1996 Marshall 1959 SLP 100W

This was originally a 1990’s reissue of the 1959 100 Watt Plexi amplifier - they were well made with a similar level of quality craftsmanship and tone to the originals. At least as close to the originals as reissues generally get, which is usually about 90% in my opinion. This amp was then modified by Dave Freidman to be a JCM 800 type circuit but with unique and idiosyncratic circuit values - the "Dookie" mod. The two main complaints the customer (and myself) had was that it was overly distorted/fizzy sounding and that it lacked bass response and general heft to the sound. The other issue was that the amp was close to 30 years old and as such needed a standard restoration - this includes a cap job, new power/misc. resistors, new tubes + biasing, and cleaning. This was a huge job but it came out singing, and included a lot of custom work that I was proud to be able to include.

The amp was given a full restoration. All the axial lead electrolytic capacitors were replaced with upgraded temperature rated Vishay brand electrolytic’s. The multi-section cap can’s were replaced with new German made F&T 50/50uf 500v type units. The power dropping resistors were upgraded to 2 and 3 Watt Metal Film type units. The Screen Grid Resistors were changed to 5 Watt Cement type units. This was done for better noise floor and reliability. Also the Screen Grid Resistors showed signs of heat damage. The Presence pot was replaced with an Alpha brand 24mm Marshall style unit due to the original having volume drop and cutout.

The second half of the job was the multiple modifications done to the circuit. V1B's coupling cap was enlarged from .001uf 600v unit, to a .022uf 630v Mallory brand polyester cap - this added bass response. V1A's entire biasing and tonal response was changed. In it's previous state Freidman had taken what is usually a cold clipper circuit and made it into a hot biased grid clipping gain stage with a bypass cap emphasizing treble frequencies. This was a big part of why the tone and character of the distortion was fuzzy/fizzy. This was changed into a cold clipper circuit with a mini-switch that can select between 3 different styles of clipping/OD. The switch selects either: "M" - A Marshall JCM 800 circuit with lots of mids and medium-high gain crunch; "S" - A Soldano SLO 100 circuit which is extremely cold biased with medium-low gain clarity and bite; and lastly "G" - A positive grid clipping stage with high gain fuzz but no fizz, my take on what Freidman had done originally. I added a "Deep" mod which addressed the bass response of the amp. This is a mini-switch that controls a CE brand 22uf 50v cathode bypass cap on V2A and is either Off (0) or On ($). It adds full frequency boost to the second gain stage increasing both bass/depth and to a lesser degree, gain. The switches were placed where the input jacks were originally, Freidman had disconnected 3 out of 4 of them for his mod, so I decided to use the space for something functional and cover up the remaining input jack that was not used. Other changes included lowering the bright cap from a 4700pf 500v ceramic disc cap to a 1000pf 500v silver mica cap - this was to smooth out the high end. Bypass caps were taken out to induce local negative feedback and emphasize mids but not presence. Attenuation resistor dividers were scaled back to not overwhelm the gain stages with volume which can create unpleasant distortion. The Presence circuit was modded to focus on just presence - the original circuit was modded to roll back mids as well which made for a wonky control that introduced mud into the tone.

The V1B plate resistor increase and 50% negative feedback mod were both kept as they added hair and a pleasant rawness to the preamp and power amp. Also the anti-oscillation snubber cap circuits were kept from Freidman as they get rid of squealing in higher gain modes. Overall the changes to the circuit and user select-able mods made the amp sound much better and gave a number of options to the player so they are no longer stuck with mods that are fixed in place.

The amp was partially re-tubed with new power tubes - the preamp tubes tested good. This was the final tube compliment: V1 = SOVTEK 12AX7, V2 = JJ 12AX7S, V3 = SHUGUANG 12AX7 , V4-V7 = JJ E34L Matched Quad. I picked out this lineup specifically for this amp to give the best balance of tone, reliability, and low noise. The adjustable Fixed Bias was set to %60 Class AB Plate Dissipation with a B+ of 446VDC. All pots were cleaned and hardware tightened.