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

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

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 ...... actually here 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.

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

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

[CroTone]

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

Sorry for resurrecting this thread from the dead, after 15 years lol :-D

I've recently acquired this preamp and would like to try this mod. If you could provide some clarifications it would be great!

Speaking of 470k resistors, is there any special value (in terms of power, wattage-W) that I should be concerned about? Which ones would you recommend?

I use this website to get equipment...so which ones would be appropriate in terms of values?

https://www.banzaimusic.com/Resistors/


From my understanding there are three steps in your mod scheme:
1. R1 is removed
2. D1 and D2 are removed and replaced with 470k resistors
3. What is done with R4? I don't understand this part. Is this also replaced with a 470k resistor?

Any other suggestion on what would be good to do to improve this circuit would be more than welcome. I would wish to try it out before I close the damn thing, swap preamp tubes, and put it back in the rack!

Thank you so much, rock on!

[CroTone]

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.

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 afterward, who knows.

Ray

Your input on this issue would also be more than welcome!
So remove R1, get rid of D1/D2, and put 470k resistors instead (which value in terms of power rating?) and with respect to R4 (replace the 1M with 470k, I guess the same resistor as for D1/D2 replacement or is this something else)?
I am not an electronics guy neither technician but I can get my dad to replace this since he is an experienced electrician...I would just need exactly which components to get.

Many thanks! ;-)

[g1]

1. R1 is removed
2. D1 and D2 are removed and replaced with 470k resistors
3. What is done with R4? I don't understand this part. Is this also replaced with a 470k resistor?[/B]

Items mentioned refer to this drawing: https://drtube.com/schematics/marshall/9001-4.gif
R1 (1M) is removed.
D1 & D2 are a parallel combination. They get removed and replaced by a single 470K
R4 (1M) is replaced by a 470K

So you only need two 470K's. 1/4 watt type will be fine as they are not handling any power.

[CroTone]

Items mentioned refer to this drawing: https://drtube.com/schematics/marshall/9001-4.gif
R1 (1M) is removed.
D1 & D2 are a parallel combination. They get removed and replaced by a single 470K
R4 (1M) is replaced by a 470K

So you only need two 470K's. 1/4 watt type will be fine as they are not handling any power.

Thanks for clearing this up! Great! :-)

[teemuk]

IMO, there's really not much point in removing the negative feedback resistor since rhythm channel's "Balance" control blends between "clean" signal path (the SRPP gain stage with NFB) and dirtier signal path (another SRPP gain stage, now without NFB). "Drive" control is global for both modes. Removing the negative feedback loop beats the purpose of such design.

You want more gain? Just adjust the balance control instead of clipping the feedback resistor. You want less "fizz" at higher gain settings? Turn your focus to that cathode bypass resistor in the SRPP stage sans NFB.

"Lead" channel mode inserts the third SRPP gain stage in series with the one having no NFB, and also changes to channel-specific drive settings. "Drive2" adjusts magnitude of overdrive of this 3rd stage. There's plenty of real tube overdrive going on in this design.

The diodes in this circuit do clip, but most of the clipping is from SRPP gain stages, which have somewhat high gain (without NFB that is) and also run at moderately low supply voltage for lower clipping threshold. The only stage with NFB is really one that is supposed to stay rather clean in the first place. All the rest should overdrive quite nicely at higher drive settings.

Ironically though, a SRPP circuit doesn't clip much differently than a pair of diodes. Clipping is very symmetric and totally unlike that of single-ended common cathode gain stages commonly employed in tube preamps. Well... The design is what it is. Let's remember it's about same vintage than JCM900 DR and gen. 1 Valvestates, and as such shares more resemblence to those than to earlier Marshall amps, let alone 2203/2204, etc.

[CroTone]

IMO, there's really not much point in removing the negative feedback resistor since rhythm channel's "Balance" control blends between "clean" signal path (the SRPP gain stage with NFB) and dirtier signal path (another SRPP gain stage, now without NFB). "Drive" control is global for both modes. Removing the negative feedback loop beats the purpose of such design.

You want more gain? Just adjust the balance control instead of clipping the feedback resistor. You want less "fizz" at higher gain settings? Turn your focus to that cathode bypass resistor in the SRPP stage sans NFB.

"Lead" channel mode inserts the third SRPP gain stage in series with the one having no NFB, and also changes to channel-specific drive settings. "Drive2" adjusts magnitude of overdrive of this 3rd stage. There's plenty of real tube overdrive going on in this design.

The diodes in this circuit do clip, but most of the clipping is from SRPP gain stages, which have somewhat high gain (without NFB that is) and also run at moderately low supply voltage for lower clipping threshold. The only stage with NFB is really one that is supposed to stay rather clean in the first place. All the rest should overdrive quite nicely at higher drive settings.

Ironically though, a SRPP circuit doesn't clip much differently than a pair of diodes. Clipping is very symmetric and totally unlike that of single-ended common cathode gain stages commonly employed in tube preamps. Well... The design is what it is. Let's remember it's about same vintage than JCM900 DR and gen. 1 Valvestates, and as such shares more resemblence to those than to earlier Marshall amps, let alone 2203/2204, etc.

Wow, thank you for such an in-depth explanation of the circuit and for making some things easier to understand.
I am a layman for electronics, so I am far from having your level of understanding of these things. I was considering the mod described above, however, still executed none.
I can get some good tones out of this thing, it only needs some time and tweaking because of its layout and controls that are not really "orthodox" for a Marshall type of stuff. I am learning a great deal about all of that.
This brings me to the question, do you think that there are any mods worth doing to this preamp, just for the sake of trying some things? Removing or adding a diode or resistor here and there?
Personally, I am leaning more towards the traditional (Marshall JCM 800, Plexi) style of amps with respect to the sound and I am less into modern Marshall sounds.
Do you think that perhaps a certain tube selection could also help this preamp to go in such a direction?
Would you have any recommendations that might get me closer to the more vintage side of the things with this preamp?
To be frank, I don't find this preamp to have much gain at all. I had experience with some other preamps and some of those things would have much more compression and gain than this unit.
It stacks well with my OD pedals and then I get the juice I want.
Thanks for your time and valuable answers!
Cheers.

[CroTone]

Speaking of mods to this preamp...would it be possible to perform a mod that will bring this preamp to sound close to JCM 800 Jose-modded Marshalls?
Many speak about Zener diodes after the cathode follower and stuff like that...I just wondered if someone tinkered with this preamp in such a direction?

Thanks!