18W Marshall Plexi

[ChrisM]

Over the last few months I have been putting together, building and recording a new amp. The amp is derived from the classic Marshall 1974x (18W) design and shares much of the same elements. It is however modified and voiced to be more of a 20W Plexi ala JTM45/JTM50/1959. Think small package Plexi tones without the killer volume. So fat clear clean tones and singing distorted tones. Lots of chunk and grind too.

-20W
-5AR4 tube rectifier
- TMB channel with Gain, Volume, Treble, Middle and Bass controls. This is your classic Marshall channel.
- Normal channel with Volume and Tone controls. This channel comes straight from the 18W design.


Everything you wanted to know about the design and controls of the amp can be seen in the schematic
http://i1.wp.com/pdfelectronics.com/wp- ... matic.jpg?

Interesting things to note:
- VRM control (controls B+ of the entire amp for a very effective master volume control)
- 6V6GT output tubes
- Quasi cathode bias to mimic a fixed bias output section
- JTM45/Plexi switch to change the preamp from a JTM45 to a Plexi (1959) preamp
- Three way preamp boost switch, bright-flat-fat

As for some pics...
The completion of this amp coincided nicely with acquisition of my ES-335

[IMG:484:600]http://i0.wp.com/pdfelectronics.com/wp- ... -335-3.jpg[/img]

[IMG:450:600]http://i2.wp.com/pdfelectronics.com/wp- ... ring13.jpg[/img]

[IMG:718:600]http://i0.wp.com/pdfelectronics.com/wp- ... ring14.jpg[/img]

[IMG:605:600]http://i0.wp.com/pdfelectronics.com/wp- ... iring7.jpg[/img]

[IMG:486:600]http://i1.wp.com/pdfelectronics.com/wp- ... iring3.jpg[/img]

[IMG:550:600]http://i1.wp.com/pdfelectronics.com/wp- ... wiring.jpg[/img]



For more info, pics and sound clips check out http://pdfelectronics.com/18w-plexi/ Interested to here some feedback on the soundclips. I am no expert in recording but I did the best I could with what I had on hand/knew.

[Reeltarded]

Beautiful build! That is a very nice amp.

[xtian]

Beautiful lead dress, man! Very nice.

[Bob S]

Nice work Chris.
If it sounds as good as it looks, it's a keeper.
And a 335 to go with it.
Life must be good lately.
Very well done.

[sepulchre]

What a beautiful build! Excellent job.

[Colossal]

Fantastic! Bet it sounds killer, and especially with the 335! Please play some Rush

[M Fowler]

Professional work there

[boots]

Beautiful workmanship! That's a work of art.

[darryl_h]

That is a remarkably well-constructed amplifier.


A couple of questions about the circuit:

1. Should there be a resistor from the grid of V2a to ground? Without it there is no voltage divider and SW1A would have no effect.

2. A very minor point: in the Normal Channel, the Volume and Tone controls appear to be mislabeled.

[cbass]

Man thats nice all my builds are crap I should just give up

[ChrisM]

Thanks for all the compliments!

That is a remarkably well-constructed amplifier.


A couple of questions about the circuit:

1. Should there be a resistor from the grid of V2a to ground? Without it there is no voltage divider and SW1A would have no effect.

2. A very minor point: in the Normal Channel, the Volume and Tone controls appear to be mislabeled.

1. That's how the switch is wired. SW1A just puts a resistor in parallel with the already existing grid stopper. Allows you to have two different grid stoppers, 470K and 270K.

2. Good catch, thanks!

[Structo]

Nice work Chris!

I had a 18w in for repair once and I really liked how it sounded.

[Matt D]

Amazing build! Excuse my ignorance, but what are the 220k resistors off of the screen grids for?

[Colossal]

Chris,

Did you use a capacitor just after the rectifier tube, but BEFORE your VRM circuit?

[tribi9]

Wow, so nice. I want to play those wires on the circuitboard like a harp.

Congrats.