I know I don’t publish on this site any more. I don’t really have plans to do so again. But I just finished a major project and I wanted this information to be available to the world. If you want to contact me you’re best off sending an e mail to: benito shavings (no space) via hotmail because I don’t remember to check the comments most of the time. Now on to the good stuff!
I own a 1970 Fender Twin Reverb and it was in need of some significant maintenance. Once I considered doing it I got bit by the amp-mod bug and here’s what happened.
My amp was built in 1970, probably early in the year judging from the parts. The control pots are dated 1969 and the speakers are Oxford speakers built in April of 1970. The tube chart labels the amp with the “AB763” schematic, but this is obviously incorrect. AB763 was the famous pre-CBS “Blackface” design. The tube callouts are identical for this model, though.
When CBS purchased Fender in 1965, the Twin Reverb design that had been used since 1963 did not change at first. The AB763 circuit design was retained through 1967 (when the Silverface cosmetics were first introduced) and through 1968 (by 1968 the Blackface cosmetics were completely replaced by the Silverface cosmetics). In 1969 the Twin received its first “post-CBS” redesign and it was a significant one. This design, seen in Schematic AA769, most notably changed the operational voltages of the tube plates and removed the adjustable bias in favor of a fixed bias with adjustable tube balance. With all circuit changes in 1969, the amp power output went from around 85W to around 100W. When 1970 hit, more design changes were introduced, as shown in Schematic AA270.
My amp was built using the AA270 design. I purchased it circa 1998 and it was completely stock (including the original RCA tubes). Over the years it has aged and was in need of an overhaul. The preamp tubes were the first things to go, they had become terribly microphonic. I replaced all the preamp tubes: The 12AT7 tubes were replaced with NOS RCA and GE 12AT7 tubes (thank you TheTubeStore.com!) and the 7025 tubes were replaces with new Svetlana 12AX7s and the vibrato 12AX7 was replaced with a new Electro Harmonix. The power tubes are all still the original RCA 6L6s.
During the overhaul, I decided to re-cap the amp since the existing capacitors were over 40 years old and some had deteriorated. Once I had committed to replacing the caps, I explored the idea of converting to the classic Blackface circuit design. The changes were relatively simple in the amp models before the Master Volume knob was introduced in 1972.
In order to make the conversion, a plan had to be developed. The first step: get copies of the electrical schematic and layout of the Blackface Twin Reverb (AB763) and the same drawings for the model of amp I was working on, my AA270. These are available online in Ampwares’ Fender Amp Field Guide. They are also published in different books. I used Aspen Pittman’s “The Tube Amp Book”. I believe they are also in Gerald Weber’s “A Desktop Reference to Hip Vintage Guitar Amps”.
To begin, I just studied the AB763 and AA270 schematics and noted all the differences that were made to get the AA270. Theoretically, you simply need to undo all those changes, and you are back to a Blackface. In Chapter 2 of Weber’s “Desktop Reference” he also has a section called “Silver to Blackface Conversion for Twin Reverb”. That was very helpful, but I thought it was incomplete in a few details, especially the bias circuit modifications, so I refined it developed my own plan.
Blackface Schematic AB763: http://www.ampwares.com/schematics/twin_reverb_ab763.pdf
My Silverface Schematic AA270: http://www.ampwares.com/schematics/twin_reverb_aa270.pdf
The cool thing with the Ampwares copy of the AB763 drawings is they includes the layout; something that is not in Aspen Pittman’s book. Notice in the bias circuit that a 3.3k resistor and a 50uF cap were added to the AA270 design. Weber’s Blackface conversion instructions retained those components, I’m not sure why. But they weren’t there in the Blackface design and, since both models used the same power transformer, I had no qualms about removing them.
Apart from installing new preamp tubes, the only other impact of the overhaul was all six original 25uF/25V capacitors in the preamp section were replaced with new 22uF/25V, because of the unavailability of 25uF.
In the vibrato circuit the AB763 had a 25uF capacitor on one of the cathode grounding connections. This was changed to a 5uF capacitor in 1969 to shunt fewer high frequencies to ground (i.e. allow more of the “lower” high frequencies to stay in the sonic circuitry). This was done because some thought the Blackface vibrato sounded muddy, I think. I like the sound of the 5uF cap, so I replaced it, but didn’t change it to a 25uF. Also in 1969 there was a 0.002uF capacitor added to the ground path on the reverb unit output. In a full Blackface conversion it should be removed (Weber calls for the removal in his Blackface conversion instructions). I tried the amp both ways, with it removed and with it in place. I preferred the sound of leaving it in place. The reverb seemed to have more punch and presence. So I didn’t do that part of the Blackface conversion either.
In the end the Blackface conversion only entailed changing 11 resistors (and removing one more), replacing one capacitor (and removing four others), then some minor rewiring around the bias adjustment pot (see pictures below). Replacing the old capacitors was more work than the conversion.
The electrolytic capacitors were replaced with new Xicon caps. My amp repair guru (Doyle from Doyle’s Broadway Music in Boise, ID) has had Sprague caps die on him a lot so he doesn’t recommend them. I got the Xicons from him for less than you can buy Sprague caps online.
If you attempt to do something similar to your amp, I must offer the ubiquitous warnings: THERE ARE VOLTAGES IN TUBE AMPS THAT CAN KILL YOU. Power tube plate voltages can be between 500 and 600 volts DC. This charge can stay in the amp for a very long time after unplugging it. If you at all doubt your ability to use a voltmeter and safely discharge capacitors – DO NOT WORK ON YOUR OWN AMP.
If you do feel comfortable with those things, use caution and be safe. Your voltmeter is your friend. If you do do the work, here are some tips for capacitors. Old ceramic caps were polarized. Modern ceramic caps are not. So you don’t have to worry about installing modern ceramic caps in backwards. However ALL ELECTROLYTIC CAP ARE POLARITY SENSITIVE. Pay attention to how you remove the old caps and install the new ones. If you get the new one in backwards it will likely explode the first time you turn on the power. The old Malory caps usually had a “+” symbol on one side. My new Xicon caps have arrows and a “-” symbol to indicate the negative side (see my filter cap pictures below).
Also be aware, you may have some parts in your amp that don’t match the spec callouts, or things may not be laid out as shown in the layout drawing. Just pay attention to what you see and if something doesn’t look right, it may not be. These amps weren’t perfect.
Interestingly, since the AA769 design you could use somewhat mismatched tube pairs in the “push” and “pull” circuits of the power amp and use the balance pot to compensate for the mismatch. With the conversion to the AB763 circuit, that feature is removed in favor of an adjustable bias. Therefore it is most desirable to use a matched quad set of tubes in this amp. If that is not possible, two matched pairs can be used. One matched pair should go in sockets V7 and V10 (outside sockets) and one pair should go in V8 and V9 (inside sockets). The stock RCA 6L6 tubes in my amp were not quite a matched quad, so they have been installed in this second fashion.
The end result of all this work? This amp used to sound (what I thought was) amazing. Now it sounds out of this world!
Perhaps now it is not as “collectible” as a bone-stock 1970 Twin Reverb amplifier would be. But how collectible was it before I got it? The Silverface Twin Reverb amps from 1967 and 1968, which contained the original Blackface design, are not nearly as collectable as the 1967 Twin Reverb with Blackface cosmetics. Even at its best, this 1970 amp was only second banana (on a good day) to the Blackfaces in a collector’s eyes. That’s being generous because the 1969, 1968 and 1967 Silverfaces are all more collectable than the 1970.
But in the end, the guitar amp is both tool and it is an instrument. To sacrifice its function – its soul – for the sake of an abstract concept like collectability is to do a great disservice to what made this amp collectible in the first place; it sounds absolutely beautiful. The modifications and maintenance we do over the life of the amp is what keeps that beauty alive.
Have fun out there and go make some noise!