Cathode Bypass Cap Question

[wattsup]

I've been looking at schematics from other amp manufacturers. Some of the higher gain amps sometimes have one or more resistors before, after and sometimes before and after the cathode bypass caps. Most of the time on a switch. Is this to change the gain or the tone or both?

[pinkphiloyd]

Being in parallel with the cathode resistor, those resistors that you switch in and out of series with the cathode bypass cap effect both the gain and bypass frequency of the stage. (As I understand it.) In his book, Merlin actually talks about using a pot in series with the cathode bypass cap to use as a sort of variable boost.

[wattsup]

Being in parallel with the cathode resistor, those resistors that you switch in and out of series with the cathode bypass cap effect both the gain and bypass frequency of the stage. (As I understand it.) In his book, Merlin actually talks about using a pot in series with the cathode bypass cap to use as a sort of variable boost.

I really need to get Merlin's books!

[pinkphiloyd]

Being in parallel with the cathode resistor, those resistors that you switch in and out of series with the cathode bypass cap effect both the gain and bypass frequency of the stage. (As I understand it.) In his book, Merlin actually talks about using a pot in series with the cathode bypass cap to use as a sort of variable boost.

I really need to get Merlin's books!

I can't recommend his preamp book enough. You can read the first chapter that covers this online.

http://www.valvewizard.co.uk/gainstage.html

[Ken Moon]

I agree about Merlins's preamp book.

What I like is that all the maths are there, in a form that doesn't require a knowledge of calculus.

Then there is metric shitload of practical info, so if you want to skip the maths, you can do that too.

In my copy, I marked all the practical info with a yellow highlighter like a school girl, then went back and re-read each section working through the formulas.

I think I'm on my 4th or 5th reading, and every time I get a clearer understanding of how it all fits together.

[V2]

if you want to switch a bypass cap in and out, you will get a 'pop' unless there is a constant path to ground. so the R that follows the cap is usually large in value so that the cap doesn't affect gain or tone but doesn't pop when switched in and out of the circuit.

[wattsup]

if you want to switch a bypass cap in and out, you will get a 'pop' unless there is a constant path to ground. so the R that follows the cap is usually large in value so that the cap doesn't affect gain or tone but doesn't pop when switched in and out of the circuit.

Now that makes sense!!

[tubeswell]

Another vote for Merlin's pre-amp book(s*) (* either edition)

See also the attached 2008 Audio xpress article on cathode bypassing by Merlin and D I James, along with a handy table from Bruce Collins of f-rolloff points with Rk and Ck values

[Tone Lover]

I will 3rd. that motion . I have both and read thru them all the time. I use them as my main referance books.
Sometimes I almost think I know what Im doing after reading them. (LOL) then there's that dose of reality when I forget the next day and have to look it up again.
Bill

[roberto]

Rk Ck file has no sense.
It's a shelving filter, and it changes with Rp.

[tubeswell]

'rp' is definitely relevant - that's why Bruce says at the bottom of the sheet that these values are for 12AX7s. In which case assume rp=62.5k for that table

Whereas changing 'Rp' and/or 'Rl' changes the overall gain, not the half-boost frequency from the combination of Rk||Ck.

[roberto]

With your last post you are confirming me that that table is a nonsense.

I was talking about Rp, not rp: raising Rp and lowering Rk you can make the 12ax7 work to never reach a -3 dB point between bypassed and unbypassed frequencies.

[tubeswell]

Oh I see where you're coming from - yes I guess the table has limits.

[roberto]

do you like Italy?

[tubeswell]

Never been fortunate enough to travel to Europe. I'm in my 50s and shackled with university-aged kids to support for at least another 5 years or so (and possibly longer), so I'm long past any aspiration to travel great distances. Hopefully California is still on my bucket list, as its closer, and its the Mecca of Surf Music and the land of vintage Fender amps and guitars, so I'd be more likely to enjoy my holiday and bring back a few souvenirs.