Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Saransk on June 25, 2020, 04:03:51 pm

Title: Blown Output Socket - Peavey Windsor Studio
Post by: Saransk on June 25, 2020, 04:03:51 pm

Picked up a Peavey Windsor Studio for basically postage that was broken.
The output socket connections had blown - looked like a short between Pin 2 (Heater) to Pin 3 (Plate)
The pad for 3 was gone as well as the two resistors on the Output heater circuit.
I noticed that the connections for the SE Output Transformer were reversed - but would that make a difference? I'm not too up on my SE Output stage design.
This one of those "universal" stages that can use everything from a 6L6 type - EL34 type - KT77/88 type.
No idea what the output transformer is - one of those Chuang Meei specials - hopefully that isn't bad.
Any ideas beyond bad tube or did something else take out the socket?
Thanks
Mike

Title: Re: Blown Output Socket - Peavey Windsor Studio
Post by: sluckey on June 25, 2020, 04:30:38 pm

A shorted socket between pin 2 and 3 is commonly caused by someone playing the amp cranked with no speaker connected or a speaker that has an open voice coil. The OT open secondary reflects a very high voltage pulse back to the primary. This high voltage pulse will often arc across pins 3 and 2 and also take out the filament ground reference resistors. The tube and/or OT may still be fine but have been stressed.

Title: Re: Blown Output Socket - Peavey Windsor Studio
Post by: PRR on June 25, 2020, 05:39:37 pm

What he said.

> connections for the SE Output Transformer were reversed - but

No NFB around OT, doesn't matter.

Title: Re: Blown Output Socket - Peavey Windsor Studio
Post by: Saransk on June 30, 2020, 05:32:03 pm

Makes sense
Consider the stuff on the output of the unit, it could be a failure in the Power Soak (large rheostat) of the tap for the balanced line output.
I checked the power transformer and got some "interesting" readings.
Without any reference on the schematic the primary voltage - no load - read 500 volts (AC) red-red - which seems high for a full-wave bridge circuit, even with no load.
When I checked across the two yellow leads for the output tube - I assumed it would be close to 6.3 VAC, I got close to 15 VAC.

I wonder now if the failure damaged the Power Transformer, or the transformer failed internally.
Since it only powers the Output tube, I can't think of why it would read so far over the needed voltage.
It's another China Special - can't find specs for them anywhere.
Mike