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In Reply to: RE: A2 Triodes posted by Triode_Kingdom on May 04, 2024 at 15:37:36
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
Follow Ups:
"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.
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