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Need help on the following issues.
I build an amp based on the Pilot AA.410 design, with mods to the original design. The amp is a basic PP amp using 6l6 tubes.
1) I can not operate the amp with the secondary of the output transformer tied to gnd (case). The speakers sound like they are about
to explode. If I remove the FB it's ok, or if the FB is connected and the gnd connection is also removed, it's ok as well.
Question: why does this happen with gnd connected to case?
2) When I probe around the HV areas with my DMM, I get a popping sound
out of speaker. This is primarily around the preamp section.
Question: do I need a analog meter to test?
J.A.Z.
Follow Ups:
Sounds to me like you've got positive feedback instead of negative.
The fix is simply to reverse the output transformer secondary connections.
i.e. ground the other side of the secondary winding, and likewise connect the feedback off the opposite wire.You should then be able to connect one side of the output transformer secondary to ground. This is an essential safety requirement even in a zero feedback design. Otherwise a possible primary/secondary short circuit would put HT voltage on the speaker wires and the speakers themselves.
Regards,
Andy.
Sounds like your case is not grounded. A lot of times cases that are painted have the paint acting as an insulator. Scrape away some of the paint to make good connections. Test the ground resistance between the chassis ground and the circuit ground.
Chris O.
Thanks
I noticed that my AC outlet has a very poor E-GND connection.
Could this be the problem?
Juan
Try reversing the feedback as the other poster suggested - this is much more likely the problem. -jd
Gentlemen,
Thank you for the help, the problem was with the feedback.
corrected the problem and it sounds really good....
But now I have oscillation in the signal path, I was able to eliminate
most of the oscillation by adding a small amount of capacitance
between the 12au7 and the 6l6 signal path. But I still hear a small
amount from my speakers, hear is what I have noticed, If I run my hand
around the first stage (6c4) I can very the oscillation, and if I add
a shild over the 6c4 and ground it to the case, I can elliminate
the problem.
My question is where is the ocsillation coming from and how do I get rid of it without adding a shild around the 6c4, this would require that I replace the tube socket to one that accommodates a shild.
Thanks
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