PreAmp Cathode bias Capacitor

[big_teee]

The .68uF in the cathode creates a low frequency shelf, not a pure rolloff. I always liked the sound of a fully bypassed cathode as a first stage. If you need a shelf, you can add a parallel cap/resistor in series with the coupling cap to the next stage. YMMV and will probably be less...

Thanks everyone for getting Me tuned in on the ByPass Cap.
I have been reading about it, at the Valve Wizzard Starting at page 25.
http://valvewizard1.webs.com/Common_Gain_Stage.pdf
I will experiment further.
Terry

[hasserl]

Trying to understand the effect of the Preamp Cathode Cap.
Marshall 1987 Circuit puts a .68uf Coupling cap in parallel with the 820 ohm resistor from the Cathode to ground on V2a.
What does the Cap do, and what effect do you get when you change the Cap Smaller or large in uf Value?
The Marshall 2204 puts the .68uf Cap on the Cathode of V1a, none on the 820 resistor of V2a.
Thanks for any explanation of the bias Cap on the cathode.
Thanks,
Terry

Just to clarify, on the Marshall 2204 circuit (at least on the schematic I'm looking at now and every Marshall schematic I recall seeing) the 820 ohm cathode resistor is bypassed with a 330uf cap, the 2k7 cathode resistor is the one bypassed with .68uf.

[big_teee]

Just to clarify, on the Marshall 2204 circuit (at least on the schematic I'm looking at now and every Marshall schematic I recall seeing) the 820 ohm cathode resistor is bypassed with a 330uf cap, the 2k7 cathode resistor is the one bypassed with .68uf.

Your looking at the wrong 2204.
I built the JCM800 2204 with the cascaded Input.
http://mirror.stones-amp-inn.de/mars...l_50w_2204.pdf
It is different from the JMP 2204.
http://site.triodestore.com/JCM8002204Stock.pdf
T

[tedmich]

Your looking at the wrong 2204.
I built the JCM800 2204 with the cascaded Input.
http://mirror.stones-amp-inn.de/mars...l_50w_2204.pdf
It is different from the JMP 2204.
http://site.triodestore.com/JCM8002204Stock.pdf
T

so many Marshalls...so little time!

[big_teee]

so many Marshalls...so little time!

I agree and I would like to build most of them!
Keep Rockin!
Terry

[hasserl]

Your looking at the wrong 2204.
I built the JCM800 2204 with the cascaded Input.
http://mirror.stones-amp-inn.de/mars...l_50w_2204.pdf
It is different from the JMP 2204.
http://site.triodestore.com/JCM8002204Stock.pdf
T

Hmmm, I'm still seeing a 2k7 rk bypassed by the .68uf cap.

[soundguruman]

A smaller cap produces less gain, and less low frequency response
A larger cap produces more gain, and more low frequencies
Changing the cap from .68 to 25 for example will make the input like a mesa boogie, but will also produce a lot of oscillation and ringing noise.
Taking the cap out will kill the gain, and it will be almost like a fender twin reverb, super clean...
and so it's a compromise, between too much and too little gain, and too much or too little low frequencies...
.68 turned out to be just right.

[big_teee]

Hmmm, I'm still seeing a 2k7 rk bypassed by the .68uf cap.

With All Due Respect, you need to read the OP.
I was talking about bypassing the V2a 820 resistor, like the 1987 Circuit does.
V1b is Fine.
I was trying to boost the treble and mids and get more gain.
They do this to some of the amps on some of the other Sites.
Terry

[soundguruman]

It won't give you more treble and mids specifically but it will give more gain overall, if you increase the value. Like change .68 to 2.2 uf
Change . 68 to .47 will give you LESS gain, less lows, by a small amount only. Almost unnoticeable difference.
I think what you want is to adjust the tone stack. Try the tone stack calculator:
TSC
you download and install the application, after that you can experiment with different values, and read the result in real time.

[big_teee]

I'm just tinkering with my home build.
I need to get some more parts but I do want to experiment further with the bypass Caps.
Was trying to get a little more gain out of it.
I have the extra tube socket hole I may try adding the other gain stage next.
I will play with the TSC if I can figure out how to run it on Win. 7.
T

[soundguruman]

It's not hard to run the TSC. You just click on the circuit component, and input whatever number you want.
Increase the value of the plate resistor, and you will have more gain, presto.
Happy hollidaze.

[Chuck H]

It won't give you more treble and mids specifically but it will give more gain overall, if you increase the value. Like change .68 to 2.2 uf... Change . 68 to .47 will give you LESS gain, less lows, by a small amount only. Almost unnoticeable difference.

Any change in gain will be directly related to knee frequency of the capacitor. Meaning that a larger cap will indeed produce more gain, but ONLY at frequencies corresponding to the new lower knee frequencies. Changing from .68uf to 22uf absolutely will not increase the gain of frequrncies above the knee of the .68uf cap. It will only increase the gain of frequencies below the knee of the .68uf cap.

Your statements seem a little misleading. You cannot, for example, increase treble gain by changing from a .68uf cap to a 22uf cap, or a 2.2uf cap. You can only increase gain at the frequncies not covered by the original cap.

[big_teee]

Merlin calls it partial, or fully bypassed.
The .68uf will only partially bypass.
T

[soundguruman]

Any change in gain will be directly related to knee frequency of the capacitor. Meaning that a larger cap will indeed produce more gain, but ONLY at frequencies corresponding to the new lower knee frequencies. Changing from .68uf to 22uf absolutely will not increase the gain of frequrncies above the knee of the .68uf cap. It will only increase the gain of frequencies below the knee of the .68uf cap.

Your statements seem a little misleading. You cannot, for example, increase treble gain by changing from a .68uf cap to a 22uf cap, or a 2.2uf cap. You can only increase gain at the frequncies not covered by the original cap.

I don't understand what you are trying to say.
Changing the cap (increasing the value) will in fact increase the gain across the entire frequency spectrum. Test it yourself it works.
"not covered by the original cap" WTF
Also increase the value of the plate resistor will increase the gain, but 100K is usually the limit cause the thing will start oscillating.

[J M Fahey]

I don't understand what you are trying to say.

Or you *pretend* not to, just to rock the boat.
You seem to find it fun.
Well, if you have *nothing* else to do, maybe itīs a way of filling long, boring days (and nights).

Changing the cap (increasing the value) will in fact increase the gain across the entire frequency spectrum. Test it yourself it works.

No, it does not.
We (and countless others) have already tested it.
Chuck h īs is the accurate description.

"not covered by the original cap" WTF

You may WTF as much as you want, but it will still be true.

Also increase the value of the plate resistor will increase the gain, but 100K is usually the limit cause the thing will start oscillating.

Absolutely FALSE.
100K is popular because itīs a good compromise value.
220K is about the max. used, simply because of the law of diminishing returns.
In fact, going for a too high load resistor value will slightly *decrease* high frequency response (thus making it less likely to oscillate, all other things equal) , because stray capacitances will still be about the same.
Going to 470K will increase gain only a very small amount and start to sound noticeably dull.
Music Manīs "limiter" tube clipping circuits did exactly that, and had that undesirable side effect.
Because in any RC circuit, leaving C constant and raising R *lowers* the crossover point.
Just in case you did not remember it.

Good luck