Solid State Guitar Amp Forum | DIY Guitar Amplifiers

That does happen, so it bears checking.  But in my 30 some years as an authorized Peavey repair center, I have to say I only saw it a very few times.

Certainly won't hurt to disassemble those magnets and clean the gaps, regardless.

Start at the start.  Pull the power tubes and check voltages at the socket pins.  Pins 3 and 4 of each should measure B+, pins 5 should measure bias voltage.  If those look reasonable, THEN install the power tubes.

Plug a cord into the speaker jack set for the impedance you will be using.  Now measure resistance from tip to sleeve.  You are measuring through the output transformer secondary.  Do you see a nice low resistance?  or do you see an open?

You can get a tube to do SOMETHING at 15v, but don't expect it to work like a 12AX7 in a real circuit at real voltages.

OK then, find the two high voltage wires from the power transformer, should be on posts W104 and W105.  Pull them off the posts so they are in free air.  Now does the bulb glow bright when powered on?  If so, the transformer is suspect.

Is F102 open?

There should be two wires to the standby switch, are they on posts W103 and W106?

Frankly I would experiment with speakers before going into the circuits.  Nothing will have more effect on tone.  A brighter speaker might be for you.

With the existing speaker, try this experiment.  Cut a piece of ply or just use heavy cardboard, cut to cover the open rear of the amp.  Not sealed up, just mostly covered.  How does that change sound to you?  If wrong direction, get rid of it.  If it seems to help, think of ways to more permanently cover the rear some.

Things seem to be changing, but I had to sign a non-disclosure.  Behringer was always tight with schematics, and didn't sell parts.  So in many minds that means not reliable.  it isn't the gear's fault they won't support it to users.I would sure try one.

At $20, I'd buy a set for all my vocals and a couple extras, just in case. Assuming my test went OK.

And all that said, the real, indestructible SM57 and SM58 are not expensive, under $100 new with warranty, and half that used in good shape.

Transformer unlikely to damage diodes.  SOmething downstream drawing excess current through the diodes burns them up.

Have you removed the old power tubes?  What exactly is the amp doing wrong at this point.

I was an authorized service center for Behringer for a good number of years, and my impression was that overall their stuff performs well enough, is reliable.  What you can't do is pound nails into a board with it.  SO I would expect if someone decides to be "cool" and drop their mic on the floor, it might not hold up after that.