1968 Fender Twin Reverb
This is a very rare AC568 circuit Twin Reverb. Only made for a few months in 1968 this Twin Reverb has an extremely odd mix of cathode and fixed bias with a power transformer putting out a significantly lower B+ voltage. This was not a great design and resulted in many returns and problems for Fender - they also just don’t sound as good as the other versions made before or after. The AC568 has a darker and more compressed tone that does not hold up well at high volumes. It by no means sounds bad as is, but it isn’t great either. After a 1/4 year of production Fender changed to the AA769 circuit that has the familiar fixed bias and is more of a continuation/evolution of the iconic AB763 circuit. Because of this and the issue of the original spec non-polar electrolytics used on the AC568 power tube cathodes not being produced anymore we decided to change this amp to basically be a AA769 circuit with the AB763 adjustable bias control. This resulted in a cleaner and clearer sounding amp with better response at high volumes. The customer wanted this as a “pedal-platform” type amp and modding it to these specs helped attain that vision. This amp will be gigged so all possible preventative maintenance was done to make it bullet-proof.
The amp functioned on arrival… but the Tremolo did not work due to the bypass caps failing and weak tubes. The amp was given a full restoration. All filter/bias/bypass caps were replaced with upgraded voltage and temperature rated MOD brand electrolytic’s. New 2 Watt Metal Oxide/5 Watt Cement 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. All plate resistors were replaced with 2 Watt Metal Oxide type units for preventative maintenance and reliability. All screen resistors were replaced with 3 Watt Cement 470 ohm type units for same reasoning. 1 ohm resistors were installed on pin 8 of the power tubes to provide an easy way to check the bias and was a convenient replacement for the cathode bias resistors previously in place.
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 Balance control that Fender moved to with many of their Silver-Panel amps only allows adjustment of how even the bias is between each pair of 6L6GC’s, it provides no way to adjust how hot or cold the bias is to the power tubes. This obviously is not a great design choice and is something I change right away anytime I see it present.
The original tubes did not test good and were all replaced: V1/V2 = JJ 12AX7MG, V3/V6 = EHX12AT7, V4 = TAD ECC83-cz, V5 = JJ 12AX7S, V7-V10 = JJ 6L6GC Matched Quad. These tubes were picked for best tone, reliability, and increased headroom. The 6L6GC's Fixed Bias was set to %60 Class AB Plate Dissipation with a B+ of 395VDC. The pots were sprayed out and the amp was cleaned inside and out. Amp now functions well - including the Tremolo.
