How important is the common cathode resistor/capacitor to the 5E3?

On a 5E3 does it make much difference if both inputs have their own 1.5k resistor and bypass capacitor or is the shared 820 ohm resistor and 22uF capacitor part of the sound of the amp? With the resistors bypassed I am thinking there is no local negative feedback to worry about and the resistor is only a concern for dc bias. Am I missing something?

That's right - the bypass cap is there to boost gain/prevent cathode-current-feedback to the point at which the RC filter is set to roll-off.

No problem with each side having its own RK/CK combination - just a matter of real estate. And it provides flexibility for having different Rk/Ck combinations on each side.

Good. Now one little wrinkle to my question. Can you share the resistor/capacitor when you have the two gain stages cascaded? In my thinking all the two gain stages care about is the bias point and the capacitor shorts the AC to ground so you should not get any feedback between the two as long as the capacitor is big enough.

Paralleled or cascaded, it does not matter. The cathode resistor's function is to set the bias point of each triode. And the presence of a bypass capacitor only affects negative feedback due to voltage drop secondary to current flow. So even in the situation on cascaded triodes sharing a common cathode R, if the cap is large enough to be full range, the effect will be full range, less than full range and the result on each triode will be a shelving effect based on the gain of each stage (because you can still go with different plate resistors, if you desire.)

If you have a shared Rk/Ck with cascaded stages, you'll get oscillation (because the stages are inverting). You really need separate cathode resistors/caps for that.

I was pretty sure you would get feedback between the stages but the floating paraphase circuit caused me to think twice, mind you I have not really looked at that circuit too deeply. Maybe after a cup of coffee.

If you have a shared Rk/Ck with cascaded stages, you'll get oscillation (because the stages are inverting). You really need separate cathode resistors/caps for that.

Yeah, this. Been there, done that!

Cascaded gain stages seperate the men from the boys. Lead dress issues that don't matter in low gain amps become critical when you max the gain.

I was pretty sure you would get feedback between the stages but the floating paraphase circuit caused me to think twice, mind you I have not really looked at that circuit too deeply. Maybe after a cup of coffee.

Paraphase circuits with a common Rk can oscillate if you don't have the gain balance right. A lot of "D" series fender amps have separate Rks in the paraphase inverters.

Will not try and chance it. Just wanted to keep as close to the 5E3 circuit since I am doing enough changes some might question it still having that iconic sound.

So what have I been up to lately? Well I got a small pot with a push-pull DPDT switch on it and it got me to thinking, Could you get the right value pots with switches and use the switches to convert a 5E3 to a Blackface amp? Well it is not 100% one or the other but might be close enough.


http://i406.photobucket.com/albums/pp142/printer2_photo/5E3BFtone.jpg

Could only get 500k pots, adjusted the BF tone stack values to get reasonably close. So you would have a 5E3 amp with Volume, Tone and Volume controls, you pull each switch on the pots, and then you have a BF preamp (sorry, still the 5E3 power amp without NFB) with Volume, Treble, and Bass controls.

Still need to work out which switch to put on which pot. Do not want a combination that causes the amp to go out in La-La land. Tried to get a Harvard Volume and tone option but have not come up with it yet. Still a work in progress but seems 95% of the way there.