Blues Jr whining and humming

[mort]

The amp whines, squeals, or BBBBBBRRRRRRRRRR's once the volume level is up around 4 or 5 and the unwanted noises vary in pitch and intensity via the EQ pots and squeals very loudly when cranked.

Swapped out all tubes.

Disconnected tank.

dc voltages all appear to be normal.

On 2nd schematic here: http://www.thetubestore.com/lib/thet...-Schematic.pdf (right click and rotate clockwise) it lists 29.5vac on each outside primary lead but I'm getting ~55vac on one side and ~3vac on the other, while dc plate voltage matches on those same lines at ~325vdc.

I'm kinda puzzled by the imbalance of ac while the dc is balanced fine. Does this sound like a failure of the OT? Where to go from here?


Thank you, as always, for looking.

[The Dude]

My first thought would be that the OT primary wires are reversed. Has it been changed or does it look as if it's been messed with. Maybe just try flipping them.

[mort]

My first thought would be that the OT primary wires are reversed. Has it been changed or does it look as if it's been messed with. Maybe just try flipping them.

Nah, it's clearly marked on the PCB blue and brown for those two leads and they are correct. This is an open jam night amp, one of several Blues Jr's the guy uses and was working fine until recently.

[The Dude]

If you're sure that the transformer is wired correctly, I would temporarily unhook the flyback diodes (CR10 &11) before shooting the transformer. Most common failure is shorted for these, in which case they will pop a fuse. I have seen them do strange things like this before, though. I'm not sure why. Maybe one of them becomes "capacitive". They are certainly cheaper than an OT.

[mozz]

Are you reading it correct by putting all pots at 50%, injecting a 10mv signal of 1khz at J1? Those amps squeal like pigs with the back off. Find the late BillM's website and read all the solutions.

[Enzo]

If the tone controls affect the frequency of the noise, I then expect the amp is oscillating. Scope each B+ node to see if the oscillatory signal is riding the B anywhere. Also check lead dress, the ribbons by the preamp tubes can couple if bent wrong. The blue and brown wires may be right, but the one that crosses the ribbon can be out of place.

[mort]

This is weird. I realized that the hum had only started after lifting the boards (the whining was there before moving anything at all) so I secured the boards back to the chassis and now it works fine with no whining or weirdness.

Before writing the thread I had replaced C2 and C3 but the whining was still present and was present this whole time until securing the boards back to the chassis. I just chopsticked around trying to spot an intermittent connection but everything seems solid. the AC is still imbalanced on the OT primary so that must be just the way it is normally, I suppose.

[The Dude]

I don't recall on that particular amp, but my bet is that without the board being mounted in the chassis, something wasn't grounded- either with a screw or the pots need to be grounded to the chassis, etc.

[mort]

it's not the pots or the main board, as all those connections to the chassis are isolated from any circuitry and teh pots just have plastic shafts going through the chassis holes. If anything the little output jack board needs to be grounded and was causing the hum. But the whining was there before ever even taking the back off, and now it's gone.

[mort]

ok. so was poking around with a wooden stick and when D11 was tapped the noises returned, then went away again on their own, then came back with another tap of that diode. So tomorrow I'll replace both of those diodes 11 & 12 and report back.

In the meantime, what are those diodes even for?

[The Dude]

Those are the flyback diodes I was referring to in post #4. I was using the first schematic in your link. You must have the second one. They are for protection from large spikes through the OT. You can simply unsolder one end of them to test. Lots of amps don't even have them. You can replace them if you then determine that they are causing the problem.