Hello to the forum .. 3 weeks ago a friend of mine brought his amp for repair because it did not work (the amp is a fender super reverb 1979 diode rectifier). when I opened it it made a buzz and the guitar signal could not be heard…. finally 2 tubes 12ax7 had burned and the 1 6L6… also had the hum balance pot damaged and I replaced it .. the 2 12ax7 were replaced and I also changed the 2 6L6 I just could not find jj matched and I got 2 unmatched tung sol… the amp just worked made a buzz .. which buzz I turned off by adjusting the output maching tubes pot, as exactly FENDER mentions in the manual .. the amp was playing perfectly! .. yesterday my friend notifies me and tells me that he started making this buzz again .the amp plays normally, it just makes this buzz .. I told it to adjust the output maching tubes from behind and it will decrease… it did not disappear, it just decreased .. what is wrong? was it the fault of the 2 6L6 that are not mached? Is the main transformer power to blame because it has it at 220v? I say this because the transformer also has a 240v option. and from what I measured the current here in Greece it is 230v…. what could be the problem?
thanks
thanks
Could be the coupling caps between driver and 6L6's.
I once had a guitar amp on the bench that measured perfectly, but wore out power tubes in weeks.
Replaced the coupling caps and all was good again.
Other possible faults could be bad connections, bad electrolytics or drifted resistors.
Check out the bias circuit, since that has influence as you said.
Old amp: how are the power supply caps?
I once had a guitar amp on the bench that measured perfectly, but wore out power tubes in weeks.
Replaced the coupling caps and all was good again.
Other possible faults could be bad connections, bad electrolytics or drifted resistors.
Check out the bias circuit, since that has influence as you said.
Old amp: how are the power supply caps?
The hum/buzz is typical of out of balance idle currents in the 6L6 output tubes, which is why you were able to "dial it out" with the balance pot after your first repair.
My first guess is that something occurred to put the idle currents out of balance badly and the first suspect for that is the 470 Ohm 2 Watt Carbon film screen resistors on the output tubes. They get very stressed when a tube goes bad so may have been on the point of failure. Before doing anything else change those screen resistors. I would recommend not replacing them with the same 470 Ohm 2 Watt Carbon film resistors but instead fit 1K 5Watt wire wound resistors in their place.
Cheers
Ian
My first guess is that something occurred to put the idle currents out of balance badly and the first suspect for that is the 470 Ohm 2 Watt Carbon film screen resistors on the output tubes. They get very stressed when a tube goes bad so may have been on the point of failure. Before doing anything else change those screen resistors. I would recommend not replacing them with the same 470 Ohm 2 Watt Carbon film resistors but instead fit 1K 5Watt wire wound resistors in their place.
Cheers
Ian