Here are some things you could do:
1. Lower the value of the cathode resistors on the two preamp gain stages (R24 and R19). Try 1.5k for R24 and 820 ohms for R19.
2. Lower the value of R20. The R20/R21 voltage divider dumps 1/2 of the signal. I would tack in a 500K linear pot, with the wiper going to the grid of the second stage (V4a pin 2), and adjust until happy, then take the pot out (without adjusting it further), measure the two "halves" of the pot, and substitute fixed resistors at a value close to what you measured. For example, R20 could end up being 100k, and R21 390k.
3. Lower the value of the power tube bypass cap to 10uF. Or you could remove it completely - this should smooth out the distortion character (EL84s can be harsh), at the expense of slightly lower overall volume (but not gain).
4. You may want to add a grid stopper resistor to the input of the second gain stage (I would try 100k), and make sure it is soldered directly to the tube socket pin - this will allow you to crank the gain of the first stage more, without blocking distortion.
Sonic effect of changing the screen grid R from 1K to 100?
Moderators: pompeiisneaks, Colossal
Re: Sonic effect of changing the screen grid R from 1K to 100?
[quote="Alexo"]Why don't you just put a ground lift switch on the tonestack if you think it's what's throttling the tone?
Alexo,
Sure! How? I would try this right away. I'd guess I lift the grounds on the mid pot and/or the bass pot. But then how are the stages being coupled? Do I ground somewhere else with a spdt?
I was also thinking this: I have a dpdt push pull on the gain to switch the bright cap in and out. I used a dpdt since I need 500KA and a dpdt guitar pot is all there is out there. I think I could use it, on one side, to switch everything between the plate of V4a and the grid of V2 i.e. the entire tonestack and the MV, and, on the other side, to switch one cap and a resistor like a 5F1. But, while I understand how a switch works I'm pretty clueless as to how coupling works, what values I'd use for the cap and R, where the resistor goes and if just copying the 5F1 is correct here? Image 2.
I also worry I would have to redesign the first stage so that it sounds good played both ways. That would be too much effort. It’s a clean build I’d hate to butcher it. Image 2 and http://picasaweb.google.com/ardpan/AX84 ... directlink
Maybe I should start a new post on this with it's own header but i'd like to try the ground lift first and see what I hear.
Thanks
Alexo,
Sure! How? I would try this right away. I'd guess I lift the grounds on the mid pot and/or the bass pot. But then how are the stages being coupled? Do I ground somewhere else with a spdt?
I was also thinking this: I have a dpdt push pull on the gain to switch the bright cap in and out. I used a dpdt since I need 500KA and a dpdt guitar pot is all there is out there. I think I could use it, on one side, to switch everything between the plate of V4a and the grid of V2 i.e. the entire tonestack and the MV, and, on the other side, to switch one cap and a resistor like a 5F1. But, while I understand how a switch works I'm pretty clueless as to how coupling works, what values I'd use for the cap and R, where the resistor goes and if just copying the 5F1 is correct here? Image 2.
I also worry I would have to redesign the first stage so that it sounds good played both ways. That would be too much effort. It’s a clean build I’d hate to butcher it. Image 2 and http://picasaweb.google.com/ardpan/AX84 ... directlink
Maybe I should start a new post on this with it's own header but i'd like to try the ground lift first and see what I hear.
Thanks
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