Help debugging PWR Transformer

[tommytube]

My ST35 is about 6 years old. Using PA774 power transformer. Also using GZ34 tube rectification. Amp sat for a few months and when I went to use it again amp seems dead. Tubes have a small light, but not full on like normal.

I'm measuring 120VAc going into my primary on the power transformer. I only get 3 or 4Vac out at Pin 10 at the board. So only 3 or 4 Vac between pins 9 and 10 at J1.

Has my PA774 fried? Any more ideas on what to check?

This is my second PA774. First one lasted about a year before frying. I'm on the second one. Maybe with tube rectification, it's drawing too much?

Any help you can provide would be appreciated.

[dcgillespie]

Tommy -- I'm not sure how you're using a GZ34 with a PA774 since there is no 5 volt winding to power the tube's heater. If you're running it off one of the 6.3 volt windings with a dropping resistor, that would put the heaters of the tubes on that winding at full B+ voltage, which is not good. It would also overload that winding significantly. If you're using one 6.3 volt winding for the rectifier (with an appropriate resistor), and the other for the amplifier tubes, the winding for the amplifier tubes is being severely overloaded. Unless you are using a separate transformer to power the GZ34 heater, there is no way to properly operate the tube without overloading the transformer in some way.

Without knowing more, it's hard to give a better answer, but suffice to say, if you're operating the transformer in some fashion as indicated above, I have no doubt you are going through power transformers rather quickly!

Dave

[tommytube]

Thanks Dave for trying to help me.

I figured out my malfunction. A connection to my main B+ cap was broken and the wire was hiding in a bundle and it was hard to see. I wasn't getting the DC to the main board. Once I connected it back, it's working great now.

BTW, I'm using a Hammond 166MS transformer for the tube rectifier, so no worries on killing the main PT.