CL2 and OD1 cathode values toggled

[BobW]

I think dogears has a great idea to either toggle the CL2 or OD1 cathode cap values, so why not take it a step further and toggle both at the same time? This could possibly help single coils vs humbuckers retain the tone? Present values could be 2, 5 and toggled to 5, 2 by simply switching a 3uF in parallel with a 2 value? Something like this:

[Normster]

Hmmm...interesting. I may try that. Probably should also throw in a 4M7 to ground to keep the caps from popping when you throw the switch.

[dogears]

Actually, I toggled CL2 and OD2 at the same time. I put 1.33uf on each CL2 and OD2, I then toggled a 3.3uf on top of one or another.

Fwiw, I REALLY prefer a little more uf on the 1uf size. 1.33uf sounds fatter to me than 1uf. It is very noticeable.

[BobW]

My bad, Dogears, I thought it was OD1. I agree about the small cathode values, makes a huge difference in tone. Either way, it a great suggestion. thanks

[caglej]

Check out this table for the -3 db points for cathode bypass caps on a 12ax7.........it should be helpful..
Look at the difference between the 2.2uf and the 1.0uf values with 3.3k and 2.7k resistors
The low E on a guitar is around 80 Hz........

http://www.aikenamps.com/RC-FHz.htm

[dogears]

Would be more helpful if they used the sweeter sounding 220k and 150k plate resistor values. Thanks for the chart.

Check out this table for the -3 db points for cathode bypass caps on a 12ax7.........it should be helpful..
Look at the difference between the 2.2uf and the 1.0uf values with 3.3k and 2.7k resistors
The low E on a guitar is around 80 Hz........

http://www.aikenamps.com/RC-FHz.htm

[sgp]

Here's the equation used to get those values. Sorry to bring up an old thread, but hey, it's new to me.
Forgot to mention, you can change the tube params and plate res. values with this.

Deleted defective equation!

[dogears]

Hi SGP,

Your equation is not quite accurate. Here is the correct one.

highpass breakpoint=1/(2*PI*Ck*Rout)

Ck = bypass cap
Rout = Rk'llRk

Rk' = (Ra +ra)/ (mu+1)

Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl.  If there is no Rl, Ra = Rp.

ra = tubes internal plate resistance

Rk = cathode resistor

By paralleling the cathode resistor with the Rk' (The tubes cathode impedence) you get much different numbers. I have checked my calculations btw. Not doing it this way is a big waste of time.

Case in point- the knee of a 1uf bypass cap on CL2 with a 150k plate and 2.2k resistor. 220k resistor feeding 100k trim PreOD net.

Wrong way = 148hz
Correct way = 170hz

As you can see, the difference is substantial. Just being anal retentive....

Here's the equation used to get those values. Sorry to bring up an old thread, but hey, it's new to me.

[sgp]

Your correct, I did NOT take the following stage input impedance into account. I was actually messing with a fender 5e7 (can I say that on this board?) which has the 1 meg volume pot as the next stage.

[dogears]

FYI, the breakpoint for the circuit you mention is 211hz when taking the 1meg pot and tube resistance into account. The extra stuff only changes it from 205hz. So, you are correct. The 1meg pot and lower Ck make the figure "close enough". In the Dumble topography, the much lower next stage input impedences seen in the OD section make the calculation more critical.

Your correct, I did NOT take the following stage input impedance into account. I was actually messing with a fender 5e7 (can I say that on this board?) which has the 1 meg volume pot as the next stage.

[sgp]

This is mo better.
Thanks for catching that.