Gibson GA6 output transformer suspect bad- But how do you tell??

[cjlectronics]

I am working on a Gibson GA6 amplifier. Very simple circuit... one 5Y3; two 6V6; two 12AX7. I have replaced the DC filter caps as they were original and leaking. The ripple voltage after the cap replacement is around 2.5vp-p... I'm cool with this. I can trace my test input signal all the way up to the 6V6's and it looks good. Problem is... no output. When I scope the output of the tubes going to the output transformer I get nothing that looks like the amplified input signal. I have measured the output transformer end to end and I measure ~ 450 ohms. I swapped the output tubes with known good 6V6's and I still get the same thing. The B1 voltage measures 430vdc.
Some observations:
1. I removed the wires going to the OT from the 6V6's and scratched them on the chassis... I can here static coming from the speaker. (weird??)
2. The OT is mounted on the Jensen speaker and the primary wires have tape on them and there appears to be burn marks on the transformer where the wires enter the primary side. It looks like something shorted and the repair was to wrap black electrical tape around the wires. Basically, a bubble gum repair and McGubber's finest work!
3. As mentioned above, I have signal all the way down to the 6V6's and the output of the 6V6' looks wierd. Is there a way I can put a resistor or something similar across the output of the 6V6 pair and simulate the OT?

Thanks,
CJ

[Jazz P Bass]

In a very general way, if you have a nice healthy signal at the grid of each 6V6 & the 6V6 tubes have there proper B+, screen & cathode connections & still no output, that leaves the output transformer or the speaker.

[R.G.]

There are very simple tests which will tell you whether a transformer is bad or not.

1. Disconnect all leads so you have the transformer for sure not connected to any loading on any winding.
2. Get out your ohmmeter, and see if (a) all windings which should be continuous are in fact continuous and (b) no winding connects to something it's not supposed to, including the core. If these DC resistance checks are OK, then...
3. Do a pulse-inductance flyback test. Pick a winding, hook a neon bulb across it. Hook any one winding up to a low voltage DC source for a second, then break the circuit. If the neon flashes, the transformer does not have an internal shorted turn.

If it passes these, you can proceed to put an AC signal on it, etc. The only thing this set of tests does not tell you is whether it shorts at very high voltages or with heating.

[cjlectronics]

Thanks R.G.... I performed the test just like you stated and the neon lamp flashed. This made me think I didn't have an output transformer problem.
I then pulled the schematic and studied it, tracing the wires and components per the schematic. This is where I discovered there was no cathode bias resistor. I am not knowledgeable enough, yet, to see these things immediately, but taking the time to study the circuit paid off. I put a 200 ohm resistor on pin 8 of the two 6V6's and the amp came to life. WOW, it sounds good too. I may have to work out a trade deal with the owner on this amp!!