How can you test an output transformer?

[sumasmreq]

Is there a way? My amp is broke and it might possibly be the OT but not sure.

[JerryP]

What kind of amp? What's wrong with it? What's it doing or not doing?

Jerry

[sumasmreq]

its on a vibro champ. i really just want to know of there is a way to do this, I think something is actually broke in my power section, probably just the rectifier tube (it lights up really bright) but I just wanted to know this just to make sure the OT is working fine as well before I turn it on.

[Pepi]

sumasmreq

 

I've worked on a lot of tube amps in my time and I can only remember of a 1/2 dozen that need OT changed. I bet you just need a re-tube job. Could also be the filter caps breaking down. Good luck and be careful with the voltage stuff.

[sumasmreq]

I guess I just want to know becuase I pulled everything apart to look at it when it broke, nothing looked bad. But then I plugged it back in and still no dice and I realized there was no speaker connected (only for about 10-20 seconds)

 

I am buyin a new rec tube for it, guess we will see how it works when that gets here.

[tlp123]

So, you think you may have a shorted output transformer? Here's a very quick test that never lies, and this same tool can also be used for checking power transformer taps.

 

First, you'll need a small 6.3 V filament transformer, this can have an output of 3A or higher. 3 Amp units are cheap, so don't worry. Next, you'll need a "current limiter" made up of a 100 watt light bulb and socket. This light bulb must be wired in "series" with one of the AC "input" leads of your 6.3 volt filament transformer. If this little 6.3 V transformer has a center tap, just cut it short, and shrink wrap it. You won't be using it for any of these tests.

 

Next, you'll want to add longer leads to the 6.3 V filament transformer "secondary."

 

Best thing is to solder them, and use shrink wrap. Add INSULATED alligator clips to the other end of these leads.

 

Now, you have a test rig for checking output transformers, and checking an "unknown" power transformer for lead specs.

 

TESTING OUTPUT TRANSFORMERS:

 

It is not necessary to unwire the output transformer for this test, just pull the output tubes. Here's what you do; Take your little 6.3 V filament test transformer, and clip one alligator clip to the output transformer secondary tap.

 

This can be the 4, 8, or 16 ohm tap. It doesn't make any difference which one you use. Next, you'll take your other test clip, and install it to the output transformer "common," or "ground" wire. MAKE SURE you have the current limiting light bulb in series with "one" of the filament transformer primary leads before plugging it into the wall. If the output transformer is shorted, the light bulb will light up, case closed, replace that transformer. To confirm you have a good connection, you can use an AC voltmeter, and take a reading on the secondary plate leads. The voltage will vary, but you should see around 120 V AC on a 50 watt Marshall output transformer (plate to ground). Your other secondary taps will have voltage on them too. These readings will be like 4-15 volts between the secondary taps. Using an AC voltmeter here, you'll need to touch chassis ground with one of the probes.

 

TESTING UNKNOWN POWER TRANSFORMERS:

 

So, you have a power transformer hanging around, but you're unsure what all those wires are. This test gets a little more trickier, but is very easy once you know where to start. Usually, but not always, the 6.3 volt secondary on power transformers have heavier leads due to the amount of current these wires need to supply. If you can identify these, you can connect your little 6.3 V test rig to them, and reverse power the transformer. Then, with an AC voltmeter, you can probe all the wires to find out what the input taps are, and also the secondary taps for high voltage, bias, 5 volt heater etc.... Now, on a power transformer that has multiple primary leads for world wide voltage, you can write the colors on a piece of paper, and as you're testing them, document your readings. If you should take two "primary" leads, and measure odd readings like say 11.3 volts between them, these are taps, and neither one is a common lead.

 

Try another combination. Even if you can't figure out by "eye" which is the 6.3 secondary on the transformer you're testing, with the current limiting light bulb you are safe to connect to any combination of wires until you find them.

 

You don't need to worry about hurting any gear with these tests. The "current limiting" light bulb in series with one of the 6.3 volt tester transformer primary leads protects your gear from any shorts. It will only light up if something is shorted.