Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tcoil on August 02, 2011, 11:26:16 am
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Hi, everyone I'm having a problem with a Band master kit amp. When I turn it on w/ all volume and tone knobs at 0 the amp makes a loud oscillating tone in the low frequency range. Kind of like the sound of a transformer right before it fries. Amp was working fine. originally it stopped working, no sound at all. I took the screws out of the board and lifted it and now I get the noise. If i pull the preamp tubes I still get the noise. I replaced all the tubes, same thing. Put new filter caps in and same thing. I'm not sure what is causing the problem. I can't turn it on long enough to check voltages with the power tubes in. Any suggestions would be appreciated. :BangHead:
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Which "Bandmaster" is this? Fender has had over 6 different designs called "Bandmaster".
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Did this amp EVER work, or was it just constructed and is showing this problem upon first power up? (you said KIT) Sounds like a classic case of motorboating. Flip the output transformer plate connections. There should be brown, red, and blue wires comprising the OT primary winding. Leave the red alone...and, by the way, make sure any electrolytics are discharged because you WILL be handling the wires that carry the HV. Either measure the volts on the red wire and let them decay until they are down to 1-2 volts or zero, or, discharge said red wire to ground through an appropriate bleeder resistor if the circuit itself does not incorporate one. That would surprise me, but....measure the volts on the red wire or the plates of the 6L6 after the amp has been turned off for a minute or three and do nothing until that voltage reading is low single digit volts.
Just swap the blue and brown wires which go to the plates of the 6L6 tubes. Or if they are not blue and brown, swap the wires that feed the plates of the output tubes. There should be only one wire going to pin 3 of those tube sockets. Swap 'em around.
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Ahhh Got it. Solder connection on the copper plate below the presence control. Sneaky little sucker. Grounds the filter caps. Should have known. Now I know what it sounds like when the filter caps are not grounded....BZZZZZT
It was a kit from Weber. A good buddy bought it and asked me it I would build it. I built it last year and he gigs with it. I had swapped the plate wires and the Filter caps. Thats when I had to walk away. I sent the message and had lunch. Came back and found it right away. Sometines it good to walk away when your stumped.
Now where's my guitar, time to put it to the test
Rock on!
Thanks
Bob