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HD-130 snap crackle and pop!


itai halevi
Posted on Monday, September 20, 2004 - 06:15 pm:   

Hello all,

I have a HD-130 with the 12ax7a phase tube. I retubed it with EH bottles about 4 months ago (the orginal RCA's were smashed) and set the bias to .5 volt at point Y as per MM specs. It has been in pretty consistent moderate use as a bass amp since then.

About a week ago it started losing clarity and volume. Today while checking it out at in a quiet environment I noticed that the amp was putting out a lot of noise (hiss, snap, crackle, pop, almost like bad tubes...). I replaced the 12ax7a but there was no change.

I opened it up to see if the bias had drifted and discovered that at the extreme end of the trimpot I can barely get point Y up to 4.7 volts.

To my knowledge this amp is all stock except for the tubes. My questions are: what could be casuing the noise, the lack of voltage at the bias point and could the new tubes have been chewed up already? If you need more info please let me know.

I am grateful for any and all help.
Steve Kennedy
Posted on Tuesday, September 21, 2004 - 10:52 am:   

It could be a lot of things, but my money would be on either a cracked resistor or failing capacitors in the power supply.

Somewhere in your circuit, a voltage is not what it should be. Start at the source... the Power Supply! Make sure all the source voltages are correct (and noise free, but this requires a scope) before looking for the cause elsewhere.


itai halevi
Posted on Tuesday, September 21, 2004 - 12:07 pm:   

Thanks, I'll start investigating.

BTW, awesome site, truly an asset to the 'net.