Repair and overhaul of Marshall Master Volume 100w Valve Guitar Amplifier
Fault - The amplifier lacks volume hums and a couple of the output valves are overheating
See the above pictures, the amplifier was not in mint condition although it was a regularly used work horse.
This job started out so I though as a straight forward re-valve of the output tubes, on closer inspection found a missing PCB nut and washer and a loose PCB nut and washer - this was sorted out.
As the amplifier was needed back in service for a gig ASAP I set to work on the re valve, I then realised that I was down to my last pair of matched EL34s and would not be able to order a matched quad of EL34s in the available time, the only way to get the job finished on time would be to use 4 matched KT88s that I did have in stock, this does involve a slight mod to the bias voltage circuit to enable the KT88s to work correctly - the mod was carried out and the KT88s installed and the bias voltage was set to the correct level (you notice I have not revealed the circuit mod or the correct bias voltage, we have to keep some things secret). New EL34s could then be fitted at a later date when convenient.
I then discovered that we had some problems, basically the amplifier was very noisy ( not loud noisy which it was, but not working correctly noisy) I suspected the mustard capacitors were probably the cause, but on an amplifier this age you can never rule anything in or out, - so checked the following
All of the other valves ECC83s they were working perfectly
All the big blue electrolytics - they were working perfectly and reading the correct value
All the mustard capacitors - these were a different story 7 of them needed replacement
All the resistors - several of these were reading completely wrong values and were replaced
Also replaced were 4 of the smaller capacitors
All of the above was carried out in one go - the reason for doing it in this way is because you have to unsolder several of the cables to get to the underneath of the board, once you have the board upended it is best to test everything and replace as necessary rather than upending the board multiple times
See above the board after all the new parts had been fitted, the amplifier was now working perfectly - as good as it was when first made.
However the customer called and said that the amp was not needed for the gig, so keep the amp and fit the EL34s when they arrive, so a few days later I had to undo the bias mod fit the EL34s and re bias the amplifier, after all that the amplifier was tested - still working perfectly with the new valves, a matched quad of EL34s and returned to the customer.
Unfortunately I did not remember to take pictures with the new EL34s fitted so I will leave you with a picture of the KT88s
A word about using KT88s instead of EL34s
This Mod does give the amplifier a different sound - not to everyone's taste, some players prefer the KT88 mod with 4 KT88s fitted and some prefer to just use 2 fitted.
You can also use 5881s, KT66s, KT77s
The reality is you would have to make your own mind up on your sound, however it is sometimes desirable to experiment using different output valves.
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