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Re: weird reverb problem!

10/5/2000 2:04 AM
K Bull
Re: weird reverb problem!
Hey Fellows, Is it possible that Gary might have a bad tube causing the hum ? This should be the third tube if I'm not mistaken, a 7025 on the AB763. Have you tried swapping it yet?,  
10/5/2000 4:51 PM
Rob W.

Here is one thing to consider. I had a similar  
problem in my twin after replacing the speakers.  
Often it is the case that one of the send/  
recieve transducers in the reverb pan is located  
a bit too close to the speaker magnet.  
This is basically sets up a situation for inuced hum. I would try taking the reverb pan out of the bag, and  
try varying its orientation to the speakers.  
(for example turning the pan 180 degrees.  
Hopefully this will cure your problem.  
Rob W. ;-)
10/5/2000 8:30 PM
Rick Erickson

The power transformer will induce a hum in the output coil if the tank is installed backwards.  
Looking from the rear of the amp make sure that the reverb tank input is to the left side, near the power transformer & power tubes and that the output is to the right - the side with the preamp tubes.  
If that doesn't help and you are sure that you cables aree shielded & connections are solid, try a new tank or one that works from another Fender tube amp.  
10/6/2000 2:04 AM

Hi guys,  
Thanks for all the suggestions,here's what I've done so far: (1) Swapped tubes,(2) swapped reverb transformer,(3)swapped tank, (4) swapped reverb cables,(5)checked resistors and caps in reverb circuit, (6) checked input and output jacks on rear of amp, (7) cleaned both tube sockets in reverb circuit.When I swapped the reverb tank I left it out of the cabinet to avoid any interference from the speakers but all to no avail,this one has really got me stumped.I'll try to go into a little more detail about the hum itself.I'ts not a loud hum such as one would hear with the groung shield wire broken on the output cable of the reverb going to the back of the amp.Although it is similar and increases in volume as I turn up the reverb control and as I mentionned in the first post it completly disapears when I switch off the reverb with the foot switch or pull either of the reverb tubes out or either of the reverb cables out of the rear of the amp.  
Oh ya I even tried grounding the resistor and bypass cap on pins 3&8 of the 12ax7 (reverb driver tube) to a different point on the eyelet board because they were origanily grounded to the ground connection of the reverb output jack on the rear of the amp,no change.I should mention that the fillament hum is little louder than it used to be but not to the point of being annoying like the reverb hum.  
Hope someone can come up with a fix I've pretty much run out of ideas!  
10/6/2000 2:30 AM
Don Symes

Does anyone else think it may be time for Mr. Owen to bake his board?
10/6/2000 7:40 PM

Here's an idea I want to run by you before I do any cooking. Seeing how I have a noticably louder hum through out the whole amp but not as loud as the reverb cicuit could it be possible that the power supply filter caps are shot causing the increased hum through out the amp and when this hum reaches the reverb driver it's further amplified hence the interaction with the reverb control pot which when turned up full increases the volume of this hum to an irratably loud level.  
Just an idea!  
10/6/2000 8:52 PM

I would think that the whole amp would hum if the caps go bad as the power supply is in series with the power amp. I like your thought on the pot however. I know it would be a pain to disconect the leads and ohm it out. It's possible at this point that anything a loose ground wire or cold solder joint could be it. On the bright side your almost going to have a brand new amp by the time you find it, :)  
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