Opinions on this circuit's potential?

[dpm]

I've scored an old Marshall 9001 preamp (free) and I've been contemplating modifying the gain stages of it. I'm a noob to tube circuitry, so I'm not sure if this is a good project to start on. What I'd like to do is removing the diodes from V2 (clipping, right?), and add 2 more gain stages before V3. I'm yet to measure plate voltage, etc. There's just enough room is this thing for another 12ax7...
Is this a stupid task to undertake? or is there potential for a decent sounding pre and a (not too frustrating) lesson in amp modding here? Thanks!

http://www.drtube.com/schematics/marshall/9001-4.gif
http://www.drtube.com/schematics/marshall/9001-1.gif

[Steve Conner]

The tubes are wired up in a really strange way there. Marshall seem to have used one triode in each double triode tube as an ordinary common-cathode gain stage, and the other triode (the upper one in the schematic) as a sort of current source. This is known as a "SRPP" circuit. If you wanted more gain, why not rewire the current sources as extra gain stages.

Be warned though, Marshall probably put a lot of R&D into getting it working and sounding all right. Modding it may do some weird things :eek:

The diodes might be there for clipping. But they might also be there to stop high voltage signals from a preceding tube stage from blowing up the switching JFET. You'd need to check before taking them out.

[Ray Ivers]

Steve,

Be warned though, Marshall probably put a lot of R&D into getting it working and sounding all right. Modding it may do some weird things :eek:

Truer words were never written. :) I'm now in the process of undoing practically everything I've done to this model 2000, in the hopes that the original Marshall kludge-fixes (I've uncovered four so far, all compensating for extremely poor circuit layout) will again coalesce into a relatively hum-free end product that can almost drive the output stage into full clean output. ;)

OTOH, I've modded many 2203/2204's and 1987/1959's into gain monsters with no problems at all - go figure.

Ray

[dpm]

Very interesting! I've had a look at SRPP circuits on the net now (thanks Steve). Of course everything refers to hifi circuits and the math is a bit more than I can grasp right now :o ...... actually here (http://www.gatago.com/rec/audio/tubes/13125450.html) is someone experimenting with SRPP in a guitar application.

This unit has plenty of distortion, but the quality of it is bad. It responds quickly, which is pretty cool, but sounds very unmusical. It's a tad noisy too.

Voltage onto V3b's plate is 229V, with 213V for V1b and V2b. I think I'll try modding this, being careful to make it reversible. I'll have to order some parts, so if anyone has suggestions in the meantime I'd love to hear them.

I'm now in the process of undoing practically everything I've done to this model 2000, in the hopes that the original Marshall kludge-fixes (I've uncovered four so far, all compensating for extremely poor circuit layout) will again coalesce into a relatively hum-free end product that can almost drive the output stage into full clean output.


Wow, that sucks! Is that a JCM2000 you're talking about? Or is there a 2000 model number from an earlier time?

[Ray Ivers]

dpm,

It's a JCM800 model 2000 - a 250W channel-switching guitar amp from 1981. Had I known earlier what I know now, I would have biased it, cleaned it up a bit, and moved on to the next job. :D

Regarding the 9001 preamp; at a glance... it may not necessarily be the tube section that's giving the sound you don't like. IC1b can be substantially overdriven - if this overdrive is required to get you the sustain you need, I would suggest replacing the stock TL072 op-amp with an LF353, provided you're of a mind to pull chips/install sockets/etc. Also, the negative feedback around V1 (R1) and the clipping diodes D1/D2 at the input to V2 (which you mentioned) don't strike me as the very best way to obtain tube-sounding overdrive. If this were my preamp, all three of these components would be history ASAP, but YMMV - and as Steve posted, you could be opening a can of worms if Marshall added these components to quickly address problems they didn't have time to chase down themselves "the right way". OTOH, you may luck out with just these simple mods and end up really happy with the preamp afterwards, who knows.

Ray

[Enzo]

Is that a JCM2000 you're talking about?

Arrgh, pet peeve of mine. There is no JCM2000 amp, that is the name of a series of amps that don't particularly resemble one another.

The 20 watt DSL201 and the 100 watt TSL100 are both JCM2000 amps. It bugs me when someone calls the shop and tells me they have a JCM2000 and expect me to know what amp they mean.

OK, I'm over it, the pills are taking effect.

[dpm]

lol Enzo, I should have asked "Is that a JCM2000 series?", as that is what I meant :D

Ray, I'll take all those suggestions and give them a try. At worst I could gut this thing and build a nice single channel preamp in this nice sturdy rack case.

[dpm]

To update, I thought I'd try a few quick tweaks with what I had on hand (not much) and I'm really happy with the results! On the Valve Board I removed R1 as Ray suggested resulting in a big jump in level with some pretty harsh clipping particularly noticable on the clean channel. The solution was to reduce R4 from 1M to 470k, still giving more level and some light distortion when pushed (the clean channel was almost impossible to distort originally - even with an EMG 81). D1 and D2 were removed from V2a and a 470k resistor put in their place. There's still enough distortion for me, and most importantly it now sounds pretty good. I play fairly complex chords distorted, the diode clipping turned them into harsh mush. In fact even certain single notes sounded worse than others. So, two 470k resistors is all it took to give this thing better tone. I'll give it a louder test session tomorrow to be sure, but I think I'll leave it like this for a while. A cool feature on the 9001 is the trim pot on the main board (PR1) that acts like a brightness control.
Anyway, thanks guys for the help. I know it's only a small tweak but I've learned a lot about tube circuits getting to it and the result has encouraged me to continue playing around inside amps :cool: