1969 Fender Bandmaster Reverb
The Bandmaster came in a number of different circuit designs during their manufacture in the 1960’s and 1970’s. The biggest changes can be relegated to either the “Black-Panel” or “Silver-Panel” categories - this refers to the cosmetics of the front panels as well as the circuits therein. The Black-Panel amps sported diode rectifiers, two channels (one with Vibrato and the other Normal) and did not have spring reverb. The Silver-Panel Bandmaster amps came with 5U4GB tube rectifiers and the vibrato channel gained the iconic Fender style spring reverb. To sum up the contrast: The Silver-Panel Bandmaster Reverb has less power and wattage, has a softer more compressed tone, and it overdrives sooner than the Black-Panel Bandmaster. The later model Silver-Panel amps ended up becoming cleaner and more powerful than any previous incarnations. This particular Drip-Edge Silver-Panel Bandmaster unusally has a GZ-34/5AR4 rectifier tube instead of the more common 5U4GB - It is even listed in the Tube Chart stapled to the inside of the cab. It was brought in for repair because it had popping/crackling noises and had weak tubes. The amp was previously restored by Brian Sours of Sour-Sound Amps up in my old hometown of Portland, OR and is the type of quality work I wish every tech was capable of. One of the few times I don’t have to fix the previous tech’s mistakes and tomfoolery.
The amp was given a partial restoration to complete the previous one from years earlier. As I mentioned, the competency of the older restoration (parts quality/technique) was actually good - but there was some preventative maintenance left undone. The actual symptoms however were caused by failing resistors in the Reverb and Phase Inverter circuits. All possible preventative maintenance was then completed. New 2 Watt Reduced Mass Metal Film/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 3 Watt Cement/2 Watt Metal Oxide type units due to heat damage. All plate resistors were replaced with 2 Watt Reduced Mass Metal Film type units for preventative maintenance and reliability.
The original preamp tubes did not test good and were all replaced with NOS stock that came in part from the customer. The Power tubes and Rectifier tested Good. This was the final lineup: V1/V2 = NOS Sylvania 12AX7, V3/V6 = NOS Phillips 12AT7WC, V4 /V5 = NOS RCA 12AX7A, V7-V8 = NOS RCA 6L6GC Matched Pair. These tubes were picked for best tone, and reliability. The 6L6GC's Fixed Bias was set to %55 Class AB Plate Dissipation with a B+ of 440VDC. The Death Capacitor was deleted from circuit for safety reasons, rendering the Ground Switch deactivated. The pots were sprayed out and the amp was cleaned inside and out. Amp now functions well.
