How it all
started....
For all you Fender
Amp Aficianados out there, my amp is a 100watt Twin Reverb,
Silverface with Master Volume.
Some purists wouldn't be too excited about that, but this amp
sounds sweet.
This is the story of how I acquired my Fender Twin Reverb amp: I
have a friend who had this ailing amp. As a favor, I offered to
try to fix it. I thought maybe it was a loose wire or something.
I didn't know what I was getting into. It had some problems. The
main one was that it made an unwelcome, intermittent popping
noise. The reverb also didn't work, nor did the vibrato. And it
seemed that the tubes were getting excessively hot. So I took it
home with me and did much searching on the internet. I found two
good sites that really helped me to isolate the problem. (See
links below). One was the "Fender Amp Field Guide",
found at www.ampwares.com. This is a good source for schematics
and information on dating vintage fender amps. In fact, they have
taken on the task of compiling information on how to date Fender
amps by serial number. I sent them the information on this amp,
and they told me it was manufactured in 1974. That made it a
vintage amp. So I thought that was cool.
Another good site is www.geofex.com. There is a tube amp
debugging section there which was very helpful. So armed with this information, I decided I could repair it. I
obtained the right schematics for it, and started to eliminate
the possibilities one by one. The schematics for this particular
amp are: AA270 Chassis Layout, which doesn't show the master
volume circuit, and the 100W Master Volume Schematic. You have to
read between the lines a little bit, too. I wasn't too
intimidated, since I am a mechanical engineer, I have done this
sort of thing a little bit before. But please take this suggestion: IF YOU DON'T KNOW HOW TO READ
THE SCHEMATICS OR WORK ON HIGH VOLTAGE EQUIPMENT (400+ VOLTS DC),
DO NOT ATTEMPT TO MAKE REPAIRS. EVEN UNPLUGGED, THERE CAN BE
STORED ENERGY IN THE CAPACITORS, WHICH COULD BE LETHAL IF
DISCHARGED INTO YOUR BODY. IF YOU DO NOT KNOW HOW TO SAFELY
DISCHARGE THEM, DO NOT TOUCH ANYTHING INSIDE THE AMP.
So... I started with the tubes and tube sockets. I cleaned all the
pins and sockets, and that seemed to help a little, but the
popping noise was still there. I also checked the tubes to see if
they were all working properly. I also adjusted the bias, as the
tubes were glowing like red-hot lava inside. So I knew that they
were underbiased. That also helped the amp be more stable. By the
process of elimination, I determined that there was a problem
with the vibrato circuit. I decided to change all the components
on the tube side of the circuit. I replaced the 100k and 10M
resistors, the 600V .022 uF capacitor (which literally blew off
one end of itself) and the "opto-isolator". The
opto-isolator is a photo-resistor. It is composed of a neon bulb
on one side which flashes faster and brighter with the adjustment
of the speed and intensity controls. The other side of it has
kind a screen resistor which varies the amount of current flowing
through it according to the intensity of the light it is exposed
to. Kind of cool technology for 1963, when the amp first hit the
streets. Actually, the vibrato circuit had been around since
1955! I dismantled the reverb tank and found a broken wire. A
little solder fixed that. I checked the integrity of every solder
joint in the amp. I resoldered one wire on the standby switch,
which made a noticable improvement, even with the bad vibrato
circuit. After replacing all these items, the amp sounded real
nice compared to before. The more I worked on it, the more I liked the amp. I asked my
friend if he would trade me for a nice six-string bass, and he
took me up on the offer. So we traded and the amp became mine in
June, 2001.
Finishing
Touches:
I had one small
problem after I was "done". I had a ticking coming
through the speakers from the vibrato. So I installed a .022uF
capacitor in parallel with 10M resistor at the optoisolator. This
acts as filter. Now the ticking is gone. I also decided to
replace the power tubes with some Svetlana 6L6GC's. They are new
manufacture, and are supposed to be the closest to the way the
old Sylvania's sounded. I personally can't see paying very high
prices to get some NOS (new old stock) tubes when you can get
some nice new ones for a fraction of the cost. Anyway, the power
tubes that were in my amp were Sovtek 5881's, which sounded ok,
but the Svetlana's really spiced things up. It has such a great
tone now. It really opened up the sound and gave it more life.
Don't put 5881's in your Twin Reverb, please! They sound very
flat compared to the Svetlana's. The power transformer still
seemed to be getting a little too hot, though. So I decided to
replace the power transformer AND the power filter capacitors. I
was a little bit hesitant to replace the filter caps, but I
decided in the long run, it is the best thing to do. It would be
a real drag to have things get fried if one of them blew. Since
the amp is 27 years old, I decided to just do it. That cleaned up
the tone even more. This amp is GREAT for clean, jazzy tones.
Just love playing through this amp!
Like Murphy's second minor law says: "Anything you plan will
take longer and cost you more than you thought." This
project took far more time and effort than I first thought it
would need (I even made my first web site about it), but I am
intimately acquainted with this amp now, and I learned about
every nook and cranny while I was tightening everything that was
loose and troubleshooting it and cleaning it.
In my opinion, the Fender Twin Reverb is one of THE classic tube
amplifiers. I would also like to extend a special thanks to Doug Hoffman at Hoffman Amps for the
replacement parts! Thanks for visiting. If you have any questions, comments or corrections, please email me at: diyguitarist@embarqmail.com
LINKS:
Please be patient
while the pictures download...
To see pictures of my project
A REAL Restoration
Project
Dimensioned Twin Reverb
Speaker Cabinet Drawings
Master Volume Chassis
Layout
Replacing my filter caps
Adding a Power Tube Bias Adjustment Pot
Installing a Cooling Fan
How I get killer tone
from a vintage amp
Check out my Hot Rod DeVille
Check out my Seymour Duncan Convertible
Building a Ventilated Amp Stand
Building a Tube Cooler
Dating Fender amps and
schematics
Tube amp troubleshooting
Fender amp replacement
parts
How vacuum tubes work
Replacement tubes
Differences between
Silverface and Blackface
1000 Great Guitar Sites!
Your Host
Schematics:
I have THE
cleanest schematics that you will be able to find for the AA769 /
AA270 circuits. Period.
These are the cleanest schematics that you will find, guaranteed.
These are high quality images, so please be patient while they
download. For best results, save them to your hard drive for faster
viewing. Adobe Photoshop or Printkey 2000 work very well for viewing
the images.
AA769 Schematic
AA769 Chassis Layout
AA270 Schematic
AA270 Chassis Layout
100W Master Volume Schematic
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