the 16 ohm tap of the transformer. Is this an an optimal usage of the 500C transformer (I have no idea how the secondaries are wound). I'm also not comfortable with the amount of negative bias is takes to make the 6S41S go "whoa, horsey!" at the higher plate voltages, another reason for a somewhat lower plate supply (though with a custom SMPS, anything is possible).

On another note, I have heard that the tube to tube variability can be a problem. Comments?

[Wavebourn]

I can see the reason why tubes that are not good for SE may be good for PP, but...

Speaking of the tube to tube variability, it is true for all Russian tubes, cars, trucks, etc...

http://datagor.ru/amplifiers/tubes/p...snogo-bp..html

[wrenchone]

There is some debate as to whether the tube is good for SE sdome people have loved it, and one hated it. As far as P-P applicability, I'm looking at it right now on a volts/amps/watts basis.

Thanks for the link - I'll try to find a translator that can make some sense out of the page.

[Wavebourn]

Originally Posted by wrenchone

There is some debate as to whether the tube is good for SE sdome people have loved it, and one hated it. As far as P-P applicability, I'm looking at it right now on a volts/amps/watts basis.

Right. Good for SE means A Good Tube.

Thanks for the link - I'll try to find a translator that can make some sense out of the page.

Google Translate

[wrenchone]

Come out and say it. Do you think this is a bad tube, and if so, why?

[Wavebourn]

Originally Posted by wrenchone

Come out and say it. Do you think this is a bad tube, and if so, why?

I did not say that. I meant an opposite thing: if the tube is good for SE, it is a GOOD TUBE. So-so tube like 6L6 clones may be bad for SE, but it may be still good for PP. The whole your question sounds weird to me.

However, the driver for this tube needs corresponding, but you know better than anybody else how to use MOSFETs.

[wrenchone]

Not really a weird question, as two people thought the tube was good in SE, and one thought it was awful (or at least, not so good). In the Russian page you referenced (Google gave me a somewhat fanciful translation - I smiled), the author used the tubes with a rather severe transformer impedance (1.7k if I remember correctly). From my calculations so far, I think something in the 3.3-3.5k range will deliver useful power. This also allows me to experiment using some of the iron I have on hand, at least in the preliminary stages of the project. I've decided that I've bought my last output transformer for a while, as I just received another pair of Fisher 500C transformers this week, as well as a pair of transformers that came from a Knight 6BQ5-based amp. I now have quite enough stuff to play with for the time being.

I had an inspiration this afternoon, and I'm going to try a really heretical approach - a switching cathode bias supply. Finally, an audio app where I can use one of my employer's SMPS chips. I won't spill the beans about this circuit until I have it breadboarded, but it will be interesting, and will no doubt elicit howls of protest (or at least a huh?) from the anti-sand purists. It may also prove useful for people biasing low-mu triodes that need both high voltage at the cathode, plus a high quiescent bias current. I really hate blowing 8-9 watts in a resistor (actually twice that if you use a common bias source for both output tubes).

Given that I am using cathode bias, I may back off from using all-sand followers to drive the output tube grids, but instead use a pair of triode/pentode duals in a differential amp/splitter and cathode follower topology. A pentode differential stage followed by cathode followers should give me enough drive to tickle the output tubes.

[Wavebourn]

Going from PWM cathode bias you may end up with high frequency output transformer and synchronous rectifier after it.

[wrenchone]

Not really - I know where to stop. Wait 'till I actually do the breadboard and post the circuit. It's not all that radical in complexity, but does some fairly radical things in reducing power dissipation for big triodes with cathode bias (Now, if I could get rid of that pesky filament, we'd really have something).

Actually, I have an idea about that as well, but it's much harder to bring to reality. If it worked, it would drastically increase the efficiency of your average tube, but at the same time, demote the tube to the same status of just another part that gets loaded into a PCB.

I don't know about you, but the romance factor of tubes (glass envelopes, glowing filaments, maybe a touch of blue fluorescence for dessert) is one of the things that make them so fun to play with. I'd like to extend their useful lifetime into the next century, but if they look the same as any other lah-di-lah part that gets loaded into a board, where's the fun?

[smoking-amp]

"I'm going to try a really heretical approach - a switching cathode bias supply"

Hmmm,
HF inductor in the cathode path, pulsed grid pos. bias + input signal, continuous analog tube conduction. Or maybe HF inductor in the plate, pulsed grid pos. bias + input signal, discontinous tube conduction but continous plate inductor current. Analog pulse height switchmode instead of PWM? Seems that would have limited dynamic range without pulse width control too. Norman Crowhurst's last article in Glass Audio was for a circuit that combined analog to pulse height and width modulation. He never got to finished part 2 unfortunately. A revolution shorted out.

There are a couple of technologies that could put a tube into a ceramic chip package. One is the nano-rod cold field emission cathode, the other is the titanium thin film tunneling emitter. Graphene grid maybe. Or laser perforated thin film grid. Very good current densities, no filament. Low voltage operation. Hard to get funds though with Mosfets around.