73 Blackfaced Twin buzzing on low notes

[ctcpete]

I have a 73 Fender Twin that I blackfaced and refurbed a few years back (all electrolytic caps replaced as well as all tubes).

Haven't been using it much, last few times I noticed a buzz on some bass notes, even at moderate volumes. Amp is quiet with guitar unplugged. It occurs on both channels and through different speakers. I've tried swapping out all the pre amp tubes. Any chance a power tube could cause this? Haven't got much time to pull the chassis and start poking around but will if I have to.

Thanks in advance

[Jag]

First things I would check would be lead dress and filter caps. You may have replaced the caps a few years back but one could still have gone bad, especially if it's been sitting unused for a while.

[ctcpete]

Thaks, Jag. Come to think of it I left all but one of the blue Mallory caps installed (these are the filter caps right..trouble with my memory I haven't read up on this stuff in a few years since I worked on it). Any easy way to test the Mallorys? I have a good multi meter with a cap reader.

Thanks again

[ctcpete]

ok, I,ve done a little research, the filter caps are the big electrolytics located in the square metal housing all in the amp where replaced, as well as all other electrolytics. The blue Mallorys are the coupling AKA signal caps correct?

[pdf64]

Mallory are a brand mainly asociated with electrolytics, see the brown cathode bypass caps on the board
http://www.regiscoyne.com/Blackface_...ster-stock.jpg
Have we heard from Regis recently?
You can see the blue coupling caps on the board also - they aren't branded, don't know who made them.
Checkout
Tube Amplifier Debugging Page
for possible causes. Pete.

[ctcpete]

The Blue Mallorys I am referring to are not electrolytics, they are tubular shaped and where used before fender started using the "choclate drop" brown ones. I hear they don't go bad too often. I'm wondering if one of the Sprague Blue Atom filter caps I installed may have failed. I've been reading that they are not as good as they used to be.

[m-fine]

If the amp is quiet with the instrument unplugged and the volume turned up,and also with the instrument plugged in but not playing, then it is not a problem with the filter caps. You would be hearing the 60 hz hum without having to play a bass note.

I would suspect a tube first. If it is not a tube, are there any of the cheap ceramic caps in the circuit? You might have a cap that has gone microphonic and is buzzing with the bass vibration. You can CAREFULLY tap around on the inside if you pull the chassis, but don't electrocute yourself or short out the amp.

[Jag]

The Blue Mallorys are the coupling caps.

[ctcpete]

If the amp is quiet with the instrument unplugged and the volume turned up,and also with the instrument plugged in but not playing, then it is not a problem with the filter caps. You would be hearing the 60 hz hum without having to play a bass note.

I would suspect a tube first. If it is not a tube, are there any of the cheap ceramic caps in the circuit? You might have a cap that has gone microphonic and is buzzing with the bass vibration. You can CAREFULLY tap around on the inside if you pull the chassis, but don't electrocute yourself or short out the amp.

Thanks m-fine, I now have a good starting point. I will try the tubes (I assume only the pre amp) again first. I believe I replaced some but not all of the ceramic caps (the round tan colored ones correct?) . I will go to them next when I can get some time to pull the chassis.

A few months back when I noticed this problem I plugged the Twin into the other speaker I have here (my Blues Jr's) and I did not hear the buzz but I was afraid to turn it up very much, swapped the speakers (Eminence Red White and Blues), turned out to not be the speakers but the Red White and Blues sound great, . Of well...

Will update you guys soon, thanks so much.