Peavey Classic 50 - Hum and current draw with no power tubes

[glebert]

Guitar shop I do repairs for had an Peavey Classic 50 come in that was blowing main fuses. Don't know how many they fed into it before bringing for repair, but sounded like a few at least. I pulled the power tubes and put in a main fuse and brought it up on the variac with LBL. Immediately on bringing up the voltage there was some hum in the speakers. It didn't necessarily get that much stronger as the voltage went up, but it did start to make the 300W bulb limiter start to glow a little so I shut it down. I tried it again with the amp in standby, hum and current draw still there. Pulling the HV leads to the recifier allows me to bring the voltage up with no hum and no real current draw. I didn't have time to check any of the voltages in this condition. I am assuming this is either a problem with the bridge rectifier diodes or the caps, causing lots of AC ripple that can then couple out through the OT to the speakers? I put my DMM across the diodes and caps and nothing was shorted but that was all I had to time to do today. Does diode or cap problem sound right based on the symptoms?

Here is a schematic, don't know if it is exactly this revision but seems close.

Thanks,
Greg

[The Dude]

Did you check (or temporarily remove) the flyback diodes?

[Helmholtz]

The OT might have a primary to core or secondary short (check with Ohmmeter).

[glebert]

I check those diodes and the OT tomorrow, thanks for the ideas.

[xtian]

This is a question of HOW MUCH CURRENT the amp is drawing. A healthy PT will draw 10-15 watts when energized. And if the amp has HT bleeders and preamp tubes installed, it will draw, ballpark, 20-30 watts. You say your bulb is lighting dimly. This may be perfectly normal.

I suggest you disconnect all PT secondaries and try again. It's certainly possible that the PT is radiating EM into the OT and you hear this as hum.

[Helmholtz]

I don't see much sense in using a 300W bulb limiter.
A 300W bulb can pass up to 2.5A (turn-on surge current can be several times larger), so not much current limiting effect.

I'd either use a 40W to 60W bulb limiter or a variac with a current meter.

Correctly interpreting lamp brightness requires some experience and comparison with good amps.

[glebert]

I don't see much sense in using a 300W bulb limiter.
A 300W bulb can pass up to 2.5A (turn-on surge current can be several times larger), so not much current limiting effect.

I'd either use a 40W to 60W bulb limiter or a variac with a current meter.

Correctly interpreting lamp brightness requires some experience and comparison with good amps.

I agree, on my home bench I use anywhere from 25-150 watt, depending on the amp, but usually 60. In this case I have to use someone else's setup and it is what it is. They do actually have an ammeter in series with the bulb, but I always forget about it and I don't know if it is accurate. I've done a few amps with the 300w and only a few amps (usually bigger class A) will get any glow at all when working correctly, but most have no visible glow. In this case the glow I was getting was with the variac maybe at 50%, so it was drawing some pretty substantial current.

[Helmholtz]

I have to admit that the 300W bulb provides enough current limiting to prevent the mains fuse from blowing.

[glebert]

I have to admit that the 300W bulb provides enough current limiting to prevent the mains fuse from blowing.

It has worked OK in a situation where a tube was drawing lots of current and as soon as I saw the light come on I cut the power in a flash (pun intended).

[Greg Robinson]

I'd highly recommend anyone that hasn't already got something similar to assemble something like I use. Eight 75W halogens in parallel, each individually switched, plus master on and bypass. The Kill-a-watt style meter is also handy, but required modification to seperate the sense and power lines, else it'll brown out and lock up or not display at all (simple modification).
Works for anything from tiny 5W solid state practice amps to multi-killowatt PA's depending on setting. Much easier than keeping a stash of different wattage bulbs around that need to be changed and risk breakage due to handling.

[glebert]

As a follow-up, it was one of the flyback diodes. Amp working now. Thanks for the suggestions!