output transformer broken?

[prune]

Hi, newbie here 

I've got here a small homemade amp (not homemade by me, so no schematic available, though I have drawn a quick one out my self - attached) amp (two 12AX7's, one EL34) that I've been tasked with repairing. I know roughly what goes on in a tube head but it's not a very deep understanding...

I have been asked to repair it as it's recently lost a lot of volume. Different valves / cab checked out OK, and after testing inside I can see that the two first gain stages have a gain of 22 and 9, and the input stages saturate when the signal is about 30V - so far so good. There then seems to be some kind of buffer (i'm not familiar with this arrangement), which I'm assuming is working OK as I see about around 30V max on the grid (pin 5) of the EL34. The HT measures 366V.

I didn't perform the following measurements with the full 30V as the cathode resistor on the EL34 gets very hot and something buzzes.. but with 16.58V on the EL34 pin 5, there is 20.52V over the output transformer primary and 0.976V over the secondary, feeding an 8ohm load.

My first impression was that the problem was the output transformer as the output is a very low voltage, but when I worked out the reflected impedance from the information above I get 2857 ohms, which sounds OK from what I've gathered about the load impedance for a single EL34.

I don't want to spend the money on a new output transformer without being sure that it's definitely going to sort the problem! Could anyone point me in the right direction?

[hesamadman]

The OT is generally one of the first things the B+ sees. I'm guessing that 20v is extremely low for your primary.

[2deaf]

What is the DC voltage on the EL34 screen grid (pin 4)?  As long as you're at it, maybe you could post the voltages for all of the EL34 pins.

[PRR]

Is 20.5V the most it will do? Or just where you tested it?

You do not say what your supply voltage is. But you expect RMS voltage at plate to be over half of B+. (Ideally 0.707, but we never get there, but we should get close.)

I would start by exploring ALL possible shorts on the loudspeaker side.

Actually I would want to know "all" voltages around the EL34. Including grids. A failed screen grid connection will be very low output.

The cathode resistor should NOT be getting much hotter with signal. The voltage at the cathode may rise 10%-20%, not more.

[Paul1453]

What is the DC voltage on the EL34 screen grid (pin 4)?  As long as you're at it, maybe you could post the voltages for all of the EL34 pins.

Voltage readings from pin 3 and pin 4 would tell us more about the OT.
Pin 3 would tell us how much B+ is getting through the OT.
If pin 4's voltage was significantly higher than pin 3's,

That would indicate an increase in resistance in the OT's primary winding, and a likely problem with the OT.   
That was the symptom when I killed my last unknown Philco OT.
Primary resistance went from 410 ohms to 82 K.
Output dropped dramatically, and voltage readings from my plate and grid confirmed OT problem.