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preamp cathode bypass cap benefits

I've designed a fairly simple guitar amp that has uses a 12AX7 preamp tube with the tone stack in the middle, a 12AU7 long-tail-pair phase inverter, into a pair of 6V6's.

With cathode bypass caps (25µF with 1.5K) on the two preamp stages, there's too much gain. I could put in a simple resistive voltage divider.

Without the bypass caps, the gain is just right.

I've heard that the bypass caps help reduce noise, but I really don't hear much noise without them.

Are there benefits to the cathode bypass caps, which compel me to use them?

Thanks!
 

kevinkr

Administrator
Paid Member
PRNDL said:
I've designed a fairly simple guitar amp that has uses a 12AX7 preamp tube with the tone stack in the middle, a 12AU7 long-tail-pair phase inverter, into a pair of 6V6's.

With cathode bypass caps (25µF with 1.5K) on the two preamp stages, there's too much gain. I could put in a simple resistive voltage divider.

Without the bypass caps, the gain is just right.

I've heard that the bypass caps help reduce noise, but I really don't hear much noise without them.

Are there benefits to the cathode bypass caps, which compel me to use them?

Thanks!

You indicate that noise performance is acceptable so they aren't needed for this reason anyway. Tone and compression behavior under deliberate overdrive may change, but again if you like the result then it is acceptable. There is no mandatory reason they have to be there - so if performance is satisfactory without them they can be left out.
 

d1camero

Member
2006-11-13 2:22 am
The bypass caps bypass AC signals which allow higher gain through the tube (as you as well have found out). In combination with the cathode resistor you can forum a hi pass filter to allow only higher frequencies to pass. So if you have too much bass you can adjust the cap appropriately.

The formula is:

cut-off freq = 1/ (2*pi*R*C)
 

kevinkr

Administrator
Paid Member
d1camero said:
The bypass caps bypass AC signals which allow higher gain through the tube (as you as well have found out). In combination with the cathode resistor you can forum a hi pass filter to allow only higher frequencies to pass. So if you have too much bass you can adjust the cap appropriately.

The formula is:

cut-off freq = 1/ (2*pi*R*C)

Perhaps not quite that simple, you need to know the thevenin equivalent source resistance of the cathode resistor plus the cathode's internal resistance. Internal cathode resistance is typically fairly close to 1/gm.. IIRC

Pretty sure none of this is relevant to PRNDL's original question about omitting the cathode bypass caps.. :D
 

d1camero

Member
2006-11-13 2:22 am
Perhaps you can be a little more helpful as to how to calculate the cutoff frequency? You reply is a little obscure...


Oh, and it is relevant to his original question - he asked the purpose of the caps. In amp design we use this all the time to adjust to tone of amp stages.
 

kevinkr

Administrator
Paid Member
d1camero said:
Perhaps you can be a little more helpful as to how to calculate the cutoff frequency? You reply is a little obscure...


Oh, and it is relevant to his original question - he asked the purpose of the caps. In amp design we use this all the time to adjust to tone of amp stages.


Just calculate the parallel equivalent resistance using the value derived from gm (the tube's transconductance) and the cathode resistor you are using. IIRC for the typical common cathode 12AX7 it is about 1K, (assuming gm of ~1000 uMhos) so using a 1K cathode resistor would give you 500 ohms thevenin equivalent resistance and then use the equation you posted previously. Most people not "tuning" tend to just deliberately overkill on this value rather than calculate it I suspect.

Point taken on voicing tone, was not thinking of the small value caps sometimes seen in guitar pre stages cathode circuits to fine tune the sound.
 
PRNDL said:
Are there benefits to the cathode bypass caps, which compel me to use them?

No.

If you have too much gain with these cathode bypasses, and enough without them, it's a good deal easier to eliminate them than to be making either voltage dividers or adding NFB networks to make anode followers. Removing cathode bypasses also improves linearity and reduces distortion.
 

d1camero

Member
2006-11-13 2:22 am
Craig, the electric guitar amp forms part of the instrument. High fidelity is bad. It is expected and desired that the amp changes the characteristics of the signal. Sometimes a player wants even harmonic distortion, other times odd. Sometimes heavy clipping, sometimes light clipping. Sometimes linearity is OK, other times it is undesirable (go figure). It is all about the elusive and undefinable "tone".


Lots of tricks are played to get different tone. Multiple gain stages whilst knocking down the signal each stage. Various coupling cap values. Various bypass cap values. Using carbon comp resistors in key areas, as they drift under high voltage. ad naseum.

This however does not apply to acoustic guitar amps. Those tend to be high fidelity, as the tone is in the wood and string of the instrument.
 

KSTR

Member
Paid Member
2007-07-17 2:35 am
Central Berlin, Germany
Depending on the character one wants to have, it is sometimes better to reduce gain of the stage at the output, that is, use a tap on the anode resistor or make a divider with the next stage's grid resistor, while the triode itself runs at full AC gain (down to the desired cut-off freq.) with the cathode bypass and produces the full open-loop curvature.

- Klaus
 
Re: Re: Re: preamp cathode bypass cap benefits

7N7 said:
Miles, I don't understand; surely one of the main causes of distortion is variation of Ra

Not variations in Ra, but rather Gm. This is a problem with every active device: Gm increases on positive upswings (negative for PNP and P-Channel transistors). Adding additional resistance to degenerate the cathode/source/emitter (since Gm= ~1/Rk) reduces that variation and reduces distortion. You can also look at it as a means of introducing local NFB, which also reduces distortion.
 
Re: Re: Re: Re: preamp cathode bypass cap benefits

Miles Prower said:


Not variations in Ra, but rather Gm. This is a problem with every active device: Gm increases on positive upswings (negative for PNP and P-Channel transistors). Adding additional resistance to degenerate the cathode/source/emitter (since Gm= ~1/Rk) reduces that variation and reduces distortion. You can also look at it as a means of introducing local NFB, which also reduces distortion.


Hi Miles,
Your putting it in terms of variations in Gm is one of the best explanations of the effects of an unbypassed cathode resistor on linearity I have seen in a long time.

:D
 
Miles Prower said:
^^^^

Hey Kevin,

That's a common explanation for emitter degeneration from solid state practice. Not so much seen in hollow state practice, but it's still relevant. (See Doug Self's Site).

Yeah I will admit to using cathode degeneration quite extensively and was inspired to do so from my experience designing ss amplifiers in the very early '80's before I made the definitive switch to tubes for audio.