Allen amp-loud hum

Greggy

Senior Member
Messages
13,439
When I turned on my Allen Class Act this evening, it was completely dead, no hum, no signal, but the pilot light was on. Noticed v1 was cooked (a Mullard CV4024). So I replaced it with a JJ ecc83s. Played for about 15 minutes and noticed the tone was bass heavy (could be the JJ). Then all of a sudden the amp began emitting a "loud" hum form the speaker, really deep and loud, scared the #^%@$% out of me. So I turned it on to standby and everything quieted down. I checked the tubes and nothing looked out of place. When I turned the standby off the hum returned and I noticed some blue light coming from inside the plates. I changed output tube (from a winged c el34 to a 6l6) but same thing.

Any ideas before I open her up and take a peak?
 

Greggy

Senior Member
Messages
13,439
Originally posted by RL in Fla
Try a different rectifier tube , if it's not that it's probably a power supply cap gone sour .
The rectifier tube is the only tube I didn't change. Thanks, I'm going downstairs now to see if that's the problem.
 

Greggy

Senior Member
Messages
13,439
Drats, that didn't work. I guess I'll email Mr. Allen. I built the amp last fall, and it has worked flawlessly until now. Thanks for the tip about the power cap. I'll look into it.
 

Greggy

Senior Member
Messages
13,439
Originally posted by RL in Fla
So it ended up being the cap , Greggy ? Was it a fluke or has David changed caps since yours ?
I emailed David and am going to send it to him soon. I just don't have the time to mess with it myself. David wasn't willing to speculate too much on the possible causes. Can't blame him since I built the amp myself, so from his perspective it could be any one or more of a 1,000 possible causes. The amp worked flawlessly from the get go (about 5 months), until recently. Could be a bad solder joint, but more likely a cap IMO. I'll know soon.

BTW, I posted a clip of the amp in the soundclips section. Recorded these tracks about 1/2 hour before she blew.

Thanks!
 

Greggy

Senior Member
Messages
13,439
Update:

Just got off the phone with Mr. Allen. I sent the chassis to him asking him to inspect and repair. Also to please install any upgrades to the circuit he felt were warranted.

Fortunately, all the solder joints were good (this was my first kit amp and was curious whether my soldering skills were up to snuff). Power tranny was starting to work loose from the chassis (an important ground point). He found a couple of lug connections where I didn't trim the leads close enough back towards the solder joint. Also replaced a cap with another value to provide better response from the volume pot. And found a questionable crimp connection on the power supply side. Said the amp is about 50% quieter now. Other than that, he believes my problems arise from too much plate voltage frying the output tube (winged c el34). This is a single ended class A design, and it runs very hot.

He suggested I use a 5Y3GT rectifier tube with the el34 to lower the power going to the tube. Thinks the tube may have been breaking down under the stress. He's sending it back to me this afternoon.

Final note. David Allen is a pleasure to deal with. Seems to enjoy his work, and is very helpful with questions. Responds quickly to questions and concerns of his customers. First class all the way.
 




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