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Old 25th March 2006, 12:01 PM   #1
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Default Vox tubes glowing red hot

Hello,
I have an old vox ac50 powered by two el34 in a mixed fixed/cathode bias combination. the amp is very old and has been sitting unused for a long time. it powers up and emits sound but both of the power tubes are glowing red hot and there's no way to regulate current flowing in them. I tried replacing filter caps and checked coupling caps for leakage (there was none) but tubes are still glowing. Screen resistors and grid resistors appear to be ok. I tried a new set of tubes but to no avail. The 1500V cap in parallel with the diode rectifier blew releasing a black blob and I replaced it as well as one of the two cathode resistors (47 Ohm) that had burned up.
Any suggestion or troubleshooting technique?
Thank you

F
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Old 25th March 2006, 12:07 PM   #2
SY is offline SY  United States
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Cathode resistor/cap replaced?

Bias supply operation verified?
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Old 25th March 2006, 01:05 PM   #3
Trout is offline Trout  United States
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Quote:
I replaced it as well as one of the two cathode resistors (47 Ohm) that had burned up.
Do you know what year your AC50 is?

Looking at schematic heavens drawings, There were a couple different versions/schematics and the output stages are not all identical.

You might Want to check the drawings At this site To confirm the version before going to far.

Trout
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Old 26th March 2006, 11:27 PM   #4
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To add (if unknown to you): The EL34 under some 440V h.t. requires in the order of 35V bias. As SY said, this is probably your problem. Hope you have a 10 meg DVM to measure with. Also check grid voltages, and if necessary the relevant coupling caps and grid resistors.

Lastly, feedback stability if you have feedback - oscillation there could overdrive power tubes. But that is not a common cause of your problem.
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Old 30th March 2006, 03:10 PM   #5
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it is a Vox AC50 from the early 60's. the schematics ofthe amp is not on the web. It's similar to the one posted in schematics heaven but with some minor differences. It has a solid state rectifier no bright switch and no neon bulb.
I checked all the coupling caps. all the filter caps, screen and grid resistors but I cannot get the right bias voltage anyway...
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Old 30th March 2006, 03:13 PM   #6
anatech is offline anatech  Canada
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Hi Franconio,
How did you test the coupling caps? Many will look fine on an ohmmeter and still be defective. Just replace the coupling caps.

-Chris

Edit: did you replace C24 and C25? How about C22 and C23?
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Old 1st April 2006, 02:46 PM   #7
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I tested the coupling caps with a device I built. i found it in one of Gerald Weber tube amp books...
I did not replace the small electrolitycs you refer to. I'll try that.
Thank you

F
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Old 1st April 2006, 02:58 PM   #8
anatech is offline anatech  Canada
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Hi Franconio,
I don't always trust testing caps working at higher voltages. Besides, you would have to lift one lead anyway, the caps are cheap. Why not then just replace them in this situation?

Another good way to quickly test them is to pull the outputs and run the amp up to 75~80% of it's line voltage. Watch the B+ voltage so you don't go over the rating of the filter caps. Measure the grid pins of all the tubes. There should be zero leakage (or very, very close to it). Any positive voltage on a grid referenced to ground indicates a bad coupling cap. This way you do not need to desolder anything.

-Chris
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Old 2nd April 2006, 10:42 PM   #9
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Hi Franconio,

I am worried by your statement in your post #5 that you cannot get the bias right. What exactly do you mean (what do you get)?

I had another look at the circuits in the reference given by Trout (post #3). The circuit under "AC50 - 1976" gives the clearest values and does not appear to be different from the "AC50 - 1965". If this is your circuit (at least as far as the fixed bias is concerned), do you get -200V where the shunt rectifier is connected, then about -124V between the two 33K resistors and finally -50V at the pots? If not you will have to begin with the electrolytic caps there. (I presume by this time you would have checked the coupling caps, also in the way that Chris indicated). By the way, the shunt diode in the circuit called "AC50 Power Amp" is drawn the wrong way round.

In my opinion the caps there are far too low, but you cannot change the first 8uF as a much higher value would give a lower bias, since it is serie-capacitor derived. For the second one (over the pots) I would rather have a few 100uF (it is half-wave rectification). Still, these are only suggestions and you should first get the indicated voltages with the existing setup.

Regards.
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Old 2nd April 2006, 10:47 PM   #10
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Sorry Franconio, my memory is getting bad at my age. I see that where I mentioned 8uF it should actually be 16uF (for both bias capacitors).

Regards.
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