1971 Fender Twin Reverb

There are many versions of the Twin Reverb that Fender made during the 1960’s and 1970’s. All of them were very popular with musicians then and now and consequently the Silver-Panel Fender Twin Reverb is the most common vintage amp that I get in. This 71’ Twin was made after the buyout of Fender by CBS in 1968 and is the last revision made before the addition of the Master Volume circuit - the merits of which are often debated and criticized one way or the other. There are a number of changes to this circuit from the iconic and coveted Black-Panel AB763 Twin Reverb that make this amp cleaner, and to many peoples ears, colder and harsher. The lowering of plate voltages in the preamp, increased filtering of the power supply, and the “improved” balancing of the Long Tail Pair Phase Inverter are largely responsible for the change in tone. These are great sounding amps however and with 100 watts of power and 2 x 12” speakers it will make sure you are heard. Notable users include: Alex Turner, Jack White, Allan Holdsworth, and Ty Segall.

This one came in having already been repaired many times with an absolute ton of work having been done to it. However the quality of about 75% work was terrible with non-original parts values, necessary preventative maintenance left undone, and the biggest offender being what was done to the cab. It looks like the baffle-board was replaced at some point with a pro-am made unit with both speakers being screwed in with wood screws directly to the MDF. This should have T-nuts and screw posts/locknuts like any other Fender - the use of wood screws like this is not ideal. This could not be replaced though because it was glued and Dado jointed into the cab - in other words… permanent. The speakers themselves were all wrong, they were Celestion G12T-75’s (wrong tonal style) with a total impedance of 8 ohms instead of the correct and necessary 4 ohm load the amp needs to see. Over time this would destroy the Output Transformer. The coupling cap replacement was actually okay and was left in place, but not much else. As well as a full restoration the customer also wanted this amp taken back to Black-Panel AB763 spec. It needed a cap job, new power/plate/misc. resistors, new octal tube sockets, preamp tubes, new speakers, lots of hardware replaced, and cleaning. The customer is going to use this with a Fender Rhodes piano for the most part and the restoration was focused on making it sound good for that application.

Amp was completely restored and modded to AB763 specs. All filter/bias/bypass caps were replaced with upgraded voltage and temperature rated MOD/Vishay brand electrolytic’s. New 2 Watt Metal Oxide power dropping resistors were installed for better reliability and lower noise floor. The cathode resistors for V3 and V6 were replaced with 2 Watt Metal Oxide type units due to heat damage and for preventative maintenance. All plate resistors were replaced with 2 Watt Reduced Mass Metal Film type units for preventative maintenance and reliability. All screen resistors were replaced with 3 Watt Cement 470 ohm type units for same reasoning. New Carbon Film 1/2 Watt resistors were installed for the bias and power amp section for the modification. 1 ohm 2 Watt Metal Oxide resistors were installed on pin 8 of the power tubes to provide an easy way to check the bias. A .001uf 630v Mallory brand film capacitor was used to mod the phase inverter to AB763 spec. A .01uf 630v Mallory film cap was used to mod the Trem circuit to stop it from ticking. All capacitor and resistor values were chosen to make amp compliant with AB763 values. The 3 Prong AC cord was re-installed and re-wired as the original work was shoddy.

The Bias Supply was given my standard modification to be an Adjustable Fixed Bias control rather then the original Balance control - this allows the power tubes to be set to idle at the correct current/voltage.

The customer wanted the speakers changed to a model that was more in line with what would have been in the amp originally but with high power ratings. Two new Weber Ceramic Chicago 12's, 80 Watt 12” Speakers (Light Dope @ 8 Ohm each) were installed to replace the previous Celestion units. These were installed with all new high quality wood screws often in larger sizes so that they would lock up properly in the MDF baffle board. To further ensure long term reliability, double the amount of screws were used with lock washers being installed as well.

The original tubes did not all test good and some were replaced. This was the final lineup: V1 = JJ E83CC, V2 = Tungsol 12AX7, V3 = EHX 12AT7, V4/V5 = JJ 12AX7S, V6 = JJ 12AT7, V7-V10 = JJ 6L6GC Matched Quad. These tubes were picked for best tone, and reliability. The 6L6GC's Fixed Bias was set to %62 Class AB Plate Dissipation with a B+ of 444VDC. The octal tube sockets were all replaced with high quality ceramic units and completely re-wired. This was done to stop the tubes from jiggling or falling out while they hang upside down and because of broken pins internally that were damaged by previous attempts at repair and old age. The preamp tube sockets were all treated with De-Oxit to remove/prevent corrosion. New chassis straps, with associated screws and nuts were installed as the originals were bent, rusted, and stripped. The pots were sprayed out and the amp was cleaned inside and out. This amp now sounds like something special, the Black-Panel conversion and new speakers elevated the tone to a different level that wasn’t there before.