Marshall 1959SLP (RI) Overhaul

[7thString]

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

Thanks

Link to Schematic http://www.thetubestore.com/lib/the.../Marshall-Slp-Reissue-100W-1959-Schematic.pdf

 

[gtrnstuff]

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=.

 

[aynirar27]

I've put this PPIMV in a 1987 RI and was happy with the results. Layout and instructions are clear too

http://triodeelectronics.com/mastervolumemodkit.html

 

[gldtp99]

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.

 

[pdf64]

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.