Tube Amps / Music Electronics
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|6/15/2005 8:06 PM|
|Eric C.||1959 clone debug|
My 1959 clone came out almost perfect. No loud hum, oscillation, everything works BUT it gives an ugly distortion when the volume gets louder. It sounds kind of like a dying fuzz or overdrive. Very square wave sounding or like an intense buzz kinda distortion accompanies the notes. Its biased correctly. All my voltages read normal. Its not there when I use THD Yellow Jackets but even with them it sounds flabby, and harsh like the treble and bass are really boosted. Not like it should anyway. Any help is much appreciated. Eric.
|6/15/2005 7:36 PM|
Eric, the noise you've described seems to be a leaking coupling cap.
And obviously one or both the caps to the Volumes. With a low setting, the leaking DC doesn't significantly change the bias of the next stage - but when cranked up, maybe huge DC offsets the next stage's bias a lot - and you get the noise.
If you'd still have the noise, go further.
Any leaking DC from the tone stage caps wouldt be blocked from that one to the PI's grid - but you'd hear a scratching noise, when turning at the tone pots due the DC.
So check the one to the PI's grid - or if in doubt, swap in a new one.
|6/15/2005 8:08 PM|
Wouldnt that still cause that distortion when using Yellow Jackets? As it is it only seems to happen with the EL34s (tried 2 different sets).
|6/15/2005 8:24 PM|
You're right! A leaking coupling cap doesn't care about the output tubes used or Yellow Jackets (with EL84-Cathode bias in?) instead of them.
I didn't understand your post completely by reading it the first time (I'm a German).
Your noise also couldt be a blocking distortion occuring on the output tube's grids - maybe due to too large coupling caps (of maybe .1 ?) - and the EL84 in the Jackets aren't so much sensitive to this due to they're cathode biased.
Check these two caps to the output tubes and maybe better swap in smaller ones - or diminish the values of the bias feed resistors, but that'll "brighten" your sound a bit and ripps off some gain to the power tubes.
|6/16/2005 5:41 AM|
I agree novo. Thanks for the info. Ill have to try a few things and see what happens. I have a type 2 PPIV master which, when dimed, gives bias splitter values of around 235k instead of the normal 220k. I do have .1s in there since its actually more of a 66 circuit then a 1959. Id prefer not to change that. Ill try removing the master.
|6/18/2005 7:29 AM|
Possibly parasitic oscillation when the signal level gets high enough. Put a 'scope on it to confirm. Then move wires around with chopsticks to find out what combination makes it quit.
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