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Post subject: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 11:07 am
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Aspiring Musician
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I've got a 64' Twin Reverb that was in need of help. I replaced all the electrolytic caps (13) in the entire amp with Sprague Atoms and the bias board resistors, replaced all the resistors on the sockets and checked all the voltages per the schematic. Everything is pretty close. I replaced all the tubes, new matched JJ 6L6GC's, some new Fender 12AX7's I had, and two NOS Jan Phillips 12AT7's. Tubes all tested good, sockets all cleaned and retentioned.

When I tried to bias this amp it wouldn't adjust above 29 and the plate voltage would drop to about 410 or so. By reducing the resistor on the bias pot and I can get it higher, but the plate voltage drops the higher the bias, bias-30, plate voltage-408, bias-35, plate voltage-396, etc., etc.

I thought about adding 1 ohm's to the sockets and reading it this way.

Any help/advice with this would be greatly appreciated, thank you.


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 12:38 pm
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Rock Star
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What and where are you measuring to get those numbers? Are you measuring the negative bias supply voltage at the junction of the two 220K resistors? The factory value there is -52 VDC. If you are decreasing the bias voltage (going less negative), you are increasing the cathode/plate current which will definitely cause the plate voltage to drop. Going from -52 VDC to -30 VDC may be setting the tubes way too hot. You need to get a good bias probe and actually measure the cathode current of each tube before you cause some damage.

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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 12:46 pm
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Aspiring Musician
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I actually am using a bias meter that measures the bias current and the plate voltage, Ik and Vp on the meter. It's an AMP-HEAD dual bias tester.


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 12:54 pm
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Rock Star
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Location: Natural Bridge, Virginia
I see. Then it looks like you have a serious power supply problem or at least a problem with the bias supply.

I would suggest pulling all the tubes and setting the bias supply voltage to -52 VDC. Measure all the open circuit power supply voltages. Then start installing the tubes one-by-one (starting with the preamp tubes) and measuring the power and bias supply voltages and you should see when the voltages really take a nose dive. Then troubleshoot power supply that feeds that tube and the circuit around the tube that caused the problem.

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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 1:11 pm
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Rock Star
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" I replaced all the electrolytic caps (13) in the entire amp with Sprague Atoms and the bias board resistors, replaced all the resistors on the sockets and checked all the voltages per the schematic. Everything is pretty close. "

After the re-cap job, you need to be getting much more than 410VDC on the plates of the 6L6GC. I'd go back and look at the polarity of the first two caps (whatever you replaced the 70mfd/350VDC caps with). The polarity should be with one cap positive up and the other one down.

You may also have bad 220k-ohm load-leveling resistors on those 70mfd/350VDC caps (these resistors sit behind the two caps).

Did you replace the diodes in the rectifier bridge? One or more maybe bad. Pull the board and test continuity across each --- both forward and backwards. If any is bad, replace ALL of them. I use good quality UF5408 for the TR and Dual Showmans.

After double checking all components in the power supply. Pull the 6L6GC and test the B+ at the standby switch. You should be getting a good 475-500VDC.


http://www.webphix.com/schematic%20heav ... _schem.pdf

Keep us informed! Good luck! :D :mrgreen: :D


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 1:20 pm
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Aspiring Musician
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Thanks much. I will use the posted advice and go to work on it. I did have 437 on the plates to begin with before adjusting the bias.


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 1:27 pm
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Rock Star
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Pull all tubes and measure the B+ at that junction point of the first filter cap section (where the 70/350 caps are attached). You need to getting close to 500VDC not 435VDC. Lower voltage on the B+ is an indication of wrong wiring, bad diodes, bad new caps (it happens), bad grounding point, bad 220k-ohm leveling resistors, bad choke, and/or bad resistors in the power rail (1k-ohm and 4700ohm resistor in rail).

Check all soldering under the doghouse. Esp on those 70/350 caps....

Good luck! :mrgreen:


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 1:41 pm
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Rock Star
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Photos of caps under the doghouse (note 220k-ohm resistor paralleling first stage filter caps) & rectifier diodes. HTH. Good luck!

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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 3:27 pm
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Aspiring Musician
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Hello men and women,

May I say that my face is red, very very red right now. Please don't laugh too hard at the following.

I ordered my replacement caps from a well known supplier. I won't mention any names but there initials are TD. (Ha!) I made the mistake of doing way too many projects at the same time. This supplier must have mistakenly sent me 8uf/450's instead of 80uf/450's. I neglected to look.
Guess what I did with the 8uf's? Ya......that's right...........I put em' in there!!

Wonder if this could be my bias problem? Hmmm.

Thanks for your help and if the under the hood picture hadn't been posted, I might not have second guessed myself. If nothing else this will prove yet one more time, don't assume anything!!

Thanks again for all of your willingness to help.


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 3:33 pm
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Aspiring Musician
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My apologies to TD. It was not them, it was NOS's in Arizona.


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 4:47 pm
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Aspiring Musician
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I've got some new Sprague Atom 100uf/450's. Would these work okay in place of the 80uf/450's ?

Thanks.


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 4:51 pm
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Rock Star
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Atom 100/450 will do just fine. The only time you really should use 80/450 (or some value close to 80mfd) is when you are running tube rectification (esp the GZ34/5AR4).

Tell us how they work out.


PS: Yes, 8mfd/450VDC will not work in this critical position for this amp.

Inadequate plate voltage could indicate an inadequate current source. Lower current draw through the 6L6GCs shows up as low idle bias values. The power supply not only delivers volts, but also current on demand.

:D


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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Sun Oct 27, 2013 6:16 pm
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I posted a reply to your original thread in the Fender Lounge, looks like you've found the problem anyway!

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Post subject: Re: 1964 Twin Reverb bias problem
Posted: Mon Oct 28, 2013 12:19 am
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Rock Star
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" Guess what I did with the 8uf's? Ya......that's right...........I put em' in there!! "

No Senior moment, at all. I've done many of these sort of things. I especially like the "I wonder what this is?" With the amp on. Only to get that wonderful "450VDC momento."

You were smart to realize an issue, when the amp would not bias up to snuff. If you didn't notice this & played the thing --- you'd get, at best --- the world's loudest 120Hz tone. At worst --- an epic FAIL under the doghouse.

Hope all is ok! :D :mrgreen: :D


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