Marshall 2×12 JMP ”Bluesbreaker” Tremolo Combo 1967

Here´s a piece of British amplifier history you don’t see every day, mainly because it sounds absolutely fantastic!

What we have here is a 1967 (confirmed by the date codes on the Celestion speakers and Mustard capacitors) Marshall ”Bluesbreaker combo” model 1962 featuring the classic Marshall ”Bass” circuit fitted with tremolo working on channel II. The amp is built into the big, boxy ”Series I” style cabinet which – contrary to popular belief – Marshall DID produce in 1967.

It is an early example of a plexi faceplate featuring the ”JMP” inscription. The amp has two 12” Celestion 25 Watt G12M 75 Hz. (Model T1221) ”Greenback” speakers (date codes: 12FM = june 12th, 1967 and 16FM = june 16th, 1967) , fitted with Marshall stickers on the back.

The amp has it´s original Drake 784-139 output transformer to go with it´s two EL34 output valves. The speaker impedance selector has been hardwired to 8 Ohms to match the two paralleled 15 Ohm internal speakers.

What follows is a list of the servicing work done on this amplifier in order to keep it running as happily as possible and in order to restore a few details back to as close to original as possible:

• The amp comes with a full set of TAD valves – a pair of STR EL34-B output valves and four ECC83WA preamplifier valves. These have all been tested on an AVO Mk II Valve Tester. V1 and V4 (input-valve and tremolo-valve respectively) are “a bit low”, but still within tolerance. The rest of the valves test fine, and the output valves are very well matched (within 1 mA).
• All old electrolytic capacitors have been replaced with genuine, German-made F&T (One TAD-labeled) capacitors in the power supply. For the bias supply and the cathode capacitor on V1, Sprague Atoms were installed.
• Non-original, cheap Chinese octal sockets for the output valves have been replaced with proper NOS types
• Non-original biaspot has been replaced with NOS-type. The amp probably was born without a bias pot. But at least this old type looks much more “right” than what was in there before. Two resistors in the bias circuit are new as well.
• Non-original speaker jack-socket replaced with proper NOS Cliff-type.
• 1KOhm/7Watt screen grid resistors have been installed – proper NOS Welwyn W22 type.
• One non-original 10 KOhm/5watt “cement” dropping resistor in the power supply chain replaced with a NOS 10KOhm/7watt Welwyn W22-type. The amp would probably originally have been fitted with a 10KOhm/2watt Carbon Composition here. But a 2 watt resistor is on the “flimsy” side in this position. So this is the better solution that falls in line with Marshall´s usual component choices.
• A bit of tidying up non-original (due to repairs and service over the years) wiring. Especially around the output valve sockets, speaker output jack and the switches on the front.
• At some point (many years ago) the original 250 KOhm treble-potentiometer was replaced with a 500 KOhm. It’s been left in, since this may very well play a part in this particular amp´s unique sound.
• The indicator lamp has probably been replaced at some point. And the footswitch for the tremolo is not original.

The amp is hardwired at the 240 Volt setting. Feeding it 230 Volts from the outlet this results in a plate voltage on the output valves of around 455 Volts. The EL34s have been biased at 35-36 mA per valve.