1975 Fender Twin Silverface 100w. push pull volume
STEVI...@msn.com
first 7025 tube, there are wires on both pin 9 and pin 5, then going
over to the next 7025 pin 9 and 5, then going to an adjustable pot
that is mounted on the back of the chasis. This pot burnt up when i
was playing, i quickly turned the amp off. My question is , why is
this on the amp ( humm canceling ? ), and do i even need it ? Or,
can I just take the wires on the 7025'S out. My 1965 Twin has none of
this wiring, or this pot . I checked prints on line ( Fender amp
field guide ), and in my own reference books, and cannot find this on
any schematic. Any help would be appreciated. Thank's
Don & Brenda Munkittrick
That is the hum balance control or artificial (adjustable center tap for the
6.3 VAC heater (Green wires from pilot light and power transformer). There
is a twisted pair of wires that runs from the pilot light to the all the
power tubes. Actually, pins 4 and 5 are tied together on the 9 pin sockets.
You can either buy a new hum balance pot, or, get rid of it and snip those
wires from it off at the tube socket being careful not to cut anything but
them. BUT....!!! then you have to create another artificial center tap
for the 6.3 VAC winding of the power transformer. (Filament or "heater" line
which is the twisted pair that runs from the pilot light to all tubes . )
To create this artificial center tap, simply connect 1 100 ohm resistor to
each pilot lamp terminal (2 total) and twist the other ends of each of those
resistors together and solder them to a grounding lug at the transformer
bolt and nut nearest the pilot lamp. Make sure you have a really good ground
( I put lug on transformer bolt, tighten nut, then solder the grounding lug
to the chassis and to the resistors twisted pair. It takes a big soldering
Iron to do that like a 100 Watt or 200 Watt. If you don't have one, Just
make sure the nut is tightened with a washer that "Bites" into the metal and
solder the two resistors to the lug. Remember, Each of the ends of the
resistors not twisted together goes 1 to each lug of the pilot lamp with the
filament line twisted pair and the two green transformer wires. You are
done!
The Filament (twisted pair to lugs 7 &2 on the 8 pin power tubes and pins
4-5 (tied together as one) and pin 9 on the 9 pin tubes should be of the
same polarity. That is to say that after the first 8 pin power tube (6L
GC in Fender), every time the wires are terminated at the next tube down the
line from it, the same pin number should correspond to the same wire. Pin 7
to pin 7 and then to pin 9 on preamp tubes (9 pin tubes). The other wire
should go from pin 2 on 8 pin tubes other pin 2 on 8 pin tubes and then to
pins 4-5 (tied together) on 9 pin tubes. That is keeping proper polarity and
cuts down on hum and noise. (don't criss cross wires)
In conclusion, You can buy a new Hum Balance control at:
http://angela.com/fenderamppotentiometerbycts100ohmhumbalance.aspx or,
http://www.darrenriley.com/shop/cart.php?m=product_detail&p=1344
Personally, I would do whatever one is easier for you. The 100 ohm resistors
are what most people convert back to. Just look at you twin reverb without
the hum balance control. Look at the pilot Lamp. Find the two 100 ohm
resistors and copy that.
Remember to snip off the wires from the Hum balance Pot (I would do it at
both ends....the wires from (and on) the pot at the pot end and the same
wires ( I believe they are black and white) from where each connects to the
7025 or (12ax7) if one is used there.. Do not cut any other wires so make
sure you cut a fraction away from the lug and filament wires. Leave the Hum
Pot in place as a chassis plug.
If you use a new Hum Balance control, don't touch the wires at the tube
socket. Just unslolder and resolder to new Hum pot and tighten Hum balance
pot to chassis securely. There is probably a red wire with a rasp washer on
the pot to the middle lug. Put it on the new pot the same way it was. It is
your ground.
Get in a quiet room, with a small screwdriver, adjust the pot to halfway
(equal on either side.) Turn on amp and let warm up. Turn on from standby.
Listen with volume slightly on to hum and adjust very slightly one way or
another for the quietest setting. Leave it there and enjoy playing it.
When connecting a new Hum Balance control, wire it exactly as it was. If
using the 100 ohm resistor method, same thing. Do not use both!!!!
Having said all this, Sometimes, a bad power transformer or other problem
can cause a hum balance control to burn out. They are quite fragile and
using the resistor method acts as a kind of fuse . If they burn out, there
is likely something wrong with you transformer.
On this link,
http://web.archive.org/web/20071017052309/www.schematicheaven.com/fenderamps/twin_reverb_sf_100_schem.pdf
you will find two schematics of a 100 watt master volume Fender Twin
reverb. At the bottom of the schematic, to the right of where it says Fender
Musical Instruments, you should see the words HUM BALANCE on the second
schematic and at the same place on the first schematic is a pair of 100 ohm
resistors. This is confusing at the actual amplifier because it does not
seem so logical. But, electrically, as you can see, the same result is being
met either way! One is adjustable, the other is not. Why, I assume it was
because of the crappy tube matching of the day and thus there was inherent
hum. Plus there was very bad wire dress in the newer amps (sloppy).
On this link is a ton of Amplifier schematics:
http://web.archive.org/web/20080822045126/www.schematicheaven.com/index_HTML.htm
Hope this helped,
Don Munkittrick
them...@epix.net
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Rich Giudice
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Rich Giudice
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