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In Reply to: RE: A2 Triodes posted by Triode_Kingdom on May 04, 2024 at 15:37:36
The problem with this scenario is that the additional power in A2 won't be sustainable at low frequencies. Unless you have an output transformer the size of a mini-refrigerator, output power will fall dramatically below 100Hz. This means the only remaining advantage to "capacitorless" coupling is the elimination of blocking distortion at higher signal levels.
If the OPT is designed to be able to play the bottom octaves with the power of the amp the class of operation isn't going to affect it. Admittedly, getting this sort of bandwidth with this much SET power is challenging and usually transformer designers opt to get the highs right rather than the bass.
If you are concerned about LF saturation due to a cheap output transformer, simply limit the values of the coupling caps prior to the driver (in this case, the EL34).
Follow Ups:
Only a parafeed transformer might be able to accomplish this.
"If the OPT is designed to be able to play the bottom octaves with the power of the amp the class of operation isn't going to affect it."
Show me a company claiming their standard SE transformer can produce full A2 power from a 211 or 845 at 20Hz (or even 30Hz), and I'll show you a company that's lying. Like i said earlier, it wold have to be the size of a mini-fridge.
Why couldn't you use a 2A3 or 45? The answer is, you can. And at those power levels, it might be doable.
BTW Jack Elliano has been doing some interesting work in this area and patented class A3 operation. I heard one example about 12 years ago that was pretty convincing. You might want to look into it.
It was not A2, and I would have to classify its OPT as 'using a trick' to reduce its physical size whilst run SE. Power tube was a 304... :)It took a whole pig butchering table. Not light. Not small. Capable of 40W at 30 cycles? probably.
Dowdy Llama, Dave S, and Jeffrey Jackson know of it. Dave could likely comment on the OPT's LF performance... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 05/07/24
that output had the filament current fed from a high impedance source running through a tertiary winding on the OT to offset ~95% of the anode DC so a minimal gap could be used. Kinda like what happens in PP but with a single tube.
dave
"that output had the filament current fed from a high impedance source running through a tertiary winding on the OT to offset ~95% of the anode DC so a minimal gap could be used. Kinda like what happens in PP but with a single tube."
That's a really good idea. I've thought about something similar using a simple resistor to offset the current, but that really kills efficiency. When all is said and done, I think Class A push-pull leads to a better solution. Many EL84 PP amplifiers operate very near class A, which might be one reason they sound so good. The use of triodes should make this mode even better.
"I've thought about something similar using a simple resistor to offset the current, but that really kills efficiency."
I think a CCS would do it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
right.... like the CCS comonnly used to power a filament. The amp referenced by douglas did it old skool with LCLCL filaments.
dave
It does not hurt at all to have 25A of filament current to drive through the counter winding either... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I am using a pair for my 304TL amp. 28 pounds, very large. However, I am using them from 100hz and above because I never can get enough big bad solid state bass.
Edits: 05/07/24
Good distortion performance for a single 211 or 845 requires a primary Z at least 10KΩ. And flat response to 20Hz at full power (say 30W in A2) would require a much larger core than the 1642SE if DC is flowing. I think it might not even be possible to accomplish both criteria (10K primary and much larger core) while also maintaining flat response to 20kHz. This is the weak link in SE amps at power levels above 12W or 15W.
Edits: 05/07/24 05/07/24
How much primary inductance is required for a 10K primary, 20Hz?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think a bit more specification is required. Such as defining the amount of loss between 20 Hz power output and that which can be delivered at say, 2000 Hz. That if you want a real answer to your question anyway... LOLA2 will of course be just like A1 in the need to balance the idle current in between minimum and maximum. The extension of the maximum is no magic in the power output.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Edits: 05/08/24
"defining the amount of loss between 20 Hz power output and that which can be delivered at say, 2000 Hz."Can we start with no loss (due to lack of primary inductance)?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/08/24
Only with a large pile of primary L.
Seems we are talking about what is possible, so how much response at 20 or 30 cps is left compared to low-midband? Seems to me the question would be for say, -3 dB at 20 cps compared to something like a 1000 cps output, could be a part of determining just how much primary L is needed.
Once you have figured out what you need, then you can try figuring out how to go about getting it. Soooo, picking that 10k number for convenience, and putting down 30W at its -3dB point at 20 cps, how much are you going to need? Then at the same time, confirm the AC flux does not push you over the edge either...LOL
I will stick with PP for my A2 amp. Effectively a zero bias one. An S265 should do just fine for the power output a pair of DHT's with 65W plates can deliver. Off to get a small handful of P-channel, lateral MOSFET's to drive the cathodes with( source followers ). If I get totally nuts I will do voltage amplification and phase splitting with a pair of HY40's. That amp will have a lot of 7.5V running around heating cathodes.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I was trying to understand what is considered acceptable response at 20Hz and how much inductance is needed to get that in a single ended amp.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/08/24
well... FWIW here is what spice says.
the basics are 10K OT 211 operating at 1kV -45V bias and around 90ma and inductance is stepped 40, 80 and 160Hy for both 20Hz and 50Hz.
The third "penne" plot has 40Hy of inductance and combination of 20Hz and 1000Hz each at -6dB as the excitation source. This appears to encompass an envelope a lot closer to the 50Hz one than the 20Hz one which is in line with what Partridge claimed was the realistic lowest frequency of interest for music.
personally i would be fine with 40Hy in this case with music being the source.
dave
That operating point (1000v/90mA/10K load) is more suited to A2 operation; in A1 as shown it only make about 8 watts.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
I measured the low frequency -1dB point for the FS-100 (driven by a 211) at 19Hz. I've never measured the inductance of these transformers though. Whatever it is, core flux degrades it considerably when output is increased above a few watts.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
20H [edit - I meant 80H] to match the DS-025; 135H to match the FS-030. For 20Hz, I think you'd want the latter.
Edits: 05/08/24
135Hy with a reactance of 17k ohms for a 10,000 ohm primary.
What would be "normal" if we were talking about a PP transformer?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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