Marshall Master 50W Mk2 Lead Rebuild

In August 2023 I was asked to restore a Marshall Master 50W Mk2 Lead back to its original specifications. The unit had beed modified quite a lot and had been purchased ‘as is’ with no cabinet. The new owner bought a new cabinet with a white tolex finish which I was given as part of the rebuild.

The chassis I received had a lot of modifications including: an extra tube for the preamp, fitted on the side of the chassis, the voltage selector had been replaced by a power outlet, an extra power switch had been added, and two extra input jacks had been added. You can see the original state of the chassis in these pictures.

In addition to doing the circuit restoration, the owner wanted the original electrolytic capacitors replaced and the 6550 power tubes replaced with EL34s. The power tube replacement would necessitate a small modification to the bias circuit.

When I examined the chassis I found that the holes for the extra input jacks and power switch were in the original steel chassis, just not part of the MK2 Lead front panel. Whoever modified it just cut openings in the panel to install the new parts.

The metal work for all the modifications was done quite nicely. I cannot say the same for the wiring changes. Take a look at the pictures below.

The soldering of the power wiring was pretty bad. One of the wires had fallen off the power switch. The power transformer wires that had been cut off from the old power selector switch were sleeved badly. I find this sort of work quite often in older amps. I also found the choice of power wire colors interesting – red and yellow. I assumed it was wire that happened to be available. Being an ex-Brit I’m used to the blue and brown EU colors in these amps, but usually install the black and white US power wire colors when I work on them.

As part of the initial work for this project I found a schematic for the amp based on the model description and the PCB issue number.

This amp was likely built in 1977, the year I graduated with my electronics degree in the UK. The summer before graduation I had a job at Daly Condensers in Weymouth, UK where I lived. I did assorted grunt jobs there for the summer including making parts for, and testing electrolytic capacitors exactly like the ones found in this amp. Based on the date stamps, these were made in 1975.

In order to restore the amp to its former configuration, and to replace the capacitors I knew I’d have to have access to the back of the PCB. For a normal re-cap job I’d just undo all the front panel jacks and pots and lift the PCB up enough to get at the capacitor under it. You can see this capacitor and the underside of the PCB below.

The first step was to remove all the components attached to the extra tube socket and the extra pot that was on the back wall of the chassis.

Once the old parts were out of the way I photographed the wire connections to the tube sockets for V1, V2 and V3. I also looked online until I found what looked like a picture of an unmodified amp. I wanted to get an idea of how the original was wired. You can do this form the schematic of course, but Marshall built several amps from the same PCB so there are extra holes for wires and components which do not get used in specific configurations. Compare the picture above with the example of an unmodified amp that I found below.

As you can see there are a few differences (ignoring the bits I’d removed).

Having photographed the wiring, I disconnected the PCB form the tube sockets so that I could get under the PCB and start the re-cap job.

When I work on old amps like these I always find traces of solder in the chassis along with spiders, dead insects, and sometimes signs of corrosion when the amp has been stored in a damp place (as here in Florida). I take a paint brush with stiff bristles and clean out as much as possible. While doing this I also often come across wires with melted coverings where someone was a bit inaccurate with a soldering iron. If that has exposed the copper wire I replace the wire.

When I examined the underside of the PCB I found a trace that had been cut and a wire soldered onto the back of the PCB. Evidently this was part of the original mod. The wire was removed and the trace repaired with some tinned copper wire. At this point I didn’t know if I’d be using that connection but it seemed reasonable to assume I would, so the repair got made while I was looking at the PCB.

Next up was to replace the three can capacitors and one axial electrolytic on the PCB. These capacitors were over 45 years old and during that time the electrolytic in the cans changes state and the aluminum foil inside can also degrade. The result is that the capacitance gets reduced a bit, and the leakage current increases. The leakage can cause significant audio noise and can also heat up the capacitor. If the leakage is bad it can cause the cans to bulge. These were not that bad but certainly due for replacement.

I always photograph the existing wiring before pulling the old caps and then photograph the replacements after the work is done. I’m a bit paranoid about getting it correct.

I usually replace one can at a time just to make it easier to put the wiring back as it was. Here are a few pictures of the re-cap work being done.

After the re-cap was completed I started to re-wire the preamp based on the schematic. There were no changes to the phase splitter (V3) so I just had to check/change the circuitry for V1 and V2 as well as the input jacks, tone, and volume controls.

I’d already removed the extra power switch with its associated wiring and the two extra input jacks. The original plan was to put plugs in place of the old parts to blank the holes. The customer decided that it would like better with them left in place, albeit not working.

I had looked into wiring up the additional input jacks in parallel with the original Hi and Lo sensitivity inputs but the switching used on the original jacks made it impossible to do that simply.

It took a few hours to go through the schematic component by component to be sure that the PCB parts were all correct and that the wires to the tube sockets were all in the right places. After that I replaced the PCB and wired the tube sockets back up.

After I had cleaned up all the power transformer wiring all the turrets normally used for transformer voltage selection were freed up so I added two one ohm resistors to the cathode circuits of the power tubes to make it easier to measure the bias without having to use an adaptor.

I debated this mod as it’s hardly original but if you change tubes you really need to reset the bias and it’s much easier to do it this way.

Now it was time to power things up and check the HT voltages and the bias circuit. I knew I’d be modifying the bias for the EL34s so I wanted to measure all the voltages in the original circuit and the range of adjustment offered by the bias pot.

I always power up the first time without and tubes and using a variac to reduce the input voltage and a 100 W lamp in series with the power line in case there is a short somewhere. If there is no smoke and the input current looks reasonable the lamp gets switched out and the input voltage is set to the full line voltage.

The bias circuit had to be modified slightly by changing a resistor value to acommodate the expected bias range I’d want for the EL34s. Once that was done it was time to add some tubes.

I started with just V1 and V2 to test the preamp since that is where the bulk of the work had taken place, and … deep sigh … I had a problem. I traced this to a resistor with a broken leg.

It had apparently been damaged during the earlier modifications but was easy to fix.

I spent some time verifying that the tone controls worked as expected.

Ideally I’d have tested the full amp with the EL34s at this stage but they had to be ordered, so in the mean time I got the new case ready for the chassis.

I removed the handle and feet from the case and drilled four holes for the M6 x 30 machine screws (new) which hold a Marshall chassis in the cabinet.

When the EL34s arrived I installed them and set the bias, then checked the amp for overall functionality. I do the bulk of my testing with a signal generator and a scope. I also sometimes do frequency plots to look at tone control function and the performance at full power. For the power tests I use a 100 W dummy load (you can find the design elsewhere on this website).

Once the amp was tested it went into its new case.

It was a shame that the previous owner had added extra components to the front panel but overall the result was not too bad. The knobs needed to be replaced but my part of the job was finished.