Please check this extra gain design for jcm 800

[leftu2]

Hey guys, I built a 6V6 jcm 800 a few yrs ago using the plans from rob robinette. I've modded the design along the way as well. I want to add a pre-amp tube for an extra gain stage, reason being is to get the amount of distortion I want. Right now I have to stack drive pedals to get distortion I want.
I want the same amount of distortion with 1 pedal instead of having to stack pedals. I want as much asymmetrical distortion as possible.  I love everything else about the design I have now minus the extra distortion I need in the amp. I know some of you guys might recommend changing values of some components in the basic design but I don't want to do that . I figure with the extra gain stage being the 1st gain stage, it will be easier to tweak that to work with what I have now and not upset the balance of what i already have.

I'll post a png file of my initial design with the added gain stage. The extra gain stage design was taken from someone off the marshall amp forum. The designer gives very specific instructions as to how to tweak the components to get where you want. I'll post his design as well that has his instructions.

**I have a question about his design. He shows the C1 capacitor bypassed with a resistor. What's the difference doing that vs bypassing the resistor with a capacitor which is what I normally see in amp design on preamp tubes? 

Thanks for looking....

[pdf64]

.. I want to add a pre-amp tube for an extra gain stage, reason being is to get the amount of distortion I want. Right now I have to stack drive pedals to get distortion I want.
 …
**I have a question about his design. He shows the C1 capacitor bypassed with a resistor. What's the difference doing that vs bypassing the resistor with a capacitor which is what I normally see in amp design on preamp tubes? 

Increasing its gain will reduce a circuit’s stability, hence you will tend to run into problems with oscillation.
The inputs are in close proximity and have 2 common cathode high gain stages arranged in cascade between them. Stray capacitance will cause unintended coupling between the inputs, and as they will be in the same polarity, that may result in oscillation.
A typical method to mitigate that is to reduce bandwidth somehow, ie roll off some high frequencies, eg with a 100 to 470oF cap between anode and cathode of V1.

Electrons don’t know their left from right, up from down, backwards from forwards  😀

Referring to any passive circuit element (resistance, capacitance, inductance) as X and Y :-

X in parallel with Y is the same as Y in parallel with X.

X in series with Y is the same as Y in series with X.

[leftu2]

Increasing its gain will reduce a circuit’s stability, hence you will tend to run into problems with oscillation.
The inputs are in close proximity and have 2 common cathode high gain stages arranged in cascade between them. Stray capacitance will cause unintended coupling between the inputs, and as they will be in the same polarity, that may result in oscillation.
A typical method to mitigate that is to reduce bandwidth somehow, ie roll off some high frequencies, eg with a 100 to 470oF cap between anode and cathode of V1.

Thanks for the reply....
When you talk about the caps being in the same polarity, i assume that refers to the orientation of the caps?
I used this process to do the caps that are in the amp already.
https://el34world.com/Forum/index.php?topic=11427.0

this is the discussion on the marshall forum I got the idea from  to do this in case your interested.
Several guys were trying to help a guy out trying to add a gain stage to his jcm.

https://www.marshallforum.com/threads/2203-extra-gain-stage-mod.79853/

[AlNewman]

You'll probably want to run a grid stopper from the input to your new v1.  Maybe the 68k + 470pf that was originally on your old circuit which isn't being used now might work fine.  You may want to remove the capacitor altogether, or perhaps run it to ground, as a high frequency shunt rather than a high frequency bypass.

The relationship between C1 and R1? is the same whether the cap bypasses the resistor, or the resistor bypasses the cap.  the only difference is, is that the tube can run without the cap, but it can't run without the resistor.  The bypass cap increases gain at a frequency reactive to the cathode resistor, which in this case is in the range of 30 hertz, or basically full guitar frequency.

[pdf64]

When you talk about the caps being in the same polarity, i assume that refers to the orientation of the caps?

No, I was referring to the signal polarity.
As stock, the gain between the tip terminals of the input sockets was about 60, and the signals were in opposing polarity (hence any feedback due to stray coupling will be negative).
By adding a gain stage, the gain between the inputs is now about 25 x 60 = 1500,  and the signals are in like polarity. Hence feedback due to stray coupling will tend to be positive, which can lead to instability, oscillation.

this is the discussion on the marshall forum I got the idea from  to do this in case your interested. 

Can you point to something specific, I’m not so interested that I want to read through a 300 post thread!

[fossilshark]

See ceriatone chupacabra. Make your first 2 gain stages to resemble that. Plenty of gain.