Any procedure to test around an output transformer?

[R.G.]

The ring tester and the neon lamp tester both look for the same thing - the release of the energy stored in the transformer core by interrupting a DC current. The neon lamp tester does this in a more quick and dirty way, and is its own comparator. I don't know the circuitry of a ring tester, but it's function is obvious from the description. It looks for ringing, rather than a sudden spike of voltage.

Both ought to work fine. Neons can't be calibrated, if calibration is needed. A ring tester as I think of it might be able to tell you more about the inductor/transformer, but in most cases for repairs, to tell if a transformer is broken or not, that may not be needed.

[R.G.]

I just want to know if the OT is causing the excess current draw.

About the only way the transformer could be causing the excess current draw is to be somehow causing oscillation inside the amp so it's eating too much current. The current in a tube output stage is not controlled by the transformer, it's controlled by the tubes. The transformer is effectively a pair of low-ish value resistors for DC conditions.

I would troubleshoot it this way.
I'm going to assume you have already subbed in new, known-good output tubes. That's always step zero in testing tube amps.

Pull the output tubes and fire it up. Still blow a fuse? If so, it's either not the output stage at all, or it's a hard short inside the OT.

Ohm out the OT, and if no windings are shorted to each other or to the core, try a neon-lamp or ring test. If the transformer passes these tests and ever worked, the transformer functions OK - cold at least.

If it's not the OT, chances are it's a power supply fault or a bias problem. Leaving the tubes out, measure the bias voltage on each output tube and see if it has bias at all, and then enough of it. If the fuses blow without output tubes in it, then you have a power supply issue (at least). If this is the case, put it on a light bulb limiter for further testing and go debug the power supply.

Once the power supply is fixed, test for bias again, and if you have correct bias and the OT was good, try it out.

[mikeydee77]

The ring tester and the neon lamp tester both look for the same thing - the release of the energy stored in the transformer core by interrupting a DC current. The neon lamp tester does this in a more quick and dirty way, and is its own comparator. I don't know the circuitry of a ring tester, but it's function is obvious from the description. It looks for ringing, rather than a sudden spike of voltage.

Both ought to work fine. Neons can't be calibrated, if calibration is needed. A ring tester as I think of it might be able to tell you more about the inductor/transformer, but in most cases for repairs, to tell if a transformer is broken or not, that may not be needed.

Had a google around and found the original article and revised version which is on sale as a kit for about 42USD / 29 GBP. Looks very useful although as others have mentioned, subbing in old OT is the only absolute test.

k7205.pdfBLUE_rt_assembly_manual.pdf

[Mick Bailey]

I changed R13 from 10k to 1k. No calibration - just test it with a few transformers to get the feel of it. With most OTs it will show which winding has a shorted turn. You could put a range switch in there instead of R13 to test a wide range of inductors.

Mine gets used all the time. Just switch it on and it repatedly pings the transformenr/inductor/choke/coil.

I do a lot more work than just amps so the extra sensitivity and bargraph type of display is useful.

Note that this is not the Blue Ring tester - that's a different circuit to the K7205

[Dave H]

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.

The usual transformer shunt method of measuring bias current effectively shorts the plates to B+ so you could completely disconnect the OT primary and give it a quick test with the plates connected to B+. If that doesn’t blow the fuse the OT must be the problem.

[R.G.]

Yep, Dave H is right - you could short the plates to B+ for a quick test. Maybe he meant to disconnect the OT and also short the plates to B+. If the OT is internally shorted to ground, it would still cause an overcurrent. If you open the OT primary, short the plates ( or use low-ish value resistors ) then the tubes control the current, and the OT can no longer overcurrent the B+. The resistors are handy, because you could then measure current per tube, which tells you more about the setup.

There is one unusual condition that can cause fuse-blowing misery - sometimes an inductor/choke shorts from winding to core and thence to the chassis. This has much the same effect as the OT shorting to chassis.

[nickb]

which is on sale as a kit for about 42USD / 29 GBP...

Here a link to a kit: - Blue Ring Tester Kit

[R.G.]

Had a google around and found the original article and revised version which is on sale as a kit for about 42USD / 29 GBP. Looks very useful although as others have mentioned, subbing in old OT is the only absolute test.

I had a look at it. That would detect an internal short as a very, very low Q, OK. It's properly a coil Q tester. It shares one of the flaws of the neon tester, in that it does not test at normal high operating voltage. But both should work fine for detecting an internal short.

[mikeydee77]

Thanks Mick,
That's was the detail I was after.

When you read the Dick Smith instructions it kind of gives some hints at how you would tweak it. I think I might knock one to see how it compares with the Neon which to be honest I really like because of its simplicity.

[Mick Bailey]

I'll point out again that mine is not the Blue Ring Tester, I'm referencing the K7205.

[Dave H]

...Maybe he meant to disconnect the OT and also short the plates to B+.

Yes, that's what I really meant to say.

[fredcapo]

Thanks to all for the feedback...direct and indirect. I made a neon tester. Very handy. I am light on direct hands on experience but the last month in the trenches and on this board have been immensely instructive. I have confirmed that the Tiny Terror and the JTM60 have shorted OT's. Have a third amp in the shop with shorted OT. A JCM900. DC looked fine as well as a nice signals at the el34 grids yet horrible weak output. Amp Parts Direct seems to have a nice OT replacement for the JCM at a decent price.

[Mick Bailey]

I've started an OT graveyard, I get so many.

[fredcapo]

I assume the OT failures are primarily spontaneous and due to poor quality. Did the older "quality" amps have a proportional OT failure rate?

[R.G.]

Thanks to all for the feedback...direct and indirect. I made a neon tester. Very handy. I am light on direct hands on experience but the last month in the trenches and on this board have been immensely instructive. I have confirmed that the Tiny Terror and the JTM60 have shorted OT's. Have a third amp in the shop with shorted OT. A JCM900. DC looked fine as well as a nice signals at the el34 grids yet horrible weak output. Amp Parts Direct seems to have a nice OT replacement for the JCM at a decent price.

Be sure to test your tester and technique by testing a known-good OT to be sure you're not getting false positive results.