High voltage MOSFET in Marshall Haze

[Enzo]

never encountered a Haze until now. Have a MHZ40C in for repair. Signal throughput very low, but peaks come cracking through very loud, as though overcoming some sort of DC it didn;t like.

The FX return buffer stage is unusual, at least to me. Return has a couple LEDs cross wired across it, I assume for limiting. Can't imagine they just want to clip the FX return. But that feeds into a MOSFET over JFET sort of totem that I guess is a level shifter. And the transistor stage runs on +200VDC!

The upper MOSFET is called out as a TN2404, and I am thinking what the heck is that?

TN2404 is a little TO92 240v MOSFET. 240v little thing? No wonder I don;t have them.

But apparently that is where the problem lies, bercause I can't get signal through the stage. Oh well, 66 cents at Mouser. SOmething new every day.

[J M Fahey]

Hi Enzo.
Our own R.G. had been experimenting here with that http://music-electronics-forum.com/t1244/ ... in 2006 !!!!
He wrote:

I did some work with some Zetex high voltage MOSFETs. These are a bit unique in that they are TO-92/E-line packages, same size as your garden variety small plastic signal transistor. They are 0.7W, 450V devices,

And Ry Ivers answered:

No worries implementing FX returns, reverb, etc. with this thing, eh?

So now you know : Marshall (and many others) probably (at least in a partial way), design by reading Music Electronics Forum, may be also SSGuitar, and a few more.
You think I'm kidding? I don't think so.
The world has come full circle: earlier tinkerers like us tried to clone or emulate "Big Name" designs in a simpler , more accessible way, or even test "crazy" designs just for kicks ; I think now at least some scrounge Forums for new, fresh ways of doing things.
R.G. mentions ZVN0540 and ZVN0545.
Also "a Supertex TN2540 ", so that TN2404 must be a very close cousin.
Everyday I am amazed at the very high level of excellence this board has.
PS: those Leds must be, as you guessed, for input protection.

[defaced]

That doesn't have a MOSFET phase inverter, so that particular MOSFET follies doesn't make much sense. There are many others, google kicks out the original and the "Third Ed".

The MOSFET over JFET is very common for a clean gain stage. I know I've seen it explained in an oddball PDF that's floating around, but I can't seem to put my hands on it now.

[Wombaticus]

You're not crazy. The Marshall Haze was designed for Marshall by James Marchant, who posts over at AX84

[Wombaticus]

The MOSFET over JFET is very common for a clean gain stage. I know I've seen it explained in an oddball PDF that's floating around, but I can't seem to put my hands on it now.

Sounds like you are describing a MOSFET/JFET cascode? It's evidently described in TUT1, but I don't have a copy.

[oc disorder]

The high power Valvestate 8280 (2 x 80 watts) used 3 VN2401L's in the preamp
running from a HT rail just for them and a 12AX7 for overdrive.
See attached

[jrfrond]

Those LED's are definitely for signal clamping. Better to have distortion than a meltdown I guess!

Hey, I don't have ANY of those Marshall MOSFET's in my database, and that's with 665 transistor types in stock! I think I'll tack these onto the next Mouser order, just in case.

[Enzo]

I found it interesting due to its function. I don't think they were going for tonal nirvana or tube emulation or anything, after all this is just a buffer stage between the FX return and the power amp in. And the power amp in is right into the PI grid. So from line level FX return right up to tube voltages. That is why I look at it and see it as a level shifter more than anything.

That and in commercial amps I don't usually expect 200v on the TO92s.

[defaced]

Sounds like you are describing a MOSFET/JFET cascode? It's evidently described in TUT1, but I don't have a copy.

Yep, found it. http://www.blueguitar.org/new/articl...er/hi_v_ss.pdf

[J M Fahey]

Hi OC DISORDER , thanks for posting that tantalizing bit.
Ahem !!, you might post it in full, cough cough!
I am quite intrigued by those Mosfets biasing supply or rail, since they refer their gates to ground.
That connection vanishes into the schematic bottom, which is not seen.
Thanks in advance.
Also thanks to Enzo for posting the Haze40.

[oc disorder]

Pre-amp Schematic link Marshall 8280 (with HV fets)

Don't you have search engines in Arch and Tina ?
Sounds like you need a fishermans friend !
(Just kiddin' JM ! )
Its available on Dr Tube
http://www.drtube.com/schematics/mar...8280-62-02.pdf
"8280 Valvestate, 2x 80W 2x12" Stereo Chorus combo with reverb
The Stereo Chorus models were introduced in 1993 and discontinued in 1996. This amp was fitted with two Celestion G12T 8Ω T4354 speakers."

[J M Fahey]

OK, thanks.
Fact is, you don't find what you don't search for, and you don't search what you do not even imagine exists.
I have repaired my fair share of the very good sounding 8240 (best VS combo in my opinion) but never saw (before) nor ever heard about the 8280.
*Loved* the bit about "Arch & Tina"
EDIT: now I see, they refer source resistors to -12V, creating 1mA constant current which drops 100V across the 100K source resistors. Clever.
I do something very similar using widely available and dirt cheap 2N7000, which support 60V, easily available for me.
No need to "make" 200+ Volts and later pad the output to something usable.
EDIT2: now I see: 2 Mosfet outputs, 2 "good" Celestions, must have been quite expensive, must have sold poorly. Sound must be killer.

[jrfrond]

That and in commercial amps I don't usually expect 200v on the TO92s.

Yeah, you wouldn't usually think that anything in a TO-92 case could beat your ass down, now would you?

Yet another good reason to always keep one hand in your pocket.

[kg]

Yeah, you wouldn't usually think that anything in a TO-92 case could beat your ass down, now would you?

roffle

[Steve Conner]

Well, how big would the case have to be before you started treating it with respect? One of these?


I need to know as I'm working on some high voltage preamp designs just now, and they do indeed have TO92s with 250V on them. Maybe I should include a neon lamp on the preamp board, as recommended by Bob Pease.