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In Reply to: RE: debugging time posted by vinnie2 on June 21, 2017 at 07:30:49
A couple of things I thought I should mention in case anyone is interested in trying this amp (it really is a nice little amp with a low parts count).
I had read on a post on another site that the cathode resistor should actually be 200 ohms instead of the 400 ohms shown on the schematic. I went to the tube manual and it says for a 6bq5 in typical PP class AB operation, the cathode resistor should be 130 ohms and the cathode current should be be between 62 and 72 mA. I was getting a cathode current of 33mA with the 400 ohm resistor, so I tried a 130 ohm resistor instead and got about 67 mA which puts me right in the grove. I also changed the coupling caps to 0.1 uf as pointed out by fender lover and as mentioned in the schem. It wasn't causing the distortion, but it did improve the overall sound. Last but not least, I decided to try a 6dj8 after following up on dak's post that it was a closer match to the 6n1p than the 12at7. Again, it was not the cause of the distortion, but I think it sounds much better than the 12at7 in this circuit.
So, if you are looking for a simple, easy to build PP amp with good sound and a beer budget, you might want to give this a whirl. I would have been able to breadboard one channel in about an hour if the stupid volume pot had not tripped me up. Again, thanks for all the help, it was a great learning experience.
look forward to your sonic impressions vinnie! which output transformers are you using?
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