Preamp kathode caps and resistors

[Bluefinger]

Hi,

Would somebody care to elaborate, please ...

Usually in a tweed amp ( I have no experience with other types) the cathodes have a 25µF cap in parallell with a 820R or 1,5K resistor. What changes could I expect from changing either of their values (or completely omitting one of those components)? I would like to try myself but can you tell me please, if there are maximum or minimum values I can use there without causing problems or damage?

Any info is appreciated!

many thanks in advance!!!

[Wilder Amplification]

Hi,

Would somebody care to elaborate, please ...

Usually in a tweed amp ( I have no experience with other types) the cathodes have a 25µF cap in parallell with a 820R or 1,5K resistor. What changes could I expect from changing either of their values (or completely omitting one of those components)? I would like to try myself but can you tell me please, if there are maximum or minimum values I can use there without causing problems or damage?

Any info is appreciated!

many thanks in advance!!!

The cathode bypass cap decouples AC across the cathode resistor when under signal. Basically it works like a filter cap in that it filters out the fluctuating voltage drop from the cathode resistor when under signal, which holds the cathode voltage constant. This removes the degenerative negative feedback caused by a fluctuating cathode voltage by holding the cathode at a constant voltage. As such, this provides an increase in gain.

A value like a 25uF basically decouples everything ALMOST down to DC. By dropping the value of it, you raise the frequency at which this gain boost starts to come into play. Frequencies above the gain boost frequency receive the gain boost (i.e. bypassed gain) whereas frequencies below the rolloff frequency receive the gain of the stage as if the cap wasn't even there (i.e. unbypassed gain). This allows you to shape the tone of the stage to a frequency range better suited for guitar frequencies rather than boosting everything in the range of human hearing.

With an 820R resistor, I'd drop down to say a 2.2uF (gain boost start frequency of 88Hz) and try lower from there. If at 2.2uF the tone lacks "body" to it, you can go higher, say to like a 4.7uF or a 10uF.

The resistor itself biases the stage. A lower value Rk will bias the stage hotter and provide a bit more symmetry/headroom to the signal to a point (i.e. the relationship of the positive/negative swings relative to the zero crossing point). Too high and the positive swing will clip before the negative swing does (i.e. asymmetrical clipping). A higher value will bias the stage colder, and will also add some asymmetry to the signal as the negative swing will clip before the positive will.

The higher the Rk value, the lower the value of Ck must be to keep the gain boost frequency the same, and vice versa.

[Bluefinger]

Hi,

Thanks a lot for the detailed explaination. This makes perfect sense.

If I split the cathodes of V1 and use different values, maybe I can use this for voicing the normal and bright channel differently. I never really liked what the bright cap across the volume pot does and different coupling caps only do not make a lot of difference.

Now I have something to tinker for tonight ...

The only caps with values between 1 and 10 µF I can find here are the ones from Conrad (which is something like Radio Shack in the States). Would you think that those cheap caps would make a lot of difference or will anything with the right value do? I do notive a small difference between different makes of the so called "tone caps" but have not compared different electrolytics yet.

all the best

[Hardtailed]

Hi,

Thanks a lot for the detailed explaination. This makes perfect sense.

If I split the cathodes of V1 and use different values, maybe I can use this for voicing the normal and bright channel differently. I never really liked what the bright cap across the volume pot does and different coupling caps only do not make a lot of difference.

Merlin has a great graph on his site showing the varying response with different bypass caps:
http://www.freewebs.com/valvewizard2/PartialBypass.jpg

This is with a 1.5K cathode resistor. The frequency will shift proportionally as you change that. A lower value (like say a 820ohm) will increase the frequency where the shift occurs, and a higher value will do the exact opposite.

Classic Marshall "Plexi" amp used this splitting trick. The schem I used for mine (a 50W Plexi from the 60's) has a 2K7 resistor with a 0.68uF cap for the bright channel, and a 820R with a 220uF for the normal channel (note that this gives a decoupling down to a fraction of an hertz, this is way bigger than necessary).

That said, I did an AC30 amp for someone recently and I mixed both channels before the Top Boost (making it sort of like a Voxified Plexi). The cathodes are shared on V1, the difference between bright and normal channels were the coupling cap (a mere 500pF on the Brilliant channel) and a really small 100pF bright cap on the volume control (500K, so that makes it equivalent to using a 200pF on a 1M).
There is still a huge difference in tone between the two channels, even though both go through the same 1K5 resistor and 22uF bypass cap.
Of course, the big factor here is the diminutive 500pF coupling cap (one of the secrets behind the Vox Top-Boost sound)

I personally love bright caps, they tighten things up imho. The problem is when you go too high (some later Marshalls were in the 4nF range, that's way too much). Try lowering it instead of removing it altogether.
The 100pF on the AC30 clone is really subtle and allowed me to get really interesting clean tones with my Les Paul.

Usually, when going above 1uF, most people will use electrolytics for size reasons. This cap will only see a couple of volts, so a small 25V part will work. Some bypass it with a smaller film cap but it's somewhat rare.

[Bruce / Mission Amps]

I've been giving this chart out for years but here it goes again...

[Bluefinger]

This is really great info and even makes sense to me .... THANKS A LOT!!!

I have already tried to tinker with these components before but did not get the desired results. I did not know that I need to vary the values this much. Not it works great.

So I have experimented with different coupling caps, cathode Rs abd Cs and bright caps. The cathode Rs and Cs and the coupling caps seem to enable me to cut bass frequencies and the bright cap across the volume pot seems to boost top frequencies. This all can help to brighten the overall tone of a channel.

So I wonder, is there anything I can do in terms of preamp voicing to go in the other direction? I mean, can I boost the low frequencies and cut the high ones? It seems like there isn't much more I can do beyond a regular .1 coupling cap, 820R/25MF cathode and no bright cap.

thx!

[Bruce / Mission Amps]

This is really great info and even makes sense to me .... THANKS A LOT!!!

I have already tried to tinker with these components before but did not get the desired results. I did not know that I need to vary the values this much. Not it works great.

So I have experimented with different coupling caps, cathode Rs abd Cs and bright caps. The cathode Rs and Cs and the coupling caps seem to enable me to cut bass frequencies and the bright cap across the volume pot seems to boost top frequencies. This all can help to brighten the overall tone of a channel.

So I wonder, is there anything I can do in terms of preamp voicing to go in the other direction? I mean, can I boost the low frequencies and cut the high ones? It seems like there isn't much more I can do beyond a regular .1 coupling cap, 820R/25MF cathode and no bright cap.

thx!

Boosting?.. well that's kinda why God invented active EQ.
But it sounds like you understand the boosting of overall gain and then shelving it, bypassing it or sending it to ground somewhere.
Mostly it's about shelving bass and sending highs to ground or around a resistance.
Remember that... (generally) .... what ever is left over from shelving, shunting or bypassing tone tweaks, is what you want to amplify some more.

[Bluefinger]

Well, boosting was a bad term to use here.

Getting rid of what you don't want and amplifying the rest ... it's as simple as that. I gotta keep that in mind, then most of my questions are already answered anyway. Thanks for getting my brain back on track

[Russ]

Hardtailed:

...and a really small 100pF bright cap on the volume control (500K, so that makes it equivalent to using a 200pF on a 1M).

Shouldn't that be a 50pF cap on a 1M pot?
double the resistance => halve the capacitance, & vice versa

[Hardtailed]

Hardtailed:


Shouldn't that be a 50pF cap on a 1M pot?
double the resistance => halve the capacitance, & vice versa

Woooops
I'm sorry, you are indeed correct!