Blackface Bassman has a crazy loud hum

[Matthews Guitars]

A friend brought me a blackface 40 watt Bassman that hasn't been plugged in for a very long time. So long that it had to be recapped before even powering it up, the originals were puking. Well, they weren't originals...most were cheap IC caps from the early 90s and they were done.

It's had a LOT done to it in the past. There's only a handful of original components still in it but the circuit looks correct.

So I recap it, with all due diligence, triple checked every cap for polarity, replaced the 25V/25uF 3 legged Mallory Monsters on the board too,
replaced the bias caps, recheck my work AGAIN....fire it up, and it hums loud and deep. Not a 60 Hz hum but way low in the bass, definitely close to 60 Hz, and the pitch does change as the caps charge up fully.

The output transformer is not original, it has three taps. Only the 8 ohm tap is connected, the other two are capped off.

DC voltages appear normal. Bias supply is working. Tubes aren't red plating. Tubes are fine, they test good, a spare pair gives the same results, too.

Usually I'm pretty good at picking up on trouble like this.

This is more challenging because the sound is full volume and I don't want to run the amp for more than a few seconds in this state.

Got any suggestions for specific things to look at?

[martin manning]

Leaky or failed rectifier diode(s)?

[xtian]

I would start by pulling out ALL the tubes, and measuring DC and AC voltage at the first filter cap when running. DC will be ~450 and AC should be less than 10V. If that looks OK, measure AC ripple of bias supply.

[nuke]

Well, get out the Variac and start by yanking out all the tubes.

Double check the previous work, including your own. (yeah, I make mistakes too...) The things that were "touched" most recently are often the most likely culprit.

First step:

Verify clean DC from all the power supply nodes, including the bias supply. Set the bias to max negative. Check the output tube sockets to be sure you have correct connections and voltage present at each pin.

Shut down, install the output tubes. Slowly bring them up on the variac to about half supply voltage. Check the output tube socket pins, make sure they're all about even between the pair. If all is good slowly bring up the variac to full line voltage. You should have no audible hum from the speaker. You should be able to adjust the bias and get even current on each plate.

Now we get to the PI stage. First thing, disconnect the feedback from the speaker to the PI stage. Put the PI tube in, slowly bring up the variac and see if it is good. If so, shut it down. Reconnect the speaker feedback line to the PI. Then bring it up and make sure it doesn't oscillate. Since someone has changed the transformer, there's a fairly good chance that the phasing is wrong. If the amp oscillates or "motorboats" then shutdown, swap the plate leads to the output tubes, bring it up slowly again.

Then you just work your back to the preamp stages, one step at a time.