1968 Fender Bassman Service +Lar-Mar Master Volume

Here’s a rundown of the service/upgrades this amp got:

-Main filter caps replaced with F&T brand capacitors: 2x 80uF, 4x 22uF
-Power voltage dropping resistors changed from metal film to metal oxide
-Cathode bypass capacitors changed to Sprague Atom 25uF 50V
-Bias capacitor changed to Vishay 47uF 100V
-New bias 1N4007 diode
-New metal oxide 470 ohm 2W bias resistor
-New carbon film 15K resistor attached to bias adjustment pot (less chance of drift and the old carbon composite had drifted to above 18K)
-New carbon comp 1.5K cathode resistors (every where a bypass capacitor was upgraded)
-New carbon comp 5% 100K plate resistors
-New 100K tone slope resistor on normal channel
-New 2W Metal Film 2x 220K and 1x 470K mixing resistors (to reduce self white noise)
-Some preamp tube sockets were dirty so I cleaned those and retentioned a loose one
-Changed faulty/worn-out bright and deep switches
-Added 1M carbon comp resistor to help prevent deep switch thump
-Removed old 220K resistors in prep for new Master Volume
-Re-wired 120 VAC mains wiring to switch hot before going to the fuse
-Removed old unused ground switch
-Added 250KA dual gang potentiometer and new wiring for Lar-Mar (Type 2) Master Volume (in place of the ground switch)
-Replaced the two 0.022uF capacitors connected to the phase inverter plates with SoZo 500V capacitors for longevity protection for power tubes
-Replaced one of the 0.01uF ceramic disc capacitors that was microphonic with an Orange Drop 6PS capacitor
-Biased the output tubes to 60% plate dissipation (matches previous bias)

One of the tricky parts of this service was tracking down some pretty strong self-noise / white noise. Sounded like a non-tuned FM radio. The main fix was switching the mixing resistors on the V2 tube to 2W Metal Film resistors (2W because that’s what I happened to have — they are the three blue ones on the board).

A couple other things would make the white noise be even better and that would be to switch the 100K plate resistors to 1W / 2W Metal Film resistors and also refresh a couple of the 7025 preamp tubes that are low-noise and non-microphonic.