Reducing Grid stopper plus raising capacitance

[Emetal]

OK as we know 12ax7 is a dual triode. First triode I will call V1A and the second triode I will call V1B. Well I am looking at an amp where capacitor was put on the V1A side but the Grid stopper value was reduced on the V1B. Why would somebody do that? Wouldn't we wanna reduce the first (input) Grid stopper?

[roberto]

Which amp? Who made the mod? What's the triodes' configuration?
The V1B's triode had already a grid stopper before the mod?

The main reason IMO is that he tried that mod and it sounded good to him.

[Emetal]

Amp is Laney VH 100R, for legal reasons will not mention who made the mod, The V1A is the first triode that guitar signal goes in to, as far as V1B there is some switching going on not shore - here's the scheme attached, both triodes had factory default 68K grid stoppers...

[Stevem]

The cap was put in for two possible reasons and the two reasons could be delt with at the same time by that one cap.
1) the small value cap will bypass some of the unneeded sharp high end response from that gain stage.
2) That gain stage was oscillating and that cap cutting the high end also snubbed the frequency of oscillation that was taking place.
A amp can oscillate at a frequency above what the speaker can output and I have worked on amps that did so to the point that the oscillation was using up all of the amps wattage for the most part and only a faction of guitar sound could be heard when played!

On V1B the value of the grid stopper resistor could now be dropped after the excess high end tone, and or oscillation was taken care of, as its best not to have a grid stopper at all if the circuit layout is done right for the amount of overall gain the amp has.
This brings up the point that if that cap on V1A was put in place to kill a oscillation issue, that is a band aid way to solve that issue, as it should be corrected by having a better wire layout, filtering or grounding in place!

[Emetal]

Stevem that's all fine but the question I asked was, why lowering down the grid stopper on V1B ? Why not lowering the stopper on V1A ?

[Merlinb]

Stevem that's all fine but the question I asked was, why lowering down the grid stopper on V1B ? Why not lowering the stopper on V1A ?

Because mods are often performed with abandon rather than forethought!

[roberto]

The VH100R stock has the 47pF across V1.

MM (the Mysterious Modder) lowered that cap and reduced the grid stopper of V1B to increase the high-mid content.

What's strange with that?

[Emetal]

The stock 47pF is still there, and the 22pF is added between Anode and Grid. That's fine, Miller capacitance and jada jada....
Let's leave capacitors out of the story.
All I wanted to know is WHY lowering grid stopper on V1B WHY not on V1A? Is there a some specific reason.. that's all I wanna know..

[Merlinb]

All I wanted to know is WHY lowering grid stopper on V1B WHY not on V1A? Is there a some specific reason.. that's all I wanna know..

Some people think 68k on the input is 'sacred' because Fender used it, so they are scared to alter it.

[roberto]

and 820R is sacred because Ken used it after the volume/gain.

[Structo]

Isn't a small cap from grid to plate a kind of feedback circuit?

Usually used for stability?

I would get back to the original schematic and go from there.

[rdjones]

Isn't a small cap from grid to plate a kind of feedback circuit?

Usually used for stability?

I would get back to the original schematic and go from there.

In a sense, yes.
I call it the "Miller Mod" because it mimics a larger Miller capacitance.
The Miller effect amplifies the grid-to-plate capacitance by the transconductance of the tube.

One popular mod places a small cap in this position, very small 5-10pF.
I did it on a 5150, by request.
If it were me I would have just upped (or placed) the grid stopper.

The address the OP's Q, 47+22pF is a lot of capacitance across a 12AX7.
I imagine it really kills the high end.
But if this is a "metal" amp that may be just the ticket ;->
I don't see any reason to go that high, though.

rd

[rdjones]

Let's leave capacitors out of the story.

It's not that simple.
The capacitance (stray capacitance + the tube's internal interelectrode capacitance + any intentionally added cap) and the series resistance interact directly with the tube's transconductance, which is quite high in a 12ax7.
This determines the overall circuit high frequency response, ie rolloff.

It really looks like an experiment to get back a bit of the high end lost due to the excessive Miller capacitance.

BTW, a high Miller cap and grid stopper work better in later stages of a high gain overdrive path, IMO.

rd

[roberto]

The VH100R has lots of highs.
Laney decided to cut lows (that is good) but not to control so much the highs.

Good with humbs, but single coils may sound too trebly and harsh.

On this amp I usually mod 2nd and 3rd stages with snubbers (I start to set the 3rd channel, then I place the other snubber to be ok with the 4th channel).

This is simply because if you act so much on the 1st stage, you affect both 3rd and 4th channel, IMO not the best option if you want a dynamic crunch and a focused lead.

[Stevem]

Ditto.
That's what I do also, as I find its better to let at least the first gain stage and tube color the tone the way it will so I can still judge the normal reaction when I audition different tubes!