How can I test a ouput trans to see if its faulty

[signalpaths]

I've got a 22W mesa and I think the OT is the culprit are there any tests I can perform??


thanks all

ERic

[MWJB]

Down & dirty test, measure the DC resistance from each power tube plate wire to the B+ centre tap...readings should be within 15%. This is just a rule of thumb test and won't catch all problems...if you have a major short in the primaries it should show up.

If you have access to a signal generator/AC voltage source and 2 voltmers apply a 0.5 VAC signal to the OT secondaries (no tubes or speaker installed) and read voltages from plate to plate at the OT primaries, divide by 0.5, square it and multiply by speaker impedance...you should end up with a figure close to your rated primary impedance for the type of power tubes used. A difference in AC voltage from either plate to the B+ centre tap will also show that something is wrong.

You don't mention any symptoms, what makes you think the OT is shot?

[signalpaths]

Well its actually a friends amp he said it smoked a bit when the incident happened. He took it to someone who couldn't find the problem and told him that he thought it was an output trans. I know from my own experience that a OT can smoke pretty good when you fry one of course so can a tube or a grid resistor or any number of things really. Once I get it in my hands tomorrow I will test the tubes. There probably fried too but at least I'll know more then.

Eric

[Rob Mercure]

SP. OT's rarely fail and I've only replaced a few in many decades of repairing stuff. But if you want to test it look for the "Tube Amp Repair FAQ" which is on RG Keene's site, I think, of should be linked to the forum somewhere. Back on the old Ampage Doc Horner provided a simple tester that used a neon bulb, a 9 V battery, a momentary contact switch and maybe another part of two. The idea is that you pulse a DC current through the tranny and if it's good the neon light will flash - if it's open or shorted the bulb won't. If I can remember where the link is I'll submit it but HD crashes have reduced me to my bad memory. You mentioned the tranny smoking but actually the power tranny is likely to smoke first - does the OT in question smell burnt or have wax, etc., dripping out of it?

Rob

[signalpaths]

I am more familiar than I would like to admit with Mr Keens tube amp trouble shooting guide

Rob good call I'll do that

I've actually fried a couple OT in my time. Mostly by stupid moves like not realizing that the speaker cable has come unplugged and cranking the volume. The power tubes arc and make noise and then burnt smell. So I would replace the tubes and the amps would work for a while but if the volume was too high they would cut out and the OT would smoke. I would replace the OT and problem solved.

thanks for the good info

Eric

[Rob Mercure]

Back on the old Ampage board there was a rolling discussion of a week or so about how to protect your OT and the consensus was that simply soldering a resistor of about 10 - 20 times the highest expected impedance across the speaker winding/jack would prevent the flyback spikes. I've done this as a standard mod in all my own stuff and when I rebuild an old Fender, etc., very cheap insurance which won't periodically short like the reverse biased diodes do.


Rob

[mac1amps]

Another "trick" test for OT's is to connect the secondary to a speaker, then take a 9-volt battery and touch the primary leads (or CT & one side) to the battery terminals. If you hear the battery "pulse" in your speaker (only when you apply the battery OR release the battery), then you know the transformer is good.
Ther ARE cases when an OT's secondary may have a "partial" short, which will result in the transformer "passing" the signal, but the volume level will be low (sounding similar to using an attenuator). If you are monitoring your current draw, you'll see that the amp is drawing much more current than it should be.

[jrfrond]

In all of the amps I've designed and built, I use a switching jack scheme on the speaker outs that automatically connects the OT secondary to an internal 8 ohm/25W power resistor to eliminate the no-load/unplugged speaker cable. The resistor, made by Arcol, is encased in a brass heat sink which is then bolted to the chassis with heat sink grease added. I also use reverse-biased R3000 diodes from each side of the OT primary to ground to catch damaging spikes and transients.

My favorite method of OT testing is the aforementioned application of a signal generator on the secondary. It is easy to check the balance of the primary to the center tap in this fashion. Remember, OT's don't always short, and can go bad on one side of the primary, which is pretty common. In this fashion, using a scope, you have a visual reference as to whether the windings have become unbalance or not.