Peavey 5150 60w draws excessive current when OT is connected

[xtian]

I picked up this 60w combo knowing that it burns mains fuse. When we tested at the seller's with a fresh fuse, amp hums loudly, immediately on power up. At home, I pulled out all the tubes and powered up with a bulb limiter and still saw high current draw. So I disconnected the OT primary, and...all good! No excess current draw, and B+ looks OK at about 500v (no load). As soon as I plug OT primary back in, it wants to draw heavy current.

So I ordered and received a replacement OT from Peavey (replacement OT is from their Vypyr series, is smaller, and has only 8/16ohm output taps, FYI). I clipped it in, and amp behaves exactly like with the original OT!

Where could the fault be, and why does it vanish when I disconnect the OT...if it's not the OT?!?

Peavey 5150 Combo.pdf

[xtian]

Photo.

[Timatmit]

Is there DC voltage present on the leads going to the OT primary?

[xtian]

Yes, the red wire (OT primary center tap) terminal has ~500vDC.

Just to make sure I'm naming things right: The OT primary is the input side, and the secondary is the output for the speakers, right?

[Timatmit]

I'm not familiar with this model amp, so I'm not sure if that is normal.. the primary coil may be providing a low resistance path to ground which would draw a lot of current from the -500 supply. Shorted cap somewhere, possibly?

[g1]

I take it you are just disconnecting the center tap of OT primary?
As soon as you connect it, the protection diodes are in circuit, have you checked them? (CR13, CR15)
Otherwise, maybe a socket is arcing or an arc on the power tube board somewhere?

[xtian]

No, I'm disconnecting all three of the OT primary wires. I will check the diodes, great idea.

[xtian]

Well! That power tube daughter card is RIVETED to the chassis. And the components are under. Yup.

[g1]

Well! That power tube daughter card is RIVETED to the chassis. And the components are under. Yup.

You can push out the old part and install the new ones on the trace side. This was factory recommended procedure for some manufacturers when disassembly was not practical. Peavey factory repair may have drilled out old rivets and installed new, but you can do it either way.

[Helmholtz]

Where could the fault be, and why does it vanish when I disconnect the OT...if it's not the OT?!?

When you disconnect the OT CT, the power tubes don't get B+ and can't draw current (current path disrupted).
I'd suspect a power tube drawing excessive current or even shorting.

Pull power tubes, re-connect the OT and see.

[Mick Bailey]

He pulled all the tubes and it still draws excess current.

[xtian]

Yes! One diode was shorted! Plugged OT back in, and no more excessive current draw. Awesome.

[xtian]

All buttoned up and sounding great. Thanks for the help.

Photo shows diodes replaced with 3x 1N4007 from top of board (each diode string is sealed in clear shrink tubing).