1967 Marshall JMP 100W Buyer’s Info

1967 Marshall JMP 100W Buyer’s Info
This amp is not my property and I am selling it on behalf of the owner via my Ebay account. Do not contact me with complaints or technical problems after sale – be prepared to get it professionally serviced before use. I am being honest and open about its condition, and am making these following suggestions to avoid your disappointment, further unnecessary damage to this amp, and most importantly – for YOUR safety.

This amp will be shipped by courier, UK wide, with a “Fragile” handling service for around 22.50 pounds paid for and/or arranged by the buyer or I can suggest a local firm that offers this service.

There will be NO RETURNS or REFUNDS or responsibility for its loss or damage by me or the current owner due to transit, or any other reason.

The amp will be powered up and videoed one last time before shipping so if DOA, then contact the courier company, not me or Ebay.

I did the basic minimal technical work to get this amp to its current working state and leave as much original working circuitry intact, as I believe much of it IS original but I have no way to verify this, but it is not perfect and there are some minor repairs and checks that should be done before it is used as a workhorse or even played further to be on the safe side. The components are in very good condition for their age, but how much longer they last is of course, unknown. The electrolytic caps are the components known to definitely deteriorate with age, and can cause more damage to other parts if they fail catastrophically. This can happen in any amp of any age at any time though, depending on…
First, if you are interested in buying this amp, then you probably know what you are looking at and realise that a classic rare beast as this will need a technical check if you value its continued existence without further expensive damage being done as well as for peace of mind and to increase its value and to make it safe. The main issues that I am aware of that should be addressed with this amp are:
1    That an original UK 240V Bulgin3 pin plug lead needs to be sourced first, as the plug used currently was made from Epoxy resin by me, for test purposes only! If it comes out then the Earth pin should be identified using a Multimeter as this plug has no key to ensure correct insertion position. You have been warned! Make sure you know what you are doing regarding mains plug connections or seek qualified advice.
You are strongly advised to source the correct 22mm size plug, which I am told, may be available on Ebay, but I have not seen one yet, or may be found with old 50s style radiograms or other era specific electrical items. An electrical tech or valve radio enthusiast may be able to help you with this. Do your research.
2    The mains indicator is a neon type and should be replaced with an original 6.3V mains lamp as in the original circuit if you wish to retain classic authenticity and operation.
This neon currently is wired across the 525V DC rail as a test aid for me when working on the amp. This is why it flickers with the dynamics of the changing DC voltage at high volumes.
3    The 16/8/4 ohm selector switch at the rear should be replaced with an original Marshall part, as the 4 ohm output transformer winding connector has broken off. This wire has been taped safely inside, so the amp is still perfectly useable with 16 or 8 ohm 100W speaker cabs.
4    One of the tone pots is crackly on watching the videos – even though these were all sprayed with IPA by me – which may be due to a high voltage connection from the original design tone stack, or it is just worn out – your guitar tech can check this.
5    The output valves bias should be checked by a qualified amp tech to ensure it is not biased too hot or cold. They have the test equipment to check this. The Push-Pull output stage waveform was symmetrical when I tested it, but the negative feedback slider pot should also be checked for its setting and function when serviced.
6    The reservoir capacitors cans are at least 30 years old from what I am told so should be checked for performance and changed accordingly. The mains secondary fuse has been re-wired so it is BEFORE these caps, not after as in some schematics such as those shown below, so it should blow before serious damage is done to the mains transformer, should any caps fail on Standby power up.
All other info on this amp and the work I have done on it to get it to this level of functionality is documented already in my previous Posts:

Here is some video of the first full volume tests over about a 2 hour period done on 10/10/13 to show this amp is working fine:

The poor sound quality is due to the Android pad mic distortion at these very loud volumes, not the amp.
Here is some of its better moments and amazing tonal possibilities and lush harmonics, with other FX pedals: