This is the summary of all the fixes in the previous posts. These old Fender amps aren't for the faint-hearted!
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Most expensive issue was a failed power tube socket. I didn’t realize it had failed, and after many other repairs I installed a set of new vintage-spec output tubes – one of which was ultimately destroyed by an intermittent break in one of the sockets (the socket pin had broken out of the back of the socket, and the heater was only intermittently connected, and arcing). The remaining functioning output tube is useless in this amp – they should be a matched pair. And when the output tube failed, it took out the rectifier tube. $150 worth of tubes down the drain… Ordered and installed a second set of vintage spec tubes (blah!).
Ultimately, I think this was the reason the amp is so clean – it must have been blowing power tubes for years, and you can’t sustain replacing them all the time. When I got the amp it came with mis-matched power tubes that were wildly different, and one of them was blown. I should have taken more notice of the blown one (I thought it was just age/unskilled previous technician).
Also…
Upgraded the 110V power:
- Replaced the 2-prong, non-grounded power cord – standard for the era, but lethal when things go wrong.
- Changed the cable entry to be a commercial cable entry gland and strain relief.
- Removed the "death cap" and associated switch – this is part of the non-grounded arrangement.
- Changed the fuse holder to a modern safe holder.
- Removed the ungrounded "service outlet" and blanked off the hole with aluminum (yes the spelling is wrong, but I bought the metal in Canada, so I guess it isn’t aluminium).
- Replaced the “standby” switch – it was arcing. A common problem because the switch is rated for 250VAC and it is switching 410VDC.
Replaced the octal tube sockets:
- Replaced the three octal sockets with Belden vintage spec sockets.
- Replaced the out-of-spec screen and grid resistors – grid resistors should be 470 Ohms, and one was 740 Ohms.
- Replaced burned cloth-covered wires in the heater supply circuit (from the failed socket) with period correct wires.
Replaced capacitors and associated components:
- Replaced the 10k dropper resistors in the doghouse (capacitor bank) – the main capacitors had been changed in 2015, so didn’t need to replace them.
- Replaced all the 25uF electrolytics. They’d also been done in 2015, but the tech had burned the plastic coating – crap soldering work. Managed to source some very nice vintage spec Sprague caps.
- Added a 0.0022uF capacitor across the 220k resistor in the reverb recovery circuit to remove noise (this is a standard problem and fix for these amps).
Bias circuit:
- Adjusted the resistance in the bias circuit – the original bias setup was running the tubes too cold.
- Added an additional filter capacitor in the bias circuit to remove hum (this is a standard problem and fix for these amps).
Vibrato did not work - it had three problems:
- One of the 100k resistors (on the back of the speed pot) had a dry joint.
- The 0.02uF capacitor was out of spec - 0.027uF.
- The optocoupler assembly (the "roach") had failed.
Reverb:
- Replaced the weak 12AT7 driver tube.
- Replaced the 12AX7 reverb recovery tube – swapped a few in to that position until I found the lowest noise one.
- Repaired the reverb tank – one of the “carrier” springs that isolate the tank from the amp cabinet had broken (in about 1965 from the look of the rubbing damage) and the reverb tank was rubbing directly on the wood cabinet, causing massive feedback in the reverb circuit.
Mechanical:
- Straightened the chassis behind the front panel (had been hit on the vibrato controls).
- Used heat to reform one of the knobs that had been hit.
- Fixed a ding in the speaker baffle.
- Fixed the “battens” that hold the front speaker baffle – they were not properly glued in the factory and had come loose. Cabinet rattle was Next Level. Cleaned the surfaces, drilled 3/8" (10mm) holes with a depth-controlled Forstner bit. Glued and doweled. They will never move again...
Miscellaneous:
- Changed the 0.047µF cap in the vibrato channel tone stack to a 0.022µF Orange Drop. That channel is now the same as the Super reverb amp from that era.
- Cleaned and lubricated every potentiometer and jack.
- Scrubbed the tolex and cleaned all the dust and debris out of the cabinet. Reglued a few lifting edges of the tolex.
The footswitch:
- It has 2 switches and neither would switch reliably. They are standard Carling ($$) SPST ON-OFF footswitches so replaced them with new switches.
- I've made a temporary footswitch using regular pedal stomp switches, so I can put the fragile 1964 footswitch away for safekeeping.
