Odd transformer wiring

dscottguitars · 9th March 2012, 06:08 PM

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

dscottguitars · 9th March 2012, 08:10 PM

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...

Cassiel · 9th March 2012, 08:24 PM

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.

dscottguitars · 08:53 PM

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...

Cassiel · 09:02 PM

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.

dscottguitars · 09:11 PM

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.

Cassiel · 09:24 PM

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.

dscottguitars · 11th March 2012, 12:17 AM

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.

9th March 2012, 06:08 PM	#1
dscottguitars diyAudio Member Join Date: Mar 2011	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

9th March 2012, 09:01 PM	#6
dscottguitars diyAudio Member Join Date: Mar 2011	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.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Odd audio wiring info needed, please?	thegrimsociety	Everything Else	5	27th February 2012 11:17 PM
Odd transformer behavior	badman	Everything Else	3	23rd August 2006 07:03 PM
Odd Gibson Wiring	audiopro	Instruments and Amps	8	19th March 2004 07:57 PM
I have an odd transformer issue (want to build the gainclone)	Jerry Parker	Chip Amps	9	22nd October 2003 12:37 AM
odd transformer design	godfather_cz	Parts	7	3rd May 2003 10:32 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

9th March 2012, 08:10 PM	#2
dscottguitars diyAudio Member Join Date: Mar 2011	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...

11th March 2012, 12:17 AM	#8
dscottguitars diyAudio Member Join Date: Mar 2011	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.