AB165 Bassman B+ voltage issues

[old tube]

Bingo!

I just lifted the yellow wire feeding the bias form the board, used it as centre tap and yes, getting 333V each side of the HT secondary.

I'll just check the bias voltage now, but it looks like it's been a PT wiring issue. Hasten to add it came to me like this ... after being "fixed" (it was blowing the mains fuse) for the previous owner, by a pro repairer!




Update, bias voltage is still 53V having swapped the relevant wires

[Phil_S]

Hooray for us! Your B+ should now be much closer to the values on the schematic. Congratulations and we should give a prize to pdf64 for his determination of the real issue.

This isn't the 1st time we've seen something "professionally repaired."

[old tube]

Hooray for us! Your B+ should now be much closer to the values on the schematic. Congratulations and we should give a prize to pdf64 for his determination of the real issue.

This isn't the 1st time we've seen something "professionally repaired."

Indeed, many thanks to you all but a special thanks to pdf64 for spotting the answer from the first!

Now to change the previously changed resistors with the wrong values, the linear volume pot, the leaking caps, hissing grid resistors ... etc. this next bit I'll enjoy

[pdf64]

Great news!
Looks like you'll have a spare PT!
Regarding volume pots, the original Fender audio taper for these was ~30%, so linear (ie 50%) is as close to that as most log types (10%).

[old tube]

Great news!
Looks like you'll have a spare PT!
Regarding volume pots, the original Fender audio taper for these was ~30%, so linear (i.e. 50%) is as close to that as most log types (10%).

Perhaps the pot is knackered, it really is on or off, full blast at 2 and no more volume after. I've got some nice 30/70 taper CTS pots that I tend use. They work very well and give nice smooth progression round the dial.

Yes, looks like I've now got the start point of a new project in the form of the replacement PT I bought

[old tube]

Last update. Soldered it all back together.

B+ at 450V, output tubes bias range OK, biased cool at 35mA.
Noticeably less hiss and electrical noise, though there is still too much hiss. Can't crank it up this late but guitar sounds pretty sweet and full at low volume, bottom end has returned, not the nasty thin tone it had when the voltages were wrong.

Best part is the transformer is no longer vibrating!

Thanks again for all your help.

[Phil_S]

For hiss you may want to look at replacing the plate load resistors, one at a time.

[old tube]

Thanks Phil_S that's what I intend doing.

A lot of the original carbon comps in the amp are over 120% of original value. I'd normally change those as a matter of course. There's also an original big 0.01uF Ceramic disk which is completely microphonic.