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Microphonic Output Transformer Housing?

DAB

Member
Messages
791
Guys,

I am chasing down a trebly buzz/hum on a 5F6A Bassman Head (Weber Kit). I just noticed that if I tap (gently) on the housing of the OT it sounds just like a microphonic tube.

Is that a grounding issue with the OT. Maybe its just the physical vibrations hitting whatever is wrong but I don't have to tap very hard to make it happen.

Also, the heater wiring is microphonic as well.

Any ideas?

Thanks...Dave
 

Jack DeVille

Member
Messages
2,456
I'd check to make sure ALL grounds are clean, tight, and well-secured. Maybe a loose pot mount, a poor chassis ground etc.
Its a long shot, but grounds are one of the first things I check whenever I get some kind of mysterious microphonics. If the circuit is searching for ground and its not readily available, it WILL find it by other means. This can cause very unpredictable behaviour from the surrounding/indirectly-coupled/adjacent portions of the circuit.

Best of luck, you'll find it.
:banana
 

JJman

Member
Messages
994
You could remove all the tubes except the output tubes and try the tap-test again. This will isolate the test to the OP tranny and the OP tubes and such.

If the noise is NOT present during the above test, the noise is NOT from the OP tranny.
 

DAB

Member
Messages
791
You could remove all the tubes except the output tubes and try the tap-test again. This will isolate the test to the OP tranny and the OP tubes and such.

If the noise is NOT present during the above test, the noise is NOT from the OP tranny.

Good tip. I did that last night and the amp was dead quiet and no microphonics with V1, V2, V3 pulled. Actually the same even with only V3 pulled. Seems the OT is okay.

I changed out the volume pots yesterday with new Alphas but no change to the hum.

What does having the amp go dead quiet with V3 pulled tell me? I think it means that the problem is somewhere in the PI, EQ, or Preamp sections. Is that correct?

I rewired the heaters yet again and didn't solve the problem. I changed out ALL of the tubes twice so I can't believe its a bad tube. Could be a bad tube socket but I have already changed them all out once so it would be a huge coincidence if I got the same problem with all new tube sockets.

Here is a strange situation for you guys. I rigged up a little tester probe where I feed music content into the amp input(s) and listen on headphones as I probe around the connections in the amp.(one side grounded and the other wire connected to the probe). I have noticed on this amp that I can touch the tip on the input and hear the sound just fine, I can hear it at the point where the 68K resistor is connected but not on the other end of the 68K resistor. In fact I can't hear any sound until I get to the PI. The sound is strong at the output jack but has the hum in it. This doesn't seem normal to me but I can't imagine what causes it. I have checked all of the solder points and reflowed them but no joy. I could believe that the signal might be so weak when it reached the Grid of V1 that I might not be able to hear it but I should certainly be able to hear it coming off the plate on V1.

Anyway if any of you have more ideas please respond.

Thanks...Dave
 

Gnarlly

Member
Messages
2,862
Your ears aren't fooling you; transformer housings can vibrate and produce a metallic ringing sound similar to a microphonic tube. I had the same exact thing happen with a THD Univalve I used to own. Try tightening the housing or using some rubber washers to dampen the vibrations.
 




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