04-25-2017, 12:08 PM
Hangman, thanks.
I finally had a chance to button everything up this weekend and test it out, and now I have a new problem to solve. The amp works fine, but the output is really low. It's not distorted, just really (really) quiet.
Cutting to the chase...
How can I test my OT to make sure it isn't blown?
For anyone who wants the details (sorry for the long post)...
I fired it up first on a dim bulb tester - all good, no shorts. Plugged it into the wall, and it works fine but it's really quiet. I can turn it up well past 12:00 and it's still not loud enough to irritate my wife upstairs. Sounds like a 20w practice amp - probably quieter. That ain't good!
Here's what has been replaced:
Power supply (C17-20; D1-4, D5; R56,57,52,53)
Bias supply (C16; R50; R49, C15 [early '71 model, no C23]; D6)
Other: R39,40,45,46; R35,36; R41,42,47,48; R37,38,43,44; D7-10, C21 with an X2 safety cap.
With it running, I took some measurements around the amp. I don't have them in front of me, but they all looked normal...
B+ from OT center tap at C20 = 550v
At pos side of C19 = 540v
Plate voltage, 540-545v on all 4 tubes.
Plate current, all 4 tubes in the high-20s mA.
Plate dissipation somewhere around 53-55% on all tubes.
All tubes are checked and known to be good (tested 7027 pairs in my B25, swapped all others, including the 6K11). I ended up replacing the flyback diodes - for what it's worth, the old ones were reading OL in both directions, so they had gone open.
Anything else I should be looking for, checking, or replacing? My fear is a bad OT - is there a way to test it?
Thanks in advance.
I finally had a chance to button everything up this weekend and test it out, and now I have a new problem to solve. The amp works fine, but the output is really low. It's not distorted, just really (really) quiet.
Cutting to the chase...
How can I test my OT to make sure it isn't blown?
For anyone who wants the details (sorry for the long post)...
I fired it up first on a dim bulb tester - all good, no shorts. Plugged it into the wall, and it works fine but it's really quiet. I can turn it up well past 12:00 and it's still not loud enough to irritate my wife upstairs. Sounds like a 20w practice amp - probably quieter. That ain't good!
Here's what has been replaced:
Power supply (C17-20; D1-4, D5; R56,57,52,53)
Bias supply (C16; R50; R49, C15 [early '71 model, no C23]; D6)
Other: R39,40,45,46; R35,36; R41,42,47,48; R37,38,43,44; D7-10, C21 with an X2 safety cap.
With it running, I took some measurements around the amp. I don't have them in front of me, but they all looked normal...
B+ from OT center tap at C20 = 550v
At pos side of C19 = 540v
Plate voltage, 540-545v on all 4 tubes.
Plate current, all 4 tubes in the high-20s mA.
Plate dissipation somewhere around 53-55% on all tubes.
All tubes are checked and known to be good (tested 7027 pairs in my B25, swapped all others, including the 6K11). I ended up replacing the flyback diodes - for what it's worth, the old ones were reading OL in both directions, so they had gone open.
Anything else I should be looking for, checking, or replacing? My fear is a bad OT - is there a way to test it?
Thanks in advance.