Re: Fender Twin Reverb Wiring Help!!!

On 16 May 2006 12:20:12 -0700, Marshall Terry typed:

Hey Everyone - first time posting here.

This is for all of the wiring and amp workers.

I'm just getting into wiring and modding amps (sortof), and I've hit a
brick wall.

The background:

I acquired a 70's Twin Reverb w/ Master Volume that I have recently
'Blackfaced' - ie, rewired the bias circuit, removed a cap or two - ie,
reverting it to the AB763 circuit. I did the majority of the work, and
it worked fine.

Actually, the original pi filter setup of that bias ckt is prolly
better. Changing from "Balance" to "Level" is a plus; I usually add a
"Level" pot to the existing "Balance" ckt, tho.

I decided to further revert it to the AB763 circuit by removing this
rogue 'Hum Control' pot on the backside of it, which is not present in
the circuit.

This pot acts as a humbucker to quiet AC hum on the heater circuit,
and does a fine job. Putting it back in won't hurt. If you want to do
without it, you should put a 100 ohm resistor to ground on either side
of the heater winding. Fender usually did this at the pilot lamp (each
R to a gnd lug under the nearest PT nut and then chassis-soldered),
although it doesn't matter a whole lot where each resistor is
installed.

See:
http://home.adelphia.net/~bethndave/ch_gnd.jpg This lug used to be
under the nut and the original solder joint was cold & had cracked, so
I soldered it to the chassis (and every other soldered chassis gnd in
this amp).

I also did a slight change here and there, and here's
what's happening to it now:

The 'Normal' channels have a loud buzzing/hum when the volume knob is
turned up. The tone knob circuits affect the tonality of this hum. It,
however, operates at the normal volume range.

See above.

The 'Vibrato' channel runs clean, but at a very low volume in
comparison to the Normal channel.

Could be a number of things.

Got plate & cathode readings preamp sections? Careful in there; it's
apt to whack ya...

I'd start measuring resistors (when those old ones drift they go up in
value; sometimes a *lot*) With some resistors, you'll need to lift one
lead.

All the tubes are correct, and new, so I know that this is not the
problem.

I assume you've tried swapping them around...

I can link to some pictures of the amp if anyone can help me, or see
what is going wrong with it, if you can help.

Please do. I don't know about anyone else, but even if it doesn't help
solve your problem, I love to look at old amp pics, and try to shoot
everything I work on. I haven't counted, but I'm guessing I have a few
thousand or so pics on the shop computer. I hate when I get so busy I
forget to get at least a mug shot.



Thanks, all!

-mt

Hi Marshall! Nice to meet you.

HTH,
-DC
.

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