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

[daz]

heres some questions regarding what i did in post #12. If i lifted C51 after the one wire mod i did in post 12, would that 1-add even more gain (seems it shouldn't because nothing in that network is grounded) and if so, much? and 2-leaving it as is with the mod in post 12, would replacing R65 with a 1M add any gain to speak of?

[g1]

Your post about the actual mod you did (post #12) is missing detail. Exactly which points are at each end of the bypass wire you added?

[daz]

Your post about the actual mod you did (post #12) is missing detail. Exactly which points are at each end of the bypass wire you added?

From where C39 and R67 meet at the connector to V1 pin 7 grid.

[g1]

Then C51 is bypassed and will not make any difference. Changing the grid resistor R65 from 470K to 1M should increase the signal level through V1b and beyond. Not sure how noticeable it will be.

[daz]

thanks. Still wanna get a bit more gain because i am also thinking of putting a 470k/470pf filter in series with the grid to get more of that marshall focus and that will lose some signal, so maybe i'll try the 1M in hopes of making up for it. One thing i am having trouble understanding is 1/2 of the boost function is that cap network I bypassed which acts to shape the boost EQ and the actual boost is the other 1/2 of the relay which removes R63 giving V1B more gain. Why is it that since i did this mod the boost function actually DROPS gain now? If anything t should add even more because the cap network is no longer there cutting frequencies, right?

[pdf64]

The signal/ gain loss of the input stage’s signal is being caused by the 100k valve of the vol and tilt pots. Swapping to 1M types would add a good chunk of dB.

Then to free up the 2nd stage, I’d rewire the V2a fx send CF as its DCCF and drive the tone stack from that, as per the classic Marshalls.
If the fx loop is essential then use a regular LND150 source follower circuit.
Another option is to drive the tone stack with the fx return signal, ie directly from the fx return jack, as the Marshall type isn’t too lossy.
Whichever, changing the 2nd stage to a DCCF will add at least 6dB gain, and enable its unusual waveshaping to take effect.

[daz]

The signal/ gain loss of the input stage’s signal is being caused by the 100k valve of the vol and tilt pots. Swapping to 1M types would add a good chunk of dB.

I thought of that, but the thing i worry about is i think the tilt function would then not work right and probably have a hug effect on the amp's gain wouldn't it? I mean, theres a cap between it and ground that is part of the tilt's dark to bright function, but when turned all the way toward that cap it would be the 900k away from the other cap. I think it;s function would be way off in the way it acts as EQ and in the middle would effect gain a lot. No? You know worlds more then I but them i know how it sounds and u aren't aware of that side of things unless u have tried one. The gain pot is the one i thought of but i'm not sure how that would affect tilt, but it would be major surgery that i don't wanna do unless i am 100% sure of the result.

Then to free up the 2nd stage, I’d rewire the V2a fx send CF as its DCCF and drive the tone stack from that, as per the classic Marshalls.
If the fx loop is essential then use a regular LND150 source follower circuit.

Yes, i need the loop and don't want that much gain. Thats what the "headfirst" mod is. He turns V2 into a 3rd gain stage and a CF i think, or maybe 3 gains not sure. It roars but i don't want that. All i want is a tad more preamp drive for low volume play. Not really necassary but if i could do it w/o major surgery i would. What is "LND150 source follower circuit" ?

Another option is to drive the tone stack with the fx return signal, ie directly from the fx return jack, as the Marshall type isn’t too lossy.
Whichever, changing the 2nd stage to a DCCF will add at least 6dB gain, and enable its unusual waveshaping to take effect.

I have tried driving the FX return side by cranking the output of the DSP i use in the loop but the drive it adds is buzzy and crappy sounding.

One other thing i'd like to do if possible is add some clean headroom to the PA. It starts driving like mad by around 4-5 on the master. Would using 6CA7's do that or maybe a change of screen R value? Thanks for your input PDF64.

[daz]

Nevermind, deleted. creating new thread for this unrelated question

[pdf64]

You really like this amp but want the preamp a bit different and the power amp a bit different?

I thought of that, but the thing i worry about is i think the tilt function would then not work right and probably have a hug effect on the amp's gain wouldn't it? I mean, theres a cap between it and ground that is part of the tilt's dark to bright function, but when turned all the way toward that cap it would be the 900k away from the other cap. I think it;s function would be way off in the way it acts as EQ and in the middle would effect gain a lot. No? You know worlds more then I but them i know how it sounds and u aren't aware of that side of things unless u have tried one. The gain pot is the one i thought of but i'm not sure how that would affect tilt, but it would be major surgery that i don't wanna do unless i am 100% sure of the result.

Apart from death and tax, nothing in life is certain
But the gain and tilt are taken from the 5F10 but with the values scaled. The 5F10 values work well, though a linear taper vol pot is only really suitable for a super low gain amp.
https://el34world.com/charts/Schemat...rvard_5f10.pdf

Yes, i need the loop and don't want that much gain. Thats what the "headfirst" mod is. He turns V2 into a 3rd gain stage and a CF i think, or maybe 3 gains not sure. It roars but i don't want that. All i want is a tad more preamp drive for low volume play. Not really necassary but if i could do it w/o major surgery i would. What is "LND150 source follower circuit" ?

I wasn’t suggesting turning V2A into a gain stage, rather to use it in its current function, a CF, but move it to buffer the 2nd gain stage from the extreme loading of the tone stack, as per any classic Marshall.
Regarding the LND150, see almost any fx loop, the send circuit is typically that.

One other thing i'd like to do if possible is add some clean headroom to the PA. It starts driving like mad by around 4-5 on the master. Would using 6CA7's do that or maybe a change of screen R value? Thanks for your input PDF64.

I hate the term headroom, guitarists use it to mean almost anything.
Technically, more headroom requires more power output, and that would require a higher HT voltage / a lower OT primary impedance.
However, beam pentode output valves tend to sound a bit different to suppressor grid types like the EL34, though part of that may be due to different bias and hence drive requirements. And guitarists might describe that change in terms of headroom, despite power output not being materially affected.
Dunno what modern 6CA7 are?
I’ve seen one brand is even offering a beam type EL34, they should be a straight swap into your amp. Other valve types such as KTxx, 6L6 etc may need the cathode bias resistor changing.

But bear in mind that there’s a NFB loop around the power amp, so unless you’re pushing the power amp hard, that will be trying to counteract such stuff.

[daz]

Well, theres not a single amp of the god knows how many i have owned that i didn't want some changes, even the ones i really liked. But this one has certain things i really like that are not available in a single package (at least not at a price a mortal can afford) and i felt i could get a sound from it that is one i have always wanted from a marshall but none ever got as close to as i wanted. This gets closer but like all other amps there are things i want to be slightly different. Thats all there is to it, not a big deal. As for headroom (sorry) i'm not sure if i will need more till i use it live but the PA starts getting really overdriven quick and while i like that sound, i am not sure if it will be too much at the volume my band plays at. Not that it;s really loud, just saying i'm not sure where the amp will be as far as PA breakup at that point. I however embarked on a new baffleboard yesterday for a 12 and putting my celestion 100 watter in there which will get me a bit more volume that may help some. By the way, it's a 4 ohm speaker and the amp only offers 8 and 16 ohm options but l saw on the schematic there is a 4 ohm tap ! All i had to do is swap the 4 and 8 ohm connectors. The 4 is only a docking place for that wire, so a simple swap and i now have a 4 ohm jack. That was a pleasant surprise.