Tube Amps / Music Electronics
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|8/10/1999 6:31 PM|
I've finished wiring a DIY amp, basically a ax84 hi-octane, with an ef86 front end. Sounds very good, but I'm running into a strange problem...
I get an intermitent crackle noise from the speaker when it's running. I've tried to trace the signal back with a small amplifier & speaker, and I can't hear the crackle from the preamp section at all, just from the power section.
I've replaced all of the plate resistors in the amp, thinking that they might have been noisy.
What things could be causing this?
I've tried shorting out the grid resistor on the el84 (5.6k), that didn't make it go away.
Could it be RF? A cold solder joint?
If the power or output transformers are going bad, would they act like this?
Thanks for any help in advance.
(By the way, outside of the crackle, the thing sounds awesome!)
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|8/11/1999 5:12 PM|
Of course, Congrats! And good trouble shooting on that also (using a small amp and a "probe" to trace the signal problem to the power stage).
Dang, that beats the heck out of me. Is the crackle there constantly or just when you play? Or maybe it goes away as you play, especially loudly?
This may need a scope. I'm wondering though if B+ is popping around at the power stage, or if that B+ is OK, but the popping appears on plate ...
On a hunch, resolder/check/futz with your EL84 screen connection and screen resistor ... I've just got this feeling that may be what's up.
- Carl B.
|8/11/1999 7:01 PM|
|Mark Buckingham||Re: Crackle Noise - Solved|
Thanks Carl! The amp really sounds good, although I've got more goofing around to do with the tone controls...
I was thinking screen resistor too, and I even resoldered the thing.
Turns out, it was the 500pf cap on the treble control... Replaced that, and the noise went away. I didn't think a silver mica cap could be noisy though... (I won't rule out a cold solder joint, although I was too lazy to look at midnight last night.)
|8/11/1999 7:49 PM|
I've run into the same problem with a silver mica in the same location on a Pro Reverb. Replaced it twice and once the volume was cranked, it would start crackling again. Used a 1000v rated cap instead.
|8/12/1999 7:59 AM|
Me too in Fender amps when using the SM 250pF treble cap also as DC blocking cap!
The 300v rated ones seem to go away within weeks.
Symptoms are very noisey volume control with pops and squeaks etc etc...
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