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In Reply to: RE: Cascode power triode, direct couple, 845 still able to reach A2 o not? posted by Eric Chan on May 04, 2024 at 11:57:05
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. One has to carefully consider whether this single characteristic is worth so much additional complication. I'll just add that I wouldn't personally attempt to accomplish A2 in the manner shown in the schematic. Also, if the 845 is as sensitive to filament-induced noise as the 211, this one will be humming.
Follow Ups:
I agree....
When I made the Sole, Mike LaFevre made a custom output transformer for it at 15W and it was 28lbs making the amp something like 105lbs for each channel.
Plus that is a pretty screwy circuit. I would suggest the 211 if you want to do A2, much easier and something I have done. A simple reactor follower is what I did with a 100H/20ma inductor to ground and a 6BL7 driving the grid though a grid stop into the 211. Yeah no top end considering the output transformer was a 10K direct feed. Parallel feed would have been better for that tube. Just a bit before it's time.
Gordon
J. Gordon Rankin
"I would suggest the 211 if you want to do A2, much easier and something I have done. A simple reactor follower is what I did with a 100H/20ma inductor to ground and a 6BL7 driving the grid though a grid stop into the 211. Yeah no top end considering the output transformer was a 10K direct feed. Parallel feed would have been better for that tube. Just a bit before it's time."That's essentially what I built (schematic below). With the standing current at 70mA (80mA possible with this transformer), the onset of visible distortion occurs at 25W at 1kHz. Low frequency output is as follows:
100Hz - 24W
60Hz - 18W
40Hz - 15W
20Hz - 6.5WThe amps measure flat from 20Hz to 20kHz at low power (5W or less) with no NFB. Adding 5dB of NFB improves the 20Hz maximum output to 10W. These measurements created a somewhat lengthy discussion on this topic over at HiFi Haven. I estimate the core of these already-large transformers would need to be about five times as large for flat 20Hz output at full power.
Schematic Edits: There is no hum pot on the filament. It is powered from a toroidal transformer with the center tap grounded through a current meter and fuse. Filament power is approximately 80kHz.
Edits: 05/07/24
If you reverse the phase of the output transformer you can send the feedback to the grid of V2B rather than its cathode.
To do that there would need to be a series resistance in the grid circuit of V2B.
The advantage of doing is this way is the resistor divider network is more linear than the cathode of V2B. So the feedback signal itself sees less distortion before mixing with the audio signal since resistors tend to be a lot more linear than tubes.
Norman Crowhurst wrote about the problem of a non-linear feedback node (the cathode of a tube) but oddly didn't propose a solution. This is one and its pretty simple.
I know you've studied this concept in some detail, and I don't doubt you're correct. It doesn't seem to translate into a real-world issue in this situation though. I say that because I measured distortion directly at the grid of the 211, and it's very low. I don't have the numbers in front of me, but distortion measured at the output of the transformer is much higher and clearly being generated by the 211 itself. More feedback would reduce it, and in that case your suggestion would be worth pursuing. However, I'm only using about 5dB at this point in time. This is sufficient to improve the damping factor and add about 3dB of usable (low distortion) power at 20Hz. I don't want to use more feedback for risk of dulling the sound (and the project is nearly complete), so the idea of rewiring the primary isn't very appealing.
I appreciate you mentioning this technique, and I'll keep it in mind for future projects.
This all makes me wonder why SET??
I've been playing with low power PP amps the last few years (5Watts, that sort of thing) and I don't hear any advantage SETs have at that power level; for example the PP amp makes better bass and is noticeably more transparent while being just as smooth and involving. Of course the PP amp is class A1; I think when people make comparisons between SETs and PP they don't bother to level the playing field.
Agree completely. And low-power push-pull could yield some fun projects! I won't build another SE amp unless it's very low power for headphones or maybe a guitar amp. At some point I'd like to try RK-34 dual triodes for this. I have a pretty good supply of those, as well as 1626, 6BL7 and other "native" triodes.
Very similar, I had a 100H choke to ground using one 1/2 of the tube. The B+ was 950V, dual power transformers.
I think I would use a switching supply for the 10V filament these days. Even though in spec I did have a few CAPS in the first proto blow because of the inrush current of that damn filament.
G.
J. Gordon Rankin
My 211 project goes back many years. The prototype started with a 10V filament transformer, but I couldn't get rid of the hum. DC didn't seem to provide the same sound quality as AC (I tried linear and switching supplies at the time), so I eventually used ultrasonic AC. This turned into a project of its own, and it consumed many hours of experimentation and construction work. The supply is detailed in the thread I linked at HiFi Haven. You might have to join the site to see the images.These are monoblocks, so like yours, they have separate power transformers. I also use a separate transformer in each amp for driver B+, and a third transformer to supply B- at the bottom of the driver choke. These separate supplies resulted in a very smooth transition from A1 to A2. I can see it on the front panel current meter, but virtually no change in THD/IM as it moves into A2.
Edits: 05/10/24
Hi TK,
Sorry to chime in, you are the very experienced DIY member. since I join 20 years ago
I am surprised seeing your 211 amp using loop NFB. IMHO global loop NFB indeed adversely affect the sonic presentation, both on clarity and speed. I tried more than 50+ times in loop NFB, although loop type feedback does reduce distortion but overall clarity and speed would be apparently affected with as little as 3dB feedback, if you have each stage having reasonably low distortion, there is no need to use global feedback
The use of SRPP in 6BL7can be replaced by much more linear 6SN7 also in SRPP, and output from bottom section plate, more natural
You will definitely be amazed with non loop feedback SE amp, much more clarity and alive
If you think using loop feedback can help 211 to have tighter bass, now you take out the loop feedback, use 4 ohm second winding terminal to connect speaker
Eric, my experience with moderate amounts of NFB has been different than yours. My listening tests (and tests with younger family members who have much better hearing) has revealed no downside to this.
Sorry, that's an older schematic that has since been revised. The current design does indeed use a 6SN7, but it was not changed for reasons of distortion. I have analyzed the distortion products at the grid of the 211 as well as at the speaker output. The majority of distortion in this amplifier is produced by the 211 itself. The driver is much cleaner, and the contribution of the 6BL7 in the earlier design was negligible. Note that this is not SRPP, because the stage is driving a very high impedance load. There is no push-pull action. It functions strictly as a totem pole, and its primary purpose is to provide the large signal swing needed to drive the 211 into class A2.
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).
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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