Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cjonesplay on August 09, 2020, 02:48:56 pm
-
First time posting, I’ve got a drip-edge Bandmaster and did some work on it (New tubes, all new electro caps, Added master volume, replaced some resistors, etc.). Everything worked fine and I went to make my measurements to bias, then turned off the amp to measure plate resistance and...
When I turned it back on no sound came out. The tubes glow but the OT center tap and DC voltages are now *negative*. -.433v. The tubes don’t get hot. I’m thinking a blown transformer, but I would like to know if this is a simpler problem first before pulling things out.
I’ve switched tubes, speaker wires and cabs, guitars and cables, scanned/shaken inside, all to no avail. Any help here would be greatly appreciated and I can do more measurements if needed.
-
... turned off the amp to measure plate resistance and...
When I turned it back on no sound came out. ... the OT center tap and DC voltages are now *negative*. -.433v. ...
You don't have the meter's Red lead to chassis & Black lead to ground, do you?
You don't have the meter attached in the amp, do you? If set wrong (as in for Current) and you poke the wrong circuit location, you would be shorting out the signal in the amp.
Are you sure the meter is set for Volts, and the meter leads are inserted in the correct sockets on the meter? They're not in the holes for Current are they?
You didn't attempt to make a Resistance measurement with voltage present in the chassis, did you? If yes, the energized circuit could have zapped your meter. You'd have to check the condition of any internal fuses in the meter.
-
the OT center tap and DC voltages are now *negative*. -.433v.
I would just call that zero volts. Not much to check... shorted filter caps, open STBY switch, rectifier diodes, PT HT secondary. Shouldn't take long to track it down.
-
the OT center tap and DC voltages ... -.433v.
I would just call that zero volts. ...
Oops!! Missed the leading decimal!
-
Thanks for the help, the voltmeter does have a blown fuse - it did make a weird beeping sound now that I remember.
That still doesn’t answer why the amp is not working, though. The tubes aren’t heating up very hot and the primaries on the OT connected to the speaker jack are showing continuity, so I’m thinking the OT is the problem. The PT seems to be working ok (supplying 430v to the rectifier at least).
I’m off to find a new fuse and take another crack at this, let me know if you have any suggestions.
-
... The PT seems to be working ok (supplying 430v to the rectifier at least). ...
Measure d.c. voltages at tube plates. Don't just to a conclusion of "what's bad." Instead, make some basic checks that will identify what is working, so you can exclude those to find what [/i]is not[/i] working.
-
Ok, I figured it out. Rob Robinetter suggested putting a 68ohm 5 watt resistor coming off the diode rectifier into the filter caps for a “sag”, but that resistor I added failed. I just pulled it, not sure if it was my error or it doesn’t work.
Thank you so much for your help! Sometimes just having a rational, patient, experienced voice is what is needed to get you through. Normally I try to do these things on my own, but if I can I also try to help others in the same boat.
-
Put a jumper across the STBY switch. Any better?