Testing a power transformer under load?

[eurekaiv]

I've been putting together a rebuild using the iron and chassis from a Calrad AP-35 (2xEL34 power section) and it looks like i have a power transformer problem. Everything comes up fine with the rectifier (tube or SS plugin), then when I add the power tubes and power on I start glowing on my lamp limiter as the filaments warm up. I did all sorts of checks on the transformer before I wired it in and everything measured out as expected when hooked to AC only (or rectified with a tube or SS plug-in). I disconnected all sorts of things (including everything upstream of the power section) to narrow it back to the power section and double checked my wiring at least 3 times so I'm pretty sure everything's good there. My last test was alligator clipping in a spare power transformer pulled from a 2x6L6 PA amp and it came up without any action on my light bulb limiter and I had appropriate voltages.

So I guess my question is... is there a way to test a transformer on the bench, disconnected from an amp, to tell whether it might fail under load? I'd like to 100% rule this thing out before I spend $125 on a new transformer. I think I'm at 99% right now.

FWIW, my process for testing old pulls has been to ohm out the sections to find the right windings and check for obvious shorts. Then hook the primary through a bulb limiter to a variac set for 12V. Measure the outputs and multiply by 10. If those look good, set the variac for 120 and carefully check again. I've rebuilt a bunch of amps now, 6 or 7, and this is the first transformer problem I've encountered.

[sluckey]

It's possible that the PT is faulty. I'm guessing you didn't try to power up the old Calrad to see if it worked? But it's just as possible that something is wrong in your power amp or bias circuit. Could we see the schematic of your actual amp?

You can test this PT on the bench but you need an assortment of big power resistors capable of loading the HT winding to about 100mA.

[eurekaiv]

It's possible that the PT is faulty. I'm guessing you didn't try to power up the old Calrad to see if it worked? But it's just as possible that something is wrong in your power amp or bias circuit. Could we see the schematic of your actual amp?

You can test this PT on the bench but you need an assortment of big power resistors capable of loading the HT winding to about 100mA.

I had the same problem before I gutted it for a rebuild but I didn't troubleshoot beyond disconnecting the PT and checking per the method outlined above. I had 1 tube red plating and assumed another problem as a bunch of caps were very definitely toast.

My schematic is a plexi style preamp mated to a cathode bias EL34 power section.

[sluckey]

I think that your PT substitution is a good test and it proves the suspect PT is bad. I would buy a new PT based on that test.

[Pietro]

draw up a diagram and post it here. No way of knowing what the problem is without it.

[eurekaiv]

draw up a diagram and post it here. No way of knowing what the problem is without it.

What is it that I should diagram?

[Pietro]

the way you have the PT, OT and HT wired.

[sluckey]

the way you have the PT, OT and HT wired.

There is no need for that. He substituted another PT and everything worked just fine. Did you miss that part?

[dude]

What about the 2.7k going to grid of V2, maybe another 470k.

[Pietro]

the way you have the PT, OT and HT wired.

There is no need for that. He substituted another PT and everything worked just fine. Did you miss that part?

So it seems...