Marshall Major DIY

[CJ]

on the bench is a quad kt 88 running at 750 volts holy cow,

interesting channel switching and clipping circuits,

NFB on stage 3

and the tone circuit is kind of strange,

and i have never seen that dual master before,

 

[CJ]

power to the people,

stacked secondaries,

 

[CJ]

parts is parts,

 

[EthanSpaulding]

Pretty cool! Preamp looks like a Marshall Silver Jubilee.

 

[CJ]

B+ came down to 714 after the bias was set, one bank had minus 73 volts on the grids,

 

[CJ]

upstream blueprint>

fixed error on 2200 pf across diodes,

added voltages

 

[heatwalk]

Lots of gain staging goin on. Designer wanted to emphasize the sound of clipping the PI with the level controls on both sides of it without going KT88s open wide

 

[zamproject]

interesting channel switching and clipping circuits

looks like a Marshall Silver Jubilee

Yep ! look 100% like a Jubilee clone, 2555 100W
with it's specific LED/diode clipper

I'm the happy owner of the 50w 2550 version since mid 90'
To me one of the best sounding Marshall ever produced.

I don't play that much guitar now, but when touring a little by that time I run it troughs a 2x12 cab fitted with old greenback, and a G&L ASAT with their big single coil (P90 like)
What-a-sound !!!

Cheers
Zam

 

[CJ]

Yeah Iommi had that overwound P90 on the first two albums IIRC.

 

[CJ]

Dang it is a Silver Jubilee! Spent a week tracing the schemo, oh well.

The dual master u do not see on the Marshall.

 

[dai h.]

FWIW, re: post-phase inverter master volume:

Dave Friedman apparently (now) likes a dual pot (wired as variable resistor) over (in parallel with) the original grid load/bias return Rs. (My novice understanding is that) there is enough drive from the PI so low end loss isn't a problem with the pot turned down (also less of a safety issue for the output tubes if something goes wrong with the pot). (That might be in the new Suhr mk.II versions of old Plexis also (I'm guessing).)

 

[Michael Tibes]

Post phase inverters are in fashion for a while now, the idea is that the p.i. can still distort before the level is being reduced on the way to the power tubes.
I wonder what's the idea behind that build? Is it supposed to be a Major? No ultralinear power amp? Looks like a mashup of several ideas.

 

[CJ]

Wow I never thought about the red plating after the dual master failure , better to run another set of coupling caps or at least use a B+ fuse,

The PI does use 47k resistors which might give more drive but I would go for a 12at7 for a quad of kt88 tubes.

 

[dai h.]

The "Lar/Mar" PPIMV`* (basically the same as that replacing the 220k bias load/grid return Rs with a dual pot) has additional 2M2 from one end to the wiper (for safety and slight change to (taper?) and overall value.

*named after Mark Abrahamian (RIP) (ex. Starship) and Larry of Larry Amplification (Larry came up with the slight alteration)

 

[Admiral-dk]

Don't like any part of the BIAS Circuit - an Accident waiting to happen

A BIAS Pot should Always Short to GND and have a Resistor between it and the rest of the BIAS Circuit. That way you don't loose BIAS when the Pot is Bad (it almost always rises - lowering the Current).

I also don't like the Stereo Master Pot idea - I had too Many Pots fail with a short to the 'house' of the Pot (or the Axle of the Pot) .... Again a remedy for Disaster on a Tube Amp

I'm not trying to tell CJ anything he don't know (as I'm sure he do) - I'm more trying to warn others with less knowledge about Tube Amps ....

Best wishes

Per