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Old 8th July 2004, 09:41 PM   #1
m0rg is offline m0rg  United Kingdom
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Default OT impedance problem

Hi,
I recently finished a fender champ like guitar amp, (6v6 output stage) using an el cheapo 5W 5K primary OT.
This all worked lovely for a few days until a few days ago, when some smoke escaped when I turned it on. I hit the power switch immedietely, so don't know exactly where the smoke was coming from, but it was around the PT. I checked the components around there and none seem blackened, and the diode's still work.
Now when I turn on the amp (there's no smoke) there doesn't seem to be a power problem. I've checked the supply as best I can (no oscilliscope) and its a steady 350V and it starts squealing if I take out any filter caps so they appear to be working.
But, the output is feeble (very low volume and distorted). I checked the bias on the power valve and its about half what it was (now 15V). so I'm guessing I'm not taking enough current through the valve (although the plate was 330V with no signal in)
A different power valve has the same problem (and the 2nd was brand new)
I've checked the OT, and it seems that the impedance has gone up!!
This was done with a fairly rubbish multimeter, but I got 1.0V on the 8ohm input, and 37V on the output, which by my maths is a impedance of over 10K?? with 0.5V on the input I get 17V so this seems to not be a meter problem.
I've tried wiring the speaker (8ohm) to the 16 ohm out to half the impedance and this made no difference. As far as I can tell the primary wiring is intact (it has a centre tap for some reason (but is definately single ended), and the output voltage of each half is exactly half the total)
I tried the preamp seperately and it seemed to work, so any ideas?
Thanks so much in advance!
Pete
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Old 8th July 2004, 10:31 PM   #2
SY is offline SY  United States
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I'm not quite following what you've been checking, so forgive me asking some dumb questions:

When you say "I got 1.0V on the 8ohm input, and 37V on the output", do you mean that you fed a 1 V signal into the 8 ohm secondary, then measured 37V across the primary?

Is the primary's DC resistance any different than original spec? Any leakage to the secondary or to ground?

When you say "I checked the bias on the power valve and its about half what it was (now 15V)", do you mean that there is 15 volts between cathode and grid or is this the voltage across a cathode resistor? If this is cathode-resistor biased, have you checked the resistor's value?
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Old 9th July 2004, 05:58 AM   #3
m0rg is offline m0rg  United Kingdom
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Sorry, badly worded post

I fed 1V into the 8 ohm secondry and got 37V accross the primary, yup.

I'll check there is no leakage of the primary to the secondry later, I don't know whether the DC resistance has changed. Its now 300Ohms.

The voltage accross the cathode resistor to ground is 15V. The resistor is defiantely 500ohms, so that means the current is 30mA.

Cheers,
Pete
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