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Old 9th March 2012, 06:08 PM   #1
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Default Odd transformer wiring

Hello,

I have an old Hammond organ amp, AO-68 that I gutted and built a PP 6L6 guitar amp. The original power tubes were 7591 and the output transformer is what I am wondering about...

The secondaries are black, green and yellow. It appears that the stock setup had the yellow tap grounded and the other two going to the speaker socket.

Is this normal? When I first tried it out it was very distorted and low power. (when I bought it, it supposedly worked, and had weird, extra added wiring in places that was not recognizable)

I grounded the black wire and used the green one on my output jack. It sounds really good, and correct now. But I don't know what impedances are with the green and or yellow taps. Any ideas?

Also, anyone know if the yellow tap being grounded is normal or maybe an error?

Thanks,

Daniel
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Old 9th March 2012, 08:10 PM   #2
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I just tested the current, pin 3 to the center tap, and get really strange readings. 4mA on one and 95 on the other. I tried a different, known used set of 6L6's and got worse results.

I'm afraid to go further without any knowledge.

Please help...
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Old 9th March 2012, 08:24 PM   #3
Cassiel is offline Cassiel  Libya
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Quote:
Please help...
Well, yellow and green would normally be 8 and 4 ohms and black would be ground but I'm just guessing. What about the global feedback loop? Did it have any? As for the strange current readings....are the tubes fixed biased? A schematic would help.
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Old 9th March 2012, 08:48 PM   #4
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here is a schematic...

That's what I thought wrt to the colors. The power stage is the like the bassman, and the feedback is variable to adjust for amount you want back into the phase inverter. I have both fixed and cathode bias on a switch.

The voltages on the schematic are from another amp and was the targets. The only ones on that page that are correct are the ones on the power supply, plate and screen voltages.

In cathode bias it is really off, so I have only been trying to get it working in fixed so far.

**ignore the -50v in the bias part, it was a target also...
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Last edited by dscottguitars; 9th March 2012 at 08:53 PM.
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Old 9th March 2012, 08:54 PM   #5
Cassiel is offline Cassiel  Libya
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Quote:
In cathode bias it is really off, so I have only been trying to get it working in fixed so far.
Alarm, alarm!

Check the voltage at the output tube's grids.
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Last edited by Cassiel; 9th March 2012 at 09:02 PM.
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Old 9th March 2012, 09:01 PM   #6
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in fixed it is set to vary from -55 to -82, in cathode I'm not sure I checked that, there was 34v on the cathode if I remember.

The other amp I built, clone but only cathode biased, only had 420 volts on the plates. This one has 480. Not sure if that's part of it.

Last edited by dscottguitars; 9th March 2012 at 09:11 PM.
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Old 9th March 2012, 09:18 PM   #7
Cassiel is offline Cassiel  Libya
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Quote:
in cathode there was about 34
31 Watts plate dissipation for the 6L6 is too much. Increase the value of the cathode resistor. Maybe you cooked the tubes while in cathode bias, did you notice red plates?



EDIT Sorry, I gotta go. Good luck.
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Last edited by Cassiel; 9th March 2012 at 09:24 PM.
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Old 11th March 2012, 12:17 AM   #8
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Well I'm not sure what the problem was, but I do have the current running 'balanced' in the tubes now. With my adjustment pot I get from 51mA to 19mA which is exactly where I want it. The plate voltage is about 460v so that 51mA is right on the edge.

I increased the cathode resistor to 440 ohm and that works great. I have about 40v on the cathode pin.

The only thing that's strange is the speaker taps. The yellow seems to be 4 ohm. The amp really sounds bad, buzzes weirdly on the front of the picking when I'm using that one through an 8 ohm speaker. I switched to the green tap and the clarity is much better, louder and does not buzz.

Thanks for all the input.
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