Marshall 1959SLP (RI) Overhaul


So my plexi is in need of a little TLC lately, and I want to really do it nicely for once as I've resisted modding it since getting it (mostly).

1) Suggestions on bright cap values? I dropped the stock one to 330pf a while ago which seems okay. Still quite bright with a strat.

2) Probably going to put in a PPIMV; who sells the best kit these days?

3) Thoughts on EHX EL34s? I have had good luck with their 12AX7 / 12AY7s many times..

4) Two of the original Ruby filter caps have a nasty bulge - is it worth just replacing them all? If so, JJ / F&T?

5) All other suggestions welcome ie 'rip out that darn fx loop' / put in a HW turret board etc


Link to Schematic


I had the 1959 and still have the 1974 reissue PCB version, before they added the FX loop.
I replaced the treble pot with a pull-switch SPST to make a brite switch, keeping stock cap .005 uF.

That way it's still there when I want it for humbuckers, running channel volume on 7 or so, and totally out for other purposes.

I always run these beasts with attenuators or load boxes, so master volume not an issue for me. Weber MASS, Marshall SE-100, Palmer.

I replaced filter caps with F&T, and also with the ones Weber sells. Both seem fine.

EH and Sovtek tubes are OK. I always liked Svetlana, later Winged =C=.


Last night I was finishing up the "rescue project" on my '73 Superlead--- I got this one inop with a blown PT, and it had a snipped off power cord, and had been sitting unused for many years:

1) Metro/Heyboer dual voltage PT----- running on the 80% power taps---- B+ 407V---- replaced one of the first 10k/1 watt voltage droppers with a 4.7k to boost PI/preamp voltages----- preamp voltages now in line with stock levels.
I want to use this one as a cleanish pedal platform, mostly, so I want a good amount of clean headroom.
I have plenty of other medium/high gain modded Marshall type amps...…..some are real Marshalls and some I've built.

2) Select old production preamp tubes---- Amprex/Holland 12ax7 in V1, British Mazda in V2, RCA 12ax7a in PI------ all tubes stable and sounding wonderful------- I went through many old and new production preamp tubes one day and these won the "tube-off".

3) JJ EL34 output tubes------ why not ?------- they sound good----- I have some quads of true NOS RFT EL34's, and I might use them in this amp later----- but the amp sounds wonderful with the JJ's so I'll leave them in.

4) Bright Cap----- was clipped out when amp came to me----- I put in a 330pF ceramic disc last night----- it brightened it up----- I like it and it's staying in, huge/chimey cleans----- I didn't want the more common .005uF that seems to boost gain----- I have other NMV Marshall amps with that Bright Cap value and it's a good value for a more aggressive sounding amp set-up.

5) Other work done back when I started this project:

A) All new F&T Filter Caps
B) Rebuilt Bias circuit---- all new components including modern Bias Pot (had the old slider type that didn't work right)----- Sprauge Atom bias caps.
C) New Power Cord-----properly installed/grounded and I disabled the original Polarity Switch (this was a 3-switcher----- switch is still there but not connected to anything).
D) Disassembled/serviced the original Speaker Impedance Switch----- I made the old style switch work properly and tightly hold the two-prong plug (so it doesn't fall out)----- took a lot of D5 and time---- I almost put in a new modern style Speaker Switch---- still might one day if the old switch starts acting up.
E) Cleaned pots/jacks/everything else----- amp was filthy and had been sitting inop for a very long time.
F) Found/fixed bad hum----- was a bad solder connection in the tonestack---- re-did that connection and re-flowed others that looked like they needed it.

I tested out the final tweak last night (the 330pF Bright Cap) through an old Orange 4x12----- using an OCD and a DD7 for flavor at times------ a friend was playing bass through a bass full stack and he asked me to turn down, at one point, because my amp was pointed right at him------ so I was playing with Vol 1 just below "2" and Vol 2 at "2.5", channels jumpered.

The amp is making plenty of clean power even with the lower B+ voltage.

I have an old THD Hotplate I have used with this amp but wasn't using it last night------ It seems to work well enough----- I was going for max Clean Headroom last night.

I'm very happy the way this "Superlead Rescue Project" came out.


As halving the voltage roughly equates to quartering the power, the CT on the HT winding on these makes it possible to implement a 1/4 power switch (DPDT switch, one for the HT, the other pole for the bias).
I did that to a couple of these a while ago, which worked out pretty well. Obviously the power tubes should last a lot longer at ~250V.
Unfortunately I didn't make notes of the detail for the bias supply low power mode, but a bit of tweaking may get you there quickly.

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