Re: Fender Super Six / Distortion / troubleshooting

"Phil S." <psymonds_no_spam@xxxxxxxxxxx> wrote in message
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"Lydmuren" <x@xxxxx> wrote in message
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Hi Steve,
Is there a sure fire way to test an output transformer?

Greg

I'm not Steve, who, BTW, is a good fella and knows a lot.

To test a transformer with just a DMM, the first thing is to put the ohm
meter on it and see if all the windings have continuity. If that passes,
then run low AC voltage into the speaker side. (I think this amp has a 4
ohm speaker load into 4 6L6, which would be 3300 ohms? You check the
facts on this to be sure and I'll use it as an example. So, please don't
hold me accountable on the ohms.) If the output tranformer is 3300 ohms
on the primary and 4 ohms on the secondary, the turns ratio is about
28.7:1. You should see about 181 volts come out on the primary side with
the 6.3v applied to the secondary. If you have continuity and the
approximate predicted voltage, chances are that the OT is OK.

To do the test, pull all the tubes and disconnect the speaker. Steal the
6.3v from the filament winding. Hook your meter onto an output tube
socket plate pin (pin 3) on one "side" and the other lead to the other
side. If you have some other source of AC that's good, too.

Transformer math:

[Reflected impendance of the output tubes (3300 ohms in this example)] /
[square of the turns ratio (28.7^2 = ~824)] = the appropriate speaker
impendance (3300/824=4).

But everyone else says do the cap job first. I'll vote with them. It is
likely to make the test unncessary.

Thanks for the answer(s).
Let me start with saying that I checked the lytics using the method
described in 'servicing your own tube amp'. Remove the lytics and apply a 9v
batt and the DMM and notice how fast the charge goes down. Only one of the
caps discharged significantly faster than the others but not as fast the
book suggests for a bad one (1 sec) - but I replaced it. No change in the
amp behaviour - still distortion.

At this point I should tell that I let the amp run with an mp3 player as
input (just turned up a little bit not to overdrive the input). I cranked
the master volume all the way up. The amp ran for about 3-4 minutes and then
both fuses blew. Again, all the voltages at the test points lined out in the
schematics are OK.

On the OT...using the method described above by Phil I removed all tubes,
unsoldered the speaker side of the OT and applied the filament voltage to
the same side.
I measured 41 volts across the amp side and 17v between the mid point of the
OT to the side points.
And then one of the patch wires from the 6.3v to the OT burned (they were
pretty thin).
Not the 180v that Phil described. Is that enough to conclude the OT is bad?

Thanks,
Greg


.

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