Peavey classic VT phase inverter voltages

[mikeydee77]

Hi, I just replaced the OT on this amp as it was short. Once back up and running there is an occasional crackle.

So was just trying to work out were it was coming from and noticed that the voltages around the PI don't match what I have. Amp sounds ok and the crackle isn't present in the pre-amp.

I have V q101 and q102 collector about 110v and 60v respectively - schematic suggest 145 so that is the big difference that bothers me.
The base is 18.75 compared to 16v on the schematic
emitters 18.5v are - compared to 17.4 on the schematic - bit of a difference here and presumably the bias conditions for the transistors is wrong.

Just wondering if I am looking that the wrong schematic.

PV_Classic_VT.pdf

PSU all looks good, B+ is 500v and the 300v is bang on, bias supply is -60
Supply is 300v. and the +/-15v are just that.
I have lifted and checked the various resistors to see if anything obvious was to be found.
And as part of the fault finding I did find a short IC24 which connects in the NFB so I guess that might have been the culprit for the crackle.

Phase inverters are a bit of a mystery to me so would appreciate any thoughts.

[g1]

How about IC18?

[mikeydee77]

Hi G1, Good point. And actually I had overlooked that path. No dice though.

I decided to remove the transistors and see what the voltages were like without the BJTs in circuit. Didn't learn much except that it is the transistor that drags it down . . . When I refitted the trannys the fault had moved to the other transistor. So I think one of them must be duff. Both buzz out fine on the meter mind you.

I'll get some 2N3439 and see.

[Enzo]

Your meter applies a volt to the parts, while the circuit applies 300v to the part. All a meter can do is give a gross indication of the part, it can tell you a part is bad. A meter cannot tell you a part is good.

[mikeydee77]

All fixed now. 2N33439 was a good substitute for the driver transistors. All voltages as per schematic are correct and background noise which wasn't that bad has all gone. Thanks all.