Any procedure to test around an output transformer?

[fredcapo]

I am working on two p-p tube amps that likely have bad output transformers...one smells burnt, and the other has a known underrated OT
...both blow fuses. Would it be reasonable to remove the OT from the circuit, and replace it with an equivalent resistor (network) to simulate
the load in order to determine if the amplifier/circuit is otherwise functional? Then, could a scope be used to actually look
at the output signal?

[Enzo]

You are looking for a gross test, a go or no-go. It doesn't matter for this test if the amp sounds great or not, all you need to know is whether the transformer is causing the problem. SO just about any OT will work for this. I have an old Fender Bassman OT I use for all test. Unsolder the primary wires from the power tube sockets, and the center tap from the B+. CLip your test transformer in its place. Just connect the secondary directly to a speaker, don't bother connecting it to the secondary circuits, forget the NFB. Does the amp now work?

Go to geofex.com and find the very simple transformer tester. Make one and use it.

[fredcapo]

OK. Thanks. I just want to know if the OT is causing the excess current draw. I just read about the neon bulb test ala RG. I will dig an old OT as per your post.

[The Dude]

I assume you already have, but if not, I would ohm out the windings first. This is kind of like using a tube tester. If it tests bad it's bad. If it tests good, it might still be bad. It could be arcing, have only a couple shorted windings, etc., and that won't show up on a DVM. However, you can test to see if you have a primary to secondary short. You can test to see if your center tapped output readings are equivalent from one side to the other, etc. Much of the time you can tell the transformer is bad just by using a meter. Other times,......not so much. It's certainly worth the couple of minutes it takes to check.

[Jazz P Bass]

OK. Thanks. I just want to know if the OT is causing the excess current draw. I just read about the neon bulb test ala RG. I will dig an old OT as per your post.

If all you want to read is the overall current draw of the amp, the amp can be turned on without the output transformer being hooked up.

[trobbins]

A bad smell is a symptom, but quite rightly may not mean a bad OT. I have a bad smelling OT in an amp on my bench at the moment, but it tests all fine.

Removing/unsoldering the OT is unnecessary work. As suggested - an ohmmeter check is first in effort, then more effort required to either quickly cut over another OT, or do a quick voltage ratio test in circuit by clipping say 5VAC to a plate-to-CT winding section. Check schematic in case any additional circuits would impact on any measured results. Of course it helps if you know what ohms to expect, and how to measure ohms, and also what volts to expect for the impedance ratios present.

[Mick Bailey]

I seem to get a lot of amps with bad transformers, so I made myself a little ring-test box in a 1st gen. Ipod Shuffle box that I had kicking around on my desk for years.

[fredcapo]

I seem to get a lot of amps with bad transformers, so I made myself a little ring-test box in a 1st gen. Ipod Shuffle box that I had kicking around on my desk for years.

[ATTACH=CONFIG]24089[/ATTACH]

OK. What does the box test for? shorted turns, etc? Thanks

[Mick Bailey]

It tests for shorted turns, shorts, opens and high resistance in transformers, chokes and other inductors.

When an unloaded inductor is given a pulse it produces a decaying waveform. Like striking a bell. The waveform decays because the energy is being lost as it oscillates. If you count the number of oscillations it gives an indication of the quality of the inductor. A shorted turn will soak up nearly all the energy and won't give any 'rings'. Other forms of leakage or resistance will also affect the number of rings.

[Sowhat]

Back in my vintage CRT monitor days I used a ring tester that I made but I was under the impression that the internal geometry of the transformer like that of a High Voltage Flyback transformer with it's gaps was the only type that was energetic enough to ring properly. This works on regular transformers too? Your board looks eerily similar to my ring tester.

[Mick Bailey]

Yours is probably based on the Dick Smith K7205 ring tester - mine is. The standard circuit won't detect the rings on a standard transformer. You're right in that there isn't enough energy (which resolves to voltage amplitude) produced by audio and mains transformers, but that's due to the sensitivity of the original design being intended for CRT flyback transformers.

All that's needed to fix this is to alter the reference voltage on the non-inverting input of the comparator.

[Steve A.]

If all you want to read is the overall current draw of the amp, the amp can be turned on without the output transformer being hooked up.

Can you elaborate on that? I can see how that might find dead shorts in wiring, etc., but I was under the impression that with the OT disconnected there would be no voltage going to the plates so the output tubes would be drawing little or no current. I remember Bruce Collins posting a good test which involved connecting a voltage source to the primary windings and measuring the voltage at the secondary. Or something like that.

Thanks!

Steve

[Sowhat]

Yours is probably based on the Dick Smith K7205 ring tester - mine is. The standard circuit won't detect the rings on a standard transformer. You're right in that there isn't enough energy (which resolves to voltage amplitude) produced by audio and mains transformers, but that's due to the sensitivity of the original design being intended for CRT flyback transformers.

All that's needed to fix this is to alter the reference voltage on the non-inverting input of the comparator.

Yeah, Dick Smith! I forgot who's circuit I used... gotta love those Aussies! Sounds like a simple tweak, I'll have to open that thing up and fiddle around. A standard transformer/inductor ring tester would be handy to have on hand. I remember that with flybacks the thing worked great, saved me a lot of time and guesswork.

[Jazz P Bass]

Yes, you are correct in the 'proper' way to test the transformer.
I simply threw it out there.

[mikeydee77]

All that's needed to fix this is to alter the reference voltage on the non-inverting input of the comparator.

Mick, Interesting; can you elaborate? Do you mean just calibrate the unit against known working transformer or do you have some measurements that you could share?