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6SN7-6L6 valve amp
6SN7-6L6 valve amp
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Old 16th June 2019, 09:49 PM   #11
6A3sUMMER is online now 6A3sUMMER  United States
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Join Date: Jun 2016
1. Use a 6L6GC. Its maximum plate dissipation is 30 Watts. A 6L6, 6L6G, 6L6GA, and 6L6GB all have maximum plate dissipation of only 19 Watts. 70mA and 300V plate to cathode is 21 Watts dissipation. Do not run your engine at 7,000 rpm all the time.
2. If you are going to parallel the input triode (and not using the cathode follower), you need to cut the cathode resistor value in 1/2, and cut the plate resistor in 1/2.
3. If you experience oscillations, try grid stopper resistors on each grid.
4. Do not forget, if the output transformer is connected out of phase, the negative feedback will actually be positive feedback, and you Will have an oscillator.

Happy Listening (report back the fun).

Last edited by 6A3sUMMER; 16th June 2019 at 09:56 PM.
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Old 5th July 2019, 03:27 AM   #12
cg45acp is offline cg45acp
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I wasnt very happy with the way the schematic posted turned out. I went with this one and like it a little better. Only issue i have is around 50% volume the sound starts to break up. I took some voltage readings while running the amp as noted on the schematic
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Old 5th July 2019, 03:47 AM   #13
Tjj226 is offline Tjj226
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You have too much gain which is why you run into audible distortion at 50% volume. My guess is you are actually running into distortion way lower than 50% volume.

You only need to swing 20v peak to peak to drive a 6l6/6l6g in triode mode and I think you only need 30v on a 6l6gc. A single half of a 6sn7 will swing that much voltage if you have 2v rms input. If you have a weaker input, then a single 6sn7 can still work, but you would need to replace the plate resistor with a CCS or a plate choke.

I would either swap out the 6sn7 for a 6j5 or I would parallel the two halves of the 6sn7 and use a really basic CCS to load the tube. That way you take a whole tube out of the signal path which will give you less distortion.
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Old 5th July 2019, 05:06 AM   #14
6A3sUMMER is online now 6A3sUMMER  United States
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How efficient are your loudspeakers? Do you have enough power and volume at 50% on the pot?

If you need a little more power, than try going from triode wired to Ultra Linear: If your output transformer primary has an Ultra Linear tap, you might try connecting the screen resistor, R6, from the screen to the ultra linear tap (disconnect R6 from the plate).

But now you will have way too much gain, just use one stage of the 6SN7 as was mentioned earlier: Disconnect pin 4 of the first 1/2 of the 6SN7, disconnect R9, and connect pin 1 to the 100k pot wiper. Keep C1. Change R5 from 22k to 1k Ohm.
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Old 5th July 2019, 01:47 PM   #15
Ketje is offline Ketje  Belgium
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Location: Flanders
How about some little? mod's
Back to pentode and cathode feedback to compensate for the higher output impedance.
Mona
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Old 5th July 2019, 09:48 PM   #16
cg45acp is offline cg45acp
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thanks Ketje, I done a little tweaking this afternoon with parts i have on hand this afternoon. Took care of most of the breakup, now I deal with a little hum. Here I am with the changes. Thinking of bumping R3 to 2200
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