freestompboxes.org

Hi everyone,
So, last year i aquired this beautiful thing:
It's a Meteoro(a brazilian manufacturer) Vulcano G100 100w 2x10, and if you look closely to its panel, it clearly shows its roots: it is basically a Valvestate 8080 copy (a weird copy, with a Valvestate preamp going into a Gallien Krueger 400RB power amp but it works).
And i've been doing all sorts of things to it like modding the clean channel to be more fenderish(wich took away a lot of the channel's headroom, and i'm still trying to figure out how to fix that) and adding a simple high pass filter to the power amp to kill the awful sub-bass, and also adding a cathode bypass cap to the tube's first stage, and so on.. And it already sounds pretty good the way it is, but i think it could be better.
So now, i wanna try this:
original schematic: https://www.thetubestore.com/lib/thetub ... ematic.pdf

The way this tube(currently a russian 6n2p, originally came with a 6bq7) stands in the curcuit right now it isn't doing much(just a clean stage followed by a buffer), so the idea here is to extract as much gain as possible from this single tube by turning it into 2 gain stages with its own gain control, followed by an opamp buffer(to substitute the cathode follower's function). And this should work, however, i have no idea of how big of a signal is gonna come out of that 2nd tube stage(and all i have to work with are a soldering iron and a cheap multimeter, so i can't really measure that preciselly), and if that is going to clip that opamp. And if does so, i wanna know about a more balsy buffer circuit with lots of headroom that would be able to withstand that without distorting the signal.

So, what do you guys think? Can this work? Suggestions?

Juarocks wrote: ↑27 Jan 2021, 13:17it isn't doing much(just a clean stage followed by a buffer),

The DC-coupled cathode follower is a bit more than that, and some say it's the key to the "Marshall Sound".
More info there : http://www.valvewizard.co.uk/dccf.html

Those bypass caps will add a lot of gain for sure.
If the tube is clipping, the signal level wont be any greater coming out of the tube stages. However, you could replace R13 and R14 with a pot to control the level, or just R13 as a variable resistor actually...

JiM wrote: ↑28 Jan 2021, 22:07

Juarocks wrote: ↑27 Jan 2021, 13:17it isn't doing much(just a clean stage followed by a buffer),

The DC-coupled cathode follower is a bit more than that, and some say it's the key to the "Marshall Sound".
More info there : http://www.valvewizard.co.uk/dccf.html

Yep. That's why i was trying to find a way to adequately replace it, and now i think i did!

Optical wrote: ↑28 Jan 2021, 23:20 Those bypass caps will add a lot of gain for sure.
If the tube is clipping, the signal level wont be any greater coming out of the tube stages. However, you could replace R13 and R14 with a pot to control the level, or just R13 as a variable resistor actually...

Oh cool! But the boost channel already has a level control (VR10), right after the countour circuit.

I made some progress: got a tip at some other forum suggesting that i could use a source follower instead of the opamp so as to boost current and not voltage. So i came up with this: I'm not sure if C7 is really needed in this case though

Right now, your feedback loop is going to create positive feedback and you will get oscillation.

Maybe you meant to connect to the first stage cathode?

From my experience, you will get unpredictable results when enclosing a gain control inside a feedback loop.

for voltage values I measured look at this post:
viewtopic.php?f=2&t=24002&p=268055&hili ... te#p268055

You could also experiment with just removing the global negative feedback loop around tube stages of the original design. It's what keeps the gain so low that the stages run clean with the given input signal range (limited by clipping LEDs).

The solid state input stages are also already introducing the archetypal "hi-pass" response of Marshall tube amps with low cathode bypass caps and treble peaking filters. I'm not sure there's need to repeat it. When signal hits those LEDs it's already pre-emphasized to near perfection.

Personally I think this series of Marshall amps has awesome distortion as is and totally sufficient amount of gain on tap. The whole thing would be fine even without any tubes. No need to layer tons of extra distortion to it just for the sake of tubes.

Anyone ever noticed that the 8100/8080 circuit is up to the shunt clipper pretty much JCM900 DR "Boost" channel, and even clipping arrangements of the two aren't all too different having nearly same forward voltages in total? (LED vs. three diodes in series). I doubt that difference is even perceivable...

teemuk wrote: ↑01 Apr 2021, 13:22 You could also experiment with just removing the global negative feedback loop around tube stages of the original design. It's what keeps the gain so low that the stages run clean with the given input signal range (limited by clipping LEDs).

The solid state input stages are also already introducing the archetypal "hi-pass" response of Marshall tube amps with low cathode bypass caps and treble peaking filters. I'm not sure there's need to repeat it. When signal hits those LEDs it's already pre-emphasized to near perfection.

Personally I think this series of Marshall amps has awesome distortion as is and totally sufficient amount of gain on tap. The whole thing would be fine even without any tubes. No need to layer tons of extra distortion to it just for the sake of tubes.

Anyone ever noticed that the 8100/8080 circuit is up to the shunt clipper pretty much JCM900 DR "Boost" channel, and even clipping arrangements of the two aren't all too different having nearly same forward voltages in total? (LED vs. three diodes in series). I doubt that difference is even perceivable...

I quit this project a long time ago after much tinkering. Got some sounds out of it(mostly oscillation, like bmx pointed out), but nothing usable. Only after i quit that i found out marshall had already done exactly what i was trying to do but in the Valvestate 2000 series(avt50 schem: https://elektrotanya.com/PREVIEWS/63463 ... .pdf_2.png ), and from what i could hear from some old bad quality videos on YT, that change didn't make a lot of difference to the sound(at least not enough to make it worth the trouble ). But anyway, learned a lot with this, the most important thing being: "If it ain't broke don't fix it" haha.

Reverted my amp back to stock and then did some smaller mods like adding a 1uF bypass cap to R70 and upping R109 to 220k. Also made some small changes to the clean channel's tone stack to make it more "responsive". Works great!

I wish there was a way i could get rid of that tube now haha

Did you try removing the feedback? Cathode bypassing and changing plate resistor have lesser effects when the stage gain is largely determined by negative feedback loop: 1uF bypass makes open loop gain higher at higher frequencies but does not alter closed loop gain much. 100k to 220k plate load increases open loop gain but closed loop gain remains the same. (You do get altered bias point, though) "Open the loop" and your mods have more effect.

i did not! But i'll try next time i open it up. Should i disconnect R69(1M) as well as C54(100n) and R95(220k)?

R69 should stay (it sets the reference voltage for the grid).
Disconnecting one leg of either R95 or C54 is sufficient to break the feedback loop, but beware of the high voltage on the side of R71.

I tried disconnecting the feedback loop and that didn't work. It gets really farty whenever you hit a low note.

But, i did add Modern/Vintage switch to the tone stack of the dirty channel that basically switches between the classic Marshall TS and the Mesa Recto Orange channel TS, and it works great! Sounds modern like the Recto but a lot tigher and less gainy. Later i can record some samples and draw some schematics of all the mods i did if you guys want. o/
up = modern, down = classic

Juarocks wrote: ↑09 Apr 2021, 00:37draw some schematics of all the mods i did if you guys want. o/

Please do !

Alright, as promised, here are some samples!
And also, i finally managed to get my gain mod working, so that's what you'll be hearing, with no pedals whatsoever except for the two samples in wich i used a modded Marshall GV2 as boost. Cheers!
Samples recorded directly from the power amp(Direct Out - my amp didn't have this, so i added it) into FL Studio, using Amplitube 4 cab simulation, without any post EQ.
Also, i demonstrate the subtle diferences between the classic and modern tonestacks.
OD2 gain can only go a little above half way before it starts oscillating, however, i added a trimpot to the circuit that adjusts that. But, since i would rather have all the gain i need in one "channel", i left it dimed.
Even though it has TONS of gain now, it still cleans up real nice by lowering the guitar's volume.

Sounds great!
What is the setting value of Trim1?

Manfred wrote: ↑05 May 2021, 06:55 Sounds great!
What is the setting value of Trim1?

You can set it as you will, it will work at any setting except 0(cuts out).

Update: I did further mods to the clean channel and to the drive channel's tonestack.
Schematics:
Clean Dirty + new TS Here's a little sample, with some generic riffing going from the clean channel to the dirty channel(OD2) and then adding the GV2.
Reverb added in post. No post EQ. I'm very pleased with how all of this turned out. The amp sounds great and can do it all: from fenderish cleans, to classic Plexi/800 rock tones, and also modern scooped high gain.

I would also like to, if possible, change the topic title to FULLY WORKING gain mod. Thanks!

Thanks for sharing, great work.
Can you please also share the read voltage values of Ub, Ua and Ug of both stages?

Manfred wrote: ↑13 May 2021, 22:45 Thanks for sharing, great work.
Can you please also share the read voltage values of Ub, Ua and Ug of both stages?

Thanks man! Glad to help!

I would if i could cus' unfortunately i don't have a multimeter handy(yes i did all of this blindly, fortunately it didn't catch on fire ).
But something around 110v can be expected on the plate of the second stage.

I did something different to my valvestate 8080...

turned it into a 2ch dirty shirley