I agree.
Big, high power tubes like GU50 or KT88 are extremely wasteful of heater power. If you run them with low plate voltage and low plate current, those big tubes will be operating in a non-linear part of their characteristics. You could run them at low voltage but high plate current, but you'll need to choose output transformers that can withstand 120mA per side of their primary windings. That's going to be a big OPT, which is more difficult to build without higher stray capacitance, worse leakage currents, and so on. The big high power tubes will require big high power OPTs, even if you run them at low voltages.
The smaller power tubes can also use smaller output transformers, which can be made to have better performance than the big output transformers required for the high plate current demands of big high power tubes.
The heater power requirement for a GU50 is 12.6V at 0.8A, or over 10 watts of heater power per tube. That's over 40W of heater power alone for a push-pull stereo amp. If you're going to use up that much electricity just to light the tubes up, you might as well use those power-hungry tubes to get a lot of audio power out. It's wasteful to only get a handful of audio watts while burning up dozens of watts on those tubes' heaters.
The heater requirements for an EL84 is 6.3V at 0.75A, or less than 5 watts per tube.
The 6V6GT is even more efficient as far as heater power goes, using 6.3V at 0.45A, or 2.8W heater power per tube.
Some people call a triode-wired 6V6GT the 'poor man's 45'. It's not really, of course, but they are reputed to sound very good in triode. They're not expensive, either. The Russian 6P6S is fine. There's even a 7-pin mini version named 6AQ5, which is available cheap.
Speaking of which, here's a proven PP 6V6-triode amp design:
RE: 6v6 pp power amp - Eli Duttman - Tube DIY Asylum
(The schematic shows 12AQ5, which is a 6V6 with a 12V heater and 7 pin base.)
Also, there was another popular design called the Musical Machine:
https://cdn.instructables.com/ORIG/FSA/A2B6/FHY0IAIF/FSAA2B6FHY0IAIF.bmp
I'd save the GU50 tubes for a monster high power amp.
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That "poor man's 300B" statement just means it's 40W triode (when strapped so) suitable for more or less the same operating conditions, nothing more.
Don't get me wrong, GU-50 is not a bad tube. It sounds OK, it is extremely rugged and it's cheap.
It's just 6V6 will eat it for breakfast in terms of sound 🙂
Those are fully OK for GU50 PP (triode).
But 300 V supply voltage is far from optimum. 400 V would be better.
But 300 V supply voltage is far from optimum. 400 V would be better.
Even 400V on g3 is not a problem for GU-50 (and it sounds better that way).
But as I said, 1.75k load per tube is too low unless you have some special speakers that don't mind the high output impedance.
Triode strapped sweep tubes (EL36/6P31S, EL500/6P36S, EL509/6P45S, 6P13S, 6P42S or probably many other European/American types I'm not familiar with) or voltage regulator triodes (6S19P, 6S33S, 6AS7, etc.) might be a better match for those OPTs.
Or you can do a parallel push-pull with more conventional tubes.
But as I said, 1.75k load per tube is too low unless you have some special speakers that don't mind the high output impedance.
Triode strapped sweep tubes (EL36/6P31S, EL500/6P36S, EL509/6P45S, 6P13S, 6P42S or probably many other European/American types I'm not familiar with) or voltage regulator triodes (6S19P, 6S33S, 6AS7, etc.) might be a better match for those OPTs.
Or you can do a parallel push-pull with more conventional tubes.
Have you noticed post 22?
What do you think that will happen with 400 V supply voltage?
And do fully understand the difference between triode and pentode connected circuit of GU50?
What do you think that will happen with 400 V supply voltage?
And do fully understand the difference between triode and pentode connected circuit of GU50?
Are those OPTs ultralinear (with screen taps)? If yes, that just screams PP-UL KT88 with about 400V to 450V B+.
Or PP-UL EL34 with 350V to 400V B+.
3.5k p-p does not look useful for GU50, unless you go class AB and employ a fair amount of negative feedback. Then you get into issues of stability, ringing, oscillation. Minor issues with high quality OPTs, but still, more work than class A, and far from a 'poor man's 300B'.
If each primary half has nominal 1.75k ohm impedance, and we want the primary impedance to be at least 3X the plate resistance of the triode it's loading, that means the rp of the triode you choose should be 1750/3 = 583 ohms.
Or let's relax that and say we need 2X the plate resistance of the triode to be loaded. That would mean 1750/2 = 875 ohms.
Those are some really low plate resistance figures.
6AS7 is a voltage regulator pass tube that has super-low rp like that. That might be a good choice for B+ of 150V with fixed bias. Your driver circuit will have to swing a lot of volts, though. There's a good Russian version of the 6AS7.
I don't know of any hi-fi output triodes that have rp below 600 ohms. Maybe someone can think of some? EL86-triode?
There are several output triodes that have rp of about 800 ohms.
What triodes have that? Well...
2A3
300B
KT88-triode
6550A-triode
EL34-triode (run at lower voltages/higher plate currents)
The other popular hi-fi output tubes have higher rp.
6L6-triode rp is close, but a little high at about 1k ohms.
6V6-triode rp is close to 2k ohms. No good with 3.5k p-p.
EL84-triode rp is also around 2k ohms. No good.
You could make a push-pull parallel ("PPP") design, using four EL34s per channel. But that means your OPT will need to be able to take 100mA per primary winding of plate current (50mA*2 per primary winding). Push-pull parallel EL34-triode could make 20+ watts per channel in class A.
Or PPP 2A3. Or PPP 6AV5GA-triode.
Isn't this fun?
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Houra!
I have four 6n5s and ten 6n8s for drivers.
And a 230v/115v x2 transformer could do the job...
I have four 6n5s and ten 6n8s for drivers.
And a 230v/115v x2 transformer could do the job...
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