My 1974 100W Silverface Fender Twin Reverb Tube Amp
By Paul Marossy, Guitarist and Engineer

Last updated 02/08/05    


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 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 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:


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!

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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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