Adjusting pre-amp tube voltage

[Tweedguy]

I'd like to increase the pre-amp gain in my (self-built) 5E8-A Fender Twin. I changed the value of the plate load resistors by half and got an immediate fat, distorted sound. So, I know that works. But I read you shouldn't change the plate load resistor to get this result but instead only the dropping resistor that supply's all 12AX7s should be changed. Why is this?

I guess there is a delicate balance of voltage going on throughout the various stages and changing only the voltage on the first pre-amp tube would throw off the other stages? Some have told me it's OK to tweak this first stage; "if it sounds good, then it's good", but I'd like to know the whole story.

Can anyone tell me if changing just the plate resistor on the first pre-amp tube is OK for modifying your tone? If not, why is it bad?

-Thanks

[mhuss]

Change anything you like -- it's your build! Doing what you've described won't harm anything, and if it sounds good, use it.

--mark

[Ears]

All you have to watch out for is that you remain within plate power dissipation limits, apart from that it's as Mark says.

Read the common cathode tutorial here
http://www.valvewizard.co.uk/

Download Terman on voltage amps this thread
https://tubeamparchive.com/viewtopic.php?t=3214&start=0

[Andy Le Blanc]

to double gain or half gain you have to times four or divide by four....
most charts were on values were made to keep you in the area of
least distortion and best freqency responce....
a higher percent distortion colors the signal thru the stage....
keep within the tubes voltage and draw ratings and tweek away....

[Phil_S]

Lowering the cathode R increases gain, and hiss with it. The 5E8, having a shared cathode R/C for all 4 sections of the first two 12AY7's makes tweaking these tubes difficult unless you separate them.

You might try 1k2 or 1k in place of the 1k5 on the cathode of the third 12AY7. Before I did this, I'd see what one or more 5751's sounds like as a sub for any or all of those 12AY7's.

You might also try 12AT7, but the plate R for this tube is considerably lower and, while plug compatible, it isn't the most appropriate sub. 5751, IMO is a much better choice.

[Firestorm]

I'd like to increase the pre-amp gain in my (self-built) 5E8-A Fender Twin. I changed the value of the plate load resistors by half and got an immediate fat, distorted sound. So, I know that works. But I read you shouldn't change the plate load resistor to get this result but instead only the dropping resistor that supply's all 12AX7s should be changed. Why is this?

This is sort of apples and oranges. When you increase a dropping resistor, you do lower the voltage at that supply node (and all subsequent nodes). This can make a tube distort easier because it reduces the available output voltage swing and the tube can clip (depending on where it's biased). But this not at all the same as increasing a plate resistor. When you do that, you increase the gain of the tube and you increase the output swing. In fact, if the tube's cathode resistor is fully bypassed (as it is in the 5E8-A), that's just about the only thing you can do to increase gain. Changing Rk will move the bias point and affect current through the tube, but with a bypass cap in place it has almost no effect on gain (it does if there's no cathode bypass cap, though).

What you did was to increase the output of the first stage thereby driving the next stage harder, an excellent starting point for tweaking. As others have pointed out, as long as you don't cross any "boundaries," you can safely do that to your heart's content.