Look at this schematic and see if u can tell me.....

[daz]

....this amp seems to have very little sag/squish in the attack. First of all, the schematic if a horrible hand drawn one and is missing much of it. Just has the key parts, but there is no available schematic from marshall (origin 20 amp) so i don't expect you will be able to tell me. It;s a very clean sounding amp that has no distortion at all in the preamp with only 2 stages. V2 is used for effects loop. Anyways, it;s dead clean till the power amp starts to gring but i am getting preamp overdrive by boosting it. Either way the notes don't have as much give or sag in the attack as other marshalls i have had so i just thought why not post the hand drawn (NOT drawn by me by the way !) incomplete schematic and see if anything jumps out at you that might cause that. There are other hand drawn jps of various missing parts that i can't seem to find now but if i can i'll post them. I was thinking power supply caps that are too big and cause a stiff sound but i don't recall if theres a PSU drawing but I will see if i can find it.

[pdf64]

So the ‘gain’ is a vol pot? What value?
And the ‘eq’ is a tone stack? Details?
Without a CF to buffer it, the tone stack is likely to be hamstringing the gain of that 2nd stage.
And no DCCF goodness.

[daz]

So the ‘gain’ is a vol pot? What value?
And the ‘eq’ is a tone stack? Details?
The tone stack is likely to be hamstringing the gain of that 2nd stage.
And no DCCF goodness.

I dunno. I saw those drawings of the different parts of the circuit like the gain and tilt but i can't remember much. I lost the link to them and i'm trying to find them. I THINK the gain pot is the usual 1M but i can't see much when i pull the chassis and theres no official schematic. I DID think about V2B and the idea that it;s not seeing much gain, but i can't find components on the board because it;s double sided and theres a lot of confusion w/o a complete official legible schematic. I saw that 470k at the V2B grid and wonder if putting a 1M in it's place would allow more gain which i want and also soften the attack. Also that "tone stack" between V1 A and B is actually a boost circuit and "tilt' cpot (not shown) with the boost thats switchable so it apparently works by lowering the signal then allowing it back for the boost albeit with some caps to change the boost tone. So i wonder how it would sound if i could bypass all that crap and go straight to the V1B grid from the gain pot. But i'd miss the tilt and to a lesser degree the boost. The way the thing is built in several respects it would be a bitch to work on so unless i felt something was a pretty sure thing i don't wanna start tearing it apart. It's also a matter of having 30+ more days on the return policy and i;'m not gonna tear into it till i'm sure i wanna keep it. Theres the catch 22....can't mod it till i decide to keep it, but don't know if i wanna keep it till i can mod it to sound like i want. LOL

[g1]

I dunno. I saw those drawings of the different parts of the circuit like the gain and tilt but i can't remember much. I lost the link to them and i'm trying to find them.

Are they the ones from here? https://www.marshallforum.com/thread...ematic.113535/

[daz]

I think so, but those don't quite match up t the schematic i posted in some ways. Some of those i'm thinking may be the modded circuits by that headfirst guy. I don't see that tone stack in the schematic i posted/.

[g1]

I think so, but those don't quite match up t the schematic i posted in some ways.

I can't expand those because I'm not a member there, but I think the one you posted is in post #16 there?

[daz]

Yes, thats the one i posted.

[nosaj]

Everyone in this thread check your PM's and then collaborate.
nosaj

[pdf64]

Right, so both 1st and 2nd stages are heavily loaded, gain will be dragged down, equivilant to swapping 5751 or ECC81 to V1&2 of a 1987 / 1959.
There’s a big 150ohm HT sag resistor, and the cathode bias may add some squish, so the potential to soften the response any further looks poor.
To me, the use of V2 for the fx loop seems a waste of a valve.
If you like it but need ‘more’, your best bet may be to use a boost pedal, or build yourself a low HT 1987.

[daz]

Right, so both 1st and 2nd stages are heavily loaded, gain will be dragged down, equivalent to swapping 5751 or ECC81 to V1&2 of a 1987 / 1959.
There’s a big 150ohm HT sag resistor, and the cathode bias may add some squish, so the potential to soften the response any further looks poor.
To me, the use of V2 for the fx loop seems a waste of a valve.
If you like it but need ‘more’, your best bet may be to use a boost pedal, or build yourself a low HT 1987.

Loaded by what....the gain pot value? This kind of schematic gives me a headache trying to follow it with all the damn connectors. If i can get that gain back i think i'd gt some preamp squish, (plus i just need some gain, it's dead clean with gain on 10) but even if not at least not have to boost the crap out of the front end till it's noise city at anything but bedroom volume. As to V2I agree and the one guy that has modded these for gain used V2 to make more gain stages then adds a SS loop. No way i'm going there, i just wanna find simple and preferably minimally invasive a way to get max gain from v1/2 so i don't need to ruin the amp's natural tone with pedal distortion.

[daz]

never mind, changed my mind on this idea. Look am my post #12

[daz]

I found a way to get more preamp drive and seems to work quite well. Not really the ideal way because i just wired a route from V1A to B's grid in parallel with that network for the boost and tilt. . Really sounds good tho. It disabled the boost more or less but the tilt still works. I could remove that network which would no doubt add a lot more gain given the 100k gain pot and the other stuff, but this is enough given that the output drives a lot at stage volume and i would not want to lose the tilt. This makes the amp worlds more usable at lower volumes especially and no need for a pedal for my main tone and just a screamer or such to boost for lead tones. Only problem is high volume squeal. The wire i used is only about 2" long but i may try shielding it to help with that but do u think 2" of unshielded wire could cause it? I can lower highs to rid it but don't wanna have to limit those if i find i need more at stage volume.

[g1]

To me, the use of V2 for the fx loop seems a waste of a valve.

That's what I was thinking as well, But with the series loop, the signal is going through V2 as well. So there are 4 triodes available (+ phase splitter) and it should at least be possible to get as much gain as a JCM800 master vol. type.

[tschrama]

The preamp is still (relatively) very linear up to the point were the PA is already clipping with very little EQ in the way. That will sound very direct with no bounce or sag/squish. A additional tube stage with more elaborate signal filtering and a MV will be needed to get a more modern higain attach/bounce.

[daz]

The preamp is still (relatively) very linear up to the point were the PA is already clipping with very little EQ in the way. That will sound very direct with no bounce or sag/squish. A additional tube stage with more elaborate signal filtering and a MV will be needed to get a more modern higain attach/bounce.

But thats the thing....i don't want modern hi gain. I just wanted some gain, about as much as a tube screamer with distortion knob full and volume around unity. Or in other words, about AC/DC rhythm gain. But with gain all the way up the stock amp is dead clean w/o a hint of overdrive. And as i said in a above post i now have that with a one wire mod. And when i boost it i now get a lot of squish and singing lead tones. just not sure how it will be in a band setting with stage volume because i can't test that much here. I may want to do further refinements but for now this is a great improvement. I won't even need this much at stage volume due to the extra drive u get with the master up high.