Have I killed my output tranny?

[Bluefinger]

Hi,

While I was trying out and swapping various parts in my harp amp build I accidently ran the amp for about 10 to 20 seconds without any speakers connected. Now the amp's volume is very low (hardly audible with volume on 10). Voltages are still correct throughout the amp. Even if I touch the plate lugs of the output tubes with my voltmeter to read the plate voltage there is only a very low crackle instead of the usual loud pop. I might have messed up something else but I can't find anything. Is there a way to check the tranny if it's dead or not?

thanks
BF

[Jazz P Bass]

Checking an output transformer can get pretty involved.
And still yield ambiguous results.
With the power supply off & drained of any residual voltage, check the dc resistance of the primary.
If it is shorted (low ohms) then it is toast.
The best results are achieved by subbing in another known good transformer.

[Bluefinger]

thanks! I'll try replacing it with my low power tweed twin tranny. it's 2x6L6, 4ohms as well so I guess it should work ...

btw: in the circuit the primary ready 97.2 ohms ... that sounds like toast ...

[MWJB]

"btw: in the circuit the primary ready 97.2 ohms ... that sounds like toast ..."

Measure from each end of the primary to the centre tap, if you have a significant mismatch it is toast, but on its own that 97ohms doesn't mean much...it's conceivable that a 50W OT could have a DCR of 100ohms accross the whole primary. You have changed the tubes?

[Bluefinger]

no, I haven't changed the tubes yet. I'll check that too ... thanks!

[R.G.]

no, I haven't changed the tubes yet. I'll check that too ... thanks!

In servicing tube amps, the first, first, first thing to do is to swap in known-good tubes. There is a reason they are in sockets.

Is there a way to check the tranny if it's dead or not?

There is a method that gets coverage for about 99% of faults. It's this:
1. Use your ohmmeter to check whether the windings are low resistance between the leads where they ought to be low resistance. Ignore trying to ferret out a shorted turn by one winding being lower resistance than it should be. You can't measure resistance to that precision with a typical meter, and halves of tapped windings are not perfectly the same resistance in typical transformers.
2. Use your ohmmeter to check whether the windings are high resistance where they should be - that is, not shorted between windings, not shorted to the core.
3. Use the magic shorted-turns connector - a battery and a neon bulb - to test for shorted turns. You ensure that all windings have open circuits outside the transformer, and then hook the neon bulb across any winding. You connect the battery across any winding, then remove it. If the bulb does not flash, there is a shorted turn. If it flashes, there is no shorted turn.

The only faults this process doesn't pick up is a dynamic one - a short that only happens when the transformer has high voltage on it or when it gets hot, or an open that only opens when it gets hot.

Subbing another OT is a better test for these last two conditions only, like replacing tubes is for general amp servicing.

[Bluefinger]

ok, resistance from center to either side of the tranny is 44 and 53 ohms ... almost exactly the same readings as on the perfectly fine tranny on my Tweed Twin.

tubes are fine ... they have been replaced with known good ones.

R.G. ... I am not quite sure if I understood everything you wrote but I'll reread it a couple of times

that pop from the fixed/kathode bias switch still is the same volume tho ... shouldn't that one be lower as well if the tranny is dead?

[txstrat]

Just a hunch... did you run the amp without a load and play it? Or did you just switch it on for 10-20 sec. with no input signal?
I've read somewhere that the chance of damaging an OT is significantly less with no input signal - if I remember correctly you might run the amp way longer than 10-20 sec. without damaging it - could the experts in this forum please verify -
I mean in that case the fault might be something else.

[Bluefinger]

it ran at idle ... then I played a note through the mic and as it stayed silent I immediately switched it off ...

somehow I have a feeling that there might some other issue but I'll know more as soon as I put in another tranny (which I won't fry hopefully)

[MWJB]

44 & 53 is quite a mismatch, that would concern me.

Do you have alternative speaker jacks (2/4/8ohm), or is your main speaker jack switched to ground with no load applied? This can save the day in this kind of situation, but needs a bit of thought to be employed with multiple taps.

[Bluefinger]

Yes, the readings are off quite a bit but my new Tweed Twin tranny has the same readings so I thought it was ok.

The trany does have a 2 and an 8 ohms tap as well but they are not connected. The speaker jack is not shortening when disconnected.

[MWJB]

Do you have a low voltage AC supply, like a signal generator or low voltage AC wall wart?

If you can apply a low voltage to the OT secondary, at the speaker jack, then you can measure the AC developed at each end of the primary with respect to the CT. This will tell you for sure if there is an issue with the OT. Voltage from each end of the OT primary to CT should be identical, voltage from one end of the primary to the other end should be in the region of x32 of the secondary voltage (which is why you only want a low voltage applied to the sec.). Measure voltage applied to sec. don't assume or guess.

What's right for one OT is right for that OT, not necessarily for "Similar" OTs. Some vintage style Fender OTs are interleaved, others (like tweed twin style) are straight wound.

[Bluefinger]

now that makes perfect sense!!! Ill try that ... Thanks!

[Jazz P Bass]

Please be aware that the low voltage test does not supply a definitive result.
I have seen OT's that test fine this way at low voltage but would absolutely not cooperate at high voltage.
The low voltage test is a real help when trying to identify the winding relationship between the primary & secondary (on a known good transformer)
I am not trying to "monkey wrench" the test.
I just felt that you need to be aware of the test limitations.
The "best test" is a swap.
John G.

[MWJB]

I'd test the OT (sub power tubes) before I swapped it, just because it takes less time & saves you potentially removing then having to refit the same OT.