The customer wanted a general servicing including, replacement of all tubes, bias check, replacement of all four output sockets, foot pedal for the tremolo, and diagnosis of reverb intermittancy issue.
Tube complement is 2 x 12AT7, 4 x 12AX7 and 4 x 6L6GC.
Old output sockets.
At the bottom left hand corner you'll see the main bias supply resistor. The schematic clearly calls for a 3k3 value here. As you can tell this one is a 33k.
A value of 33k dropped the bias voltage to approximately -38VDC, at this voltage the lowest achievable idle current was well over 70% of maximum plate dissipation.
Seeing as this component looks possibly original, this amplifier would have been cooking output tubes since 1978.
Having the opportunity to select a new value I went with 2200 ohms. This gave me around -45VDC for an idle current of near 60% at the halfway point of the potentiometer's travel. 60% is considered average. FYI, B+ was 435V.
Old preamp tubes.
New preamp tubes.
Old output socket wiring.
New Sockets. Grid resistor, 470Ω 2W Metal Oxide. Bias feed resistors, 1k5Ω 1W 1% Metal Film.
One service tech had installed 220k filter cap bleed resistors.
Faux center tap. 100R, 1%.
1 Ohm resistor from cathode to ground to measure bias.
New bias feed lines. Teflon wiring.
New output tube socket wiring. Also tidied up the AC wiring. New grounded line cord. And took the unfused utility outlet and the ground select switch out of service.