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DC coupled single ended cathode follower 6SJ7/6V6 amp project
DC coupled single ended cathode follower 6SJ7/6V6 amp project
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Old 18th July 2019, 02:50 AM   #11
Bigun is offline Bigun  Canada
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
in common cathode gain stage the 10k resistor you refer to is the load against which the tube works to generate voltage gain. for the output of a power amplifier the output transformer is the load and the voltage developed across it (current) will couple into the load through the secondary winding. A resistor in series with the output transformer will simply waste heat and contribute nothing to the load. But being direct coupled you need to lift the output tube off ground and so you end up needing this resistor.

There is some stabilization of the operating point going on as the cathode of the 6V6 is connected to the screen of the input tube and this feedback to the screen of the input will help linearize it and improve the PSRR.
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Last edited by Bigun; 18th July 2019 at 03:10 AM.
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Old 18th July 2019, 03:10 AM   #12
PRR is online now PRR  United States
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Originally Posted by 45 View Post
Not the first time this amp is mentioned. I doubt one can get the claimed 4.5W with that circuit. . .
My sim with similar tubes gives 5.5%THD at 10Vrms across 6K load: 0.016 Watts. 22%THD at 42Vrms in 6k; 0.3W output.
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Old 18th July 2019, 03:11 AM   #13
Tubelab_com is offline Tubelab_com  United States
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A resistor in series with the output transformer will simply waste heat and contribute nothing to the load.
In this case the resistor is not bypassed, so a good chunk of the AC output voltage winds up being burnt up in this resistor too, so it steals power from the load.
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Old 18th July 2019, 03:22 AM   #14
Bigun is offline Bigun  Canada
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DC coupled single ended cathode follower 6SJ7/6V6 amp project
did anybody attempt checking that the schematic matches the wiring in the photo - a thankless task given the poor quality of the photo of course.
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Old 18th July 2019, 03:54 AM   #15
PRR is online now PRR  United States
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Originally Posted by 45 View Post
....If the 6SJ7 and the 6V6 are RC coupled then you have complete freedom about quiescent conditions of the input tube and can be adjusted to what one needs.
AC-coupled, you can do a heap better. Easily 10X the power. Still seriously distorted at any useful power.

My gut says it is not worth tracing the schematic against the chassis photo. A 6V6 as Triode just CAN'T make 4 real Watts SE. (Couple watts at best.) Bates clearly did not METER TEST this amp, just "assumed" it would reach normal-operation output.
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Old 18th July 2019, 10:36 AM   #16
45 is offline 45  Italy
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AC-coupled, you can do a heap better. Easily 10X the power. Still seriously distorted at any useful power.
Yes. With a cathode follower output one just moves the troubles to the input stage and actually it's even bigger trouble. Not so many solutions to get 180V RMS swing with little distortions. If it also has to be cheap even worse..... One needs a good device with its own power supply. That's why that circuit has remained unnoticed and no one addressed those design mistakes.
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Old 18th July 2019, 10:39 AM   #17
DF96 is offline DF96  England
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It is obvious that the original author did not understand his own circuit. If he understood it he would have quietly buried it and felt embarrassed, not published it for everyone to see. Maybe he was drunk when he designed it?

He may have built it and listened to it. A 100mW can sound fine with a high efficiency speaker. I guess he didn't have an oscilloscope.
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Old 18th July 2019, 11:58 AM   #18
Dinolobe is offline Dinolobe  Canada
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I think i’ve had my answer now. The other threads i’ve read about the circuit weren’t as detailed as to why the circuit won’t work or give satisfactory results. They sounded more like haters anything else. Now it’s getting clearer that i’ll be loosing my time building it even though it could be converted into something else easily. Too bad ! I really liked the idea behind it, but too good to be true.

Any ideas for a useful amp that would make good use those parts ? It could be with different valves too. Something like a Single ended el84, el34, 6v6...
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Old 18th July 2019, 12:00 PM   #19
Sodacose is offline Sodacose  United States
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With the pentode input, it is begging for some "schade" style plate to grid feedback.
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Old 18th July 2019, 12:56 PM   #20
DF96 is offline DF96  England
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You could make a simple SE. It won't be hi-fi, but it could make pleasant noises.
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