Different Resistor/Cap Configurations at the Cathode

[shef86]

Hello,

I'm modding a four gain stage amp to have the first two gain stages of a bogner xtc and the lead section of a mark IV in the last two gain stages. While constructing this mod I ran into an interesting configuration at the cathode with a 2.2 electrolytic capacitor in parallel with a resistor. If I do the mod as planned I will have these two configurations on back to back gain stages:

4.7k in parallel with a 2.2 uF
1.6k in parallel with a 2.2 uf cap

So I'm just curious what effect this will have on the tone, and if it will be over kill to place this in back to back gain stages. My guess would be it's a low pass filter, similar to placing a cap in parallel with a plate resistor, but that's just a guess.

While we're on the topic, I'm also curious what happens when you change the resistor value on the cathode, I've heard that lowering it can make the amp to be "hotter" or more touch sensitive.

Thank you in advance for all the help!

[tubeswell]

Bypass caps

Cathode Bypass Caps hold the cathode voltage constant (against swings in voltage that you otherwise get from an unbypassed stage, as the latter tends to follow the +ve and -ve swing at plate, which changes when a signal is applied at the grid). The effect of adding the bypass caps is increased gain and decreased negative feedback. The value of the bypass cap affects the frequency of the stage. Smaller value caps will boost higher frequencies and higher value caps will boost freqs all-around. If you want to use 2.2uF in combination with 1k6 or 4k7, go right ahead. 0.68uF in combination with 2k8 is a typical 'marshallesque' set up which accentuates mids and highs more. 25uF in combination with 1k5 is typical for fender cleans.

The value of the cathode resistor affects the bias of the stage (i.e. how much -ve the voltage at the grid is compared to the voltage at the cathode), and you have to know the B+ and plate resistor value, and what type of tube you're dealing with, in order to work the effect of that out.

In general, the smaller the resistor (down to a certain minimum - usually 820R for a 12AX7 normal gain stage) the 'hotter' the bias (i.e. more tube current) - Too much current can result in exceeding the ability of the tube to dissipate energy, and death to the tube . With colder biasing (larger value resistors up to a certain value, but rarely above 39k in a 12AX7 - and in those instances is commonly un-bypassed) you get less tube current.

My understanding about how the bias affects the tone, is that it depends on the load resistance at the plate.

Higher load resistance (i.e. bigger plate resistors up to a certain point - usually 220k in a 12AX7 normal gain stage) tend to 'even out' the shape of the signal wave at the plate compared to what is at the grid, whilst increasing the amplitude of this wave up to a certain point and leads to sooner signal clipping. The bias of the tube in these cases generally makes less difference to the overall degree of harmonic distortion (than it would if you have a lower plate load).

Lower value plate resistors (not usually lower than about 47k for a 12AX7 normal gain stage) tend to decrease the amplitude of the signal at the plate. But also, depending on the bias, you can get a more asymmetrical signal wave shape at the plate compared to what is at the grid, which results in more harmonic distortion.

[shef86]

Nice and technical, just how I like it. I really appreciate the help and knowledge. It's interesting because the last gain stage has a .22uf so now I know what that is doing.

I was looking around forever for a 2.2 uf coupling capacitor, ordered it, and then realized that I needed an electrolytic...live and learn.

[tubeswell]

Nice and technical, just how I like it. I really appreciate the help and knowledge. It's interesting because the last gain stage has a .22uf so now I know what that is doing.

I was looking around forever for a 2.2 uf coupling capacitor, ordered it, and then realized that I needed an electrolytic...live and learn.

Coupling caps are different from cathode bypass caps.

Coupling caps take the signal from the plate and allow it to pass to the grid of the next stage without letting the DC voltage through (so you are just left with a signal swing going to the next stage's grid - except in some cases like a 'direct coupled pair' - gain stage and cathode follower hooked together without a coupling cap). Again smaller values of coupling cap let higher frequencies through and not lower frequencies, whereas bigger value coupling caps let a broad range of freqs through - which can result in the signal being too muddy, as bass freqs more easily distort the whole caboodle.

[camnicklaus]

while you're explaining bypass/coupling caps and their effect on frequency...Why do cathode bypass caps seem to run such a wide range of values with less impact on frequency (higher than 25uf and as low as .6) where as coupling caps seem to rarely be above .1 and the effect can be heard pretty easily just dropping down to .02? mostly I'm curious about coupling caps, cuz I think I understand that the bypass caps are seeing relatively little signal?

[tubeswell]

while you're explaining bypass/coupling caps and their effect on frequency...Why do cathode bypass caps seem to run such a wide range of values with less impact on frequency (higher than 25uf and as low as .6) where as coupling caps seem to rarely be above .1 and the effect can be heard pretty easily just dropping down to .02? mostly I'm curious about coupling caps, cuz I think I understand that the bypass caps are seeing relatively little signal?

Well my take on this, is that the cathode bypass cap is in parallel with a resistor and the whole caboodle is in series with the tube. The cathode bypass cap's job is to hold the cathode voltage constant. The part of the cathode voltage swing that they hold constant compared to the part of the voltage swing they have no effect on, depends on their capacitance. The voltage swing at the cathode is dependent on the voltage swing at the grid affecting the voltage at the plate, which in turn affects the current through the tube, which is in turn affected by the inter-electrode capacitances and the overall plate voltage and the cathode resistor voltage. So for starters a cathode bypass cap is operating under different conditions, and is doing a different job, to a coupling cap, the latter which is 'letting' A/C, of given frequencies, pass across the capacitor, which is directly hooked up to the plate on one side and directly hooked up to the grid on the other side, and is not in parallel with any one resistor. Anyone else care to chime in?

[boyer]

Yeah, thanks for the info. Helps out a bit. I've been slowly transforming the first channel of my Bassman 50 into a distortion channel. I've replaced almost all the cathode bypass caps with something like 2.2uf/16v already, but I like the idea of using the Marshall style bypass, maybe in conjunction with a bigger plate load resistor. Currently it has a 100k, but I might try 220k on the plate, and 0.047 with 2.2k on the cathode.

[Merlinb]

while you're explaining bypass/coupling caps and their effect on frequency...Why do cathode bypass caps seem to run such a wide range of values with less impact on frequency (higher than 25uf and as low as .6) where as coupling caps seem to rarely be above .1 and the effect can be heard pretty easily just dropping down to .02? mostly I'm curious about coupling caps, cuz I think I understand that the bypass caps are seeing relatively little signal?

With a typical ECC83 gain stage, a bypass cap of more than about 1uF already bypasses pretty much all frequencies covered by the guitar. It's common for manafacturers to use an arbitrarily large value, since small electrolytics cost almost nothing. Hence, values from 10uF to 47uF crop up all the time. For more treble boosting effect you have to resort to values between about 1uF and 100nF, and there aren't that many common values within that range.
You don't see much range in coupling caps because very large, high-voltage caps are expensive. No need to use anything bigger than about 22nF, to pass all useful frequencies. To audibly cut bass you're looking at values in the range of 4.7nF to 1nF, which is a narrow range, so you don't see the same range of values across the board. (It's easier to alter the value of resistances in the circuit, to get the desired freq' response, because more resistance values tend to be available.)