Question regarding a schematic

[Doommachine]

Hello all!
This forum is super cool, I have had a good look around and learned some interesting stuff, and I did search but unfortunately haven't been able to answer this one.

I have a carvin VM100, and while I've already modified the gain channel as per the very helpful Mr Hasse, I need MORE gain.

I've replaced the clipping diodes with a 1m resistor and also replaced the gain pot with a 1m value.

What I'm thinking is that I could add a "post gain" pot - overdrive V3a.

I've increased the gain by increasing the grid leak resistance for V1a and V2a, so my thinking is that this is the grid leak resistor for V3 - if I replace this with a 1M pot I can then increase gain on V3. Just confirming as I'm not certain it will work the way I think given it goes right into the phase inverter.


Thanks in advance!

[thetragichero]

r34 appears to be the tail resistor of a long tail pair phase inverter. r26 would be the place for a volume control prior to v3a but it wouldn't get any more gain (do you mean distortion?)
if you provide a full as-modified schematic folks might be able to provide pointers for more distortion

[Doommachine]

Yeah, of course, more distortion - the way I envisioned it V3a could act as another "gain" stage. There's not really room on the board to just add another valve


The amp is basically stock with some finagling of the cathode bypass capacitors. I did start with Hasse's suggestions for coupling caps but went back to the stock values as I preferred the sound. I honestly bought the amp because it was missing caps and didn't work, I didn't expect to like it as much as I do!


So I am guessing then that due to V3 being right before the phase inverter it's gain cannot really be adjusted? I sort of assumed as much.

If I replace R26 as I understand things it will have the same effect as P6, ie, the channel 2 volume control? Sort of a post reverb volume, I guess technically then I could turn the preamp volume up louder? This would then drive V2.A harder, yes?

(Note that I have a Rev.A, this is a Rev.B schematic and the earlier model does not have the additional mid boost circuitry which I have crossed out)

(Edit: I realised I have missed something major, I've rewired the heaters to suit 6n2p-ev's. I have a few that are "balanced" and they sound good in the PI, and generally speaking I like the midrange sound of them. But I don't think relevant to question asked hehe)

[thetragichero]

So I am guessing then that due to V3 being right before the phase inverter it's gain cannot really be adjusted?

i mean, r26 could be replaced with a 1M pot but all the way up would be the same as just having the resistor there

so with drive and volume 2 cranked there's not enough distortion?

[Doommachine]

i mean, r26 could be replaced with a 1M pot but all the way up would be the same as just having the resistor there

But then I could use the second pot as a master volume - turn it to 2 or 3, which would restrict output volume, and crank V2 up, which would increase preamp gain?

so with drive and volume 2 cranked there's not enough distortion?

The amp doesn't have a master volume, and I don't really have the space to turn V2 past 3

[thetragichero]

ahhhhh i get it now. i would look into the trainwreck type 2/lar mar master volume, replacing r37 and r38 with a dual-gang pot

[Doommachine]

Ahhh now that's an interesting idea. I'll look into that! Many thanks for the help.

[Doommachine]

OK, spent some time looking into the lar/mar.

It sounds like a good solution, but I have both a presence control as well as the Hasse "attitude" mod for the NFB circuit. From what I understand the Post PI MV will affect the NFB somewhat and I'm pretty happy overall with the voicing with the presence halfway, plus I can get a little more with it maxed.

Now I'm a little torn. I think perhaps I'll start by replacing R26 with a pot and see how it sounds. I'll still be able to crank both the volume controls on channel 2. If it's not enough, I'll have to sacrifice presence for distortion

Now that I think about the entire circuit, it was kinda silly to focus on V3a as it feeds the NFB circuitry anyway.

Thanks! I feel very enlightened.

[Doommachine]

OK so I've made my mods and I'm mostly pleased.

I haven't bothered making the diagram for the "master" volume, just replaced the 1m resistor with an A1m pot. I used a 1nf bright cap.

The audio taper pot isn't really sufficient to do much - attenuation only really occurs at about 1, after that it's pretty much full volume. I think I might swap to A150k and add a switch with a 1m resistor across it for "max volume" if desired.
It really does make a great difference though, cranking the now "post EQ" gain control up fattens the gain sound up nicely. It's no 5150 but I dunno.. pretty close. It's late and I haven't really had the chance to crank it up yet.

I realised that I had followed Richard Hasserls mod and replaced all the plate resistors with 100k, no wonder I had no chonk! Now she has plenty gain.

Replacing the mid pot gave slightly better control range in the 5-7 range. With the stock pot I could dime it and it was still "reasonable" sounding, I guess I have a thing about pots that go to places that are pointless

Note: I have a Rev A and only one presence control. I solved this by cutting the trace from pin 6 to earth on relay L2.A and instead bridge the 0.01uf cap to earth; then bridging pin 8 to earth. If you have a Rev A and want some pics or help let me know. I thus end up with about 70pf value of presence cap on the clean channel. Of course if you have a Rev B and like the presence on channel 1 don't change the cap, but IMO the amp needs more presence.

The rev A cathode coupling caps are also different, I swapped them out to match the Rev B as by my estimations it would be OK, and I'm pleased enough.

I admit to using unreasonably high values for bright caps, but I like it really biting and jangly with the gain at 3-4.

I played around with the plate bypass caps but Richards values honestly work really well, anything over 150pf and it just loses too much shimmer, and under that it's a little bright. YMMV, depending your guitars of course.
I highly recommend you also install the "attitude" mod.

Thanks also to Richard if you're here and reading


Any suggestions or comments welcome, please try this out especially if you have an unloved valvemaster sitting around! Even if you're not a metal guy, with the gain set about 7 it has a pretty usable classic lead tone. You just have the extra gain in reserve

[Doommachine]

Hello all,
I have done some more finagling of values.

My test rig was initially a basswood bodied greco with Lace Deathbuckers tuned to E standard which does not really have a modern metal sound. After trying it with other instruments - I have an agile 8 string with duncan blackouts, and an alder jackson with fishman fluence tuned to B standard, I realised I was sort of trapped within the sound of that specific instrument.

The three loud valve stages really just didn't provide the ideal tone - fizzy and the bass response was a bit flubby - and so I went with a cold clipped second stage with atypical values (330k plate and 39k cathode). It was interesting the difference just changing the clipped stage made to the overall tone and feel of the amp - the marshall style 100/10k didn't have enough saturation, soldano style 100/39k (obviously) gave a severe volume drop.

I swapped in a 100k "master" pot before the PI and dumped the bright cap as well which gave a much more useful range. (Will add a switch for a 1m resistor in case I need it really loud.)
Further, the extra mid control actually is useful, the extended presence control has a good deal of interplay with it.

All I really need to finish it off is a depth knob just to tweak the low end - I suspect it will have a similar interplay with the bass control.

Now with "gain 2" at about 8 with gain 1 at 10 the saturation and overall tone is excellent.

If people are interested I shall record a sound clip and provide an updated schematic.

I started modding this thing because the gain channel sounded so cruddy I figured I had nothing to lose, now I am really into it!