Marshall JTM30 output and bias question

[g1]

Have you tried both output jacks? There is some switching going on there and it might just be a jack issue.
Also check your test setup with another amp.

[Randall]

I have an 8 ohm load on it.

[Randall]

I tried different jack combinations, it's not that. I measure much less signal at pin 3 than I do at pin 5.

[Chuck H]

Is this the first time you've tested the actual output of the amp? That is, could this low output have been present all along? Rough DC measurements of the OT from ends to CT aren't adequate. First test for shorts between windings or to the chassis. If that checks out you'll need to pull the tubes and the primary CT to do a proper ratios test.

[Randall]

This is the first time since it came in that I tested output. First I repaired the burnt out AC inlet, then repaired the bias problem, then I tested for output.

I will do the neon bulb test on the OT to see where that goes, in this case that will be easy the way it is set up. I don't know how to do a proper ratios test.

[Jazz P Bass]

"I don't know how to do a proper ratios test."

Easy peasey.

Insert an ac voltage into the secondary & measure the primary voltage.

[Enzo]

The neon bulb test should do the trick. Ratios are fine for identifying but really, if it used to work, then they won't be wrong now.

But do learn how to do them.

If you have good B+, good screen voltages, and loads of signal at the grids, then the OT becomes suspect. Just takes one shorted turn and your meter can never detect that. A neon tester can though.

I always had a fender Bassman OT on the bench I could quickly sub into any circuit as a quick test.

[Chuck H]

THAT^^^^^^^^^^^^^^^^^^^

Jazz and Enzo have it. I've never done the neon test though. I just test for shorts and measure the ratio.

Plug a voltage from your signal generator into the secondary. You need to measure this voltage under testing conditions because the transformers inductance can load the signal down. Shooting for an easily mathed voltage makes getting results easier . So, a half a volt (or even one volt is better if your sig genny will handle that, most won't). If one volt at the secondary results in, say, 20 volts across the primary then you have a 20:1 ratio. Square that and you get 400. Then multiply that by the intended load to get the primary impedance. So 20x20=400. 400x16 (the load) =6.4k primary impedance.

[Enzo]

I have 6vAC handy from a number of transformers. If I want to test one, I apply 6v to it, rather than a test oscillator. the 6v winding has zero problem driving the test unit.

[nickb]

"I don't know how to do a proper ratios test."

Easy peasey.

Insert an ac voltage into the secondary & measure the primary voltage.

If you do go this way you need to measure the magnetizing current as that being high is the sign of a shorted turn. A shorted might not have much effect on the ratio.

[Randall]

I just did the neon test, and I'm seeing something I have not seen before. With the neo on the primary and the battery on the secondary, I get a flash, but in the opposite I get no flash. I thought any windings work with this test. Is what I'm seeing abnormal? This is with OT fully disconnected.

[Dave H]

Perhaps having the battery on the primary and neon on the secondary transformed the voltage down below the neon striking voltage. Try it with neon and battery on the same winding as below.

[Randall]

Doing it this way with the battery and neon in parallel I get very dim on the primary, nothing on the secondary. I am using a fresh 9 volt battery.

[Enzo]

DO you have any other OTs about? Perform the exact same test on a different transformer to get an idea what normal looks like.

[Randall]

I'm familiar with the neon test, I have a clip on jig made up, so no this doesn't look normal to me. I subbed a 30 watt 4 ohm OT that I have, and with a 8 ohm load I get about 23 watts, I didn't measure it exactly, but I think it tells me what I need to know, the Drake is faulty. I don't think I have seen an OT go partially bad like this. It works enough to play the amp at moderate volume, but I think it should be louder. Could this be the case if one or two windings shorted? Could the bias issue causing the tubes to pull 96mA have caused this?