'79 Marshall 2204 50 Watt Amp Repair
July 2020
Here is a vintage Marshall 2204 50 Watt head. The information on the data plate indicates it was built in 1979. The amp had been stored in a basement for a number of years, and was pretty dirty.
Marshall 2204 - Front
Marshall 2204 - Front Chassis Panel with Missing Knobs
Marshall 2204 - Back - Missing Back Cab Panel
Marshall 2204 - Back Chassis Panel
We were asked to check this out to see if it was worth trying to fix. The customer had plugged a guitar into one of the input jacks, and did not get any response. Also, initially, the output tubes showed signs of red plating (main part of the inside tube structure was glowing red - a sign of the bias not being properly set). While talking to them on the phone, they tested again, and this time got some sound. We found out why once we took a closer look at the inside.
We pulled the chassis to take a look, and it also looked to be in pretty good shape. There was no real sign of corrosion or over heating, other than some mildew on the bias pot. There were signs of what we initially thought were previous repairs, but these turned out be be some (somewhat sloppy) modifications. One involved disconnecting the 'Low' input jack completely. Another was a modification to the Cathode circuit of the first preamp stage, and another was a mod to how the Preamp volume control was wired.
Marshall 2204 - Input Jack Mod
Marshall 2204 - Preamp tube Cathode Mod
Marshall 2204 - Preamp Volume Mod
While the chassis was out of the cab, we cleaned the Tolex up with some mild soap and water and a good scrub brush, and then gave it a light coat of Armorall. We also removed the knobs from the chassis, and cleaned the chassis face. Once everything was fairly clean, we took a closer look at the mods, and found a schematic for this model amp. We pretty quickly decided that the best course of action was to try to get this back to as close to factory original as we could.
Using a schematic we found on-line, we undid the mods, and rewired with correct value components, including wiring the two input jacks up properly. We also discovered that this amp probably had different output tubes out of the factory. Apparently, this model originally used EL34 output tubes, but the ones that were being shipped to the US often arrived with the output tubes broken. As a result, the US distributors asked Marshall to use 6550 output tubes instead, as they were similar, but more robust. This involved a slight modification (at the factory) in order to be able to properly bias the 6550 tubes. This amp came in with EL34 tubes, but, according to the schematics, the bias resistor network looked to be setup for 6550 tubes. It appeared that someone swapped the original 6550 tubes for some EL34 tubes without modifying the bias circuit. We had already ordered a replacement set of JJ tubes, including EL34s for the output, so we ended up modifying the bias circuit so that we could properly bias the new set. This only involved changing out one resistor, so was not a big deal.
Marshall 2204 - Input Jack & Preamp Volume Back to Factory
Marshall 2204 - Preamp tube Cathode Back to Factory
Marshall 2204 - Original Bias Resistor
Marshall 2204 - New Bias Resistor
Once we had reversed the mods, we installed the new tubes, and set the bias on the new EL34s. We also replaced the power switch, as the pilot light built into the original was not working. We then reinstalled the chassis into the cab, installed a new set of knobs, and made a new back cab panel out of 3/8" plywood (forgot to take a picture of this).
Marshall 2204 - Back - Cleaned and With New Tubes Installed
Marshall 2204 - Front Panel - Cleaned and With New Knobs
Marshall 2204 - Front - Cleaned and Ready To Go
Both inputs are now working - the 'Low' input gives a quieter, cleaner sound, and the 'High' input breaks up as intended.