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cathode bias cap value

12/25/1999 9:06 PM
Ricky	cathode bias cap value have any of you seen the amp project on Angela Instruments web site? it shows a switch in the cathode bias section of the 6v6's that takes the 10uf electro cap out of the circuit, leaving the 250 ohm resistor. they call the switch the "vox-twang" switch. my question is, wouldn't that amp kind of sound dead without a cathode bias cap? to take it a step farther couldn't you have a 10uf wired in all the time and have a switch that throws a 25uf in and out of paralel with the existing cathode bias cap of 10uf? would this work? do any of you guys have a favorite value of cathode bias cap in a Deluxe 5E3type circuit?

12/25/1999 9:06 PM

Ricky

cathode bias cap value

have any of you seen the amp project on Angela Instruments web site? it shows a switch in the cathode bias section of the 6v6's that takes the 10uf electro cap out of the circuit, leaving the 250 ohm resistor. they call the switch the "vox-twang" switch. my question is, wouldn't that amp kind of sound dead without a cathode bias cap? to take it a step farther couldn't you have a 10uf wired in all the time and have a switch that throws a 25uf in and out of paralel with the existing cathode bias cap of 10uf? would this work? do any of you guys have a favorite value of cathode bias cap in a Deluxe 5E3type circuit?

12/26/1999 3:02 AM
Ken Gilbert	Switching a bypass cap in and out of the cathode circuit will alter gain IF there is appreciable signal across the shared cathode resistance. If there is no signal voltage across the cathode resistor, then there is no AC to short by the cap. There would be no signal across that resistor IF the output stage is running in class A. Switch over to class AB though and you're gonna get some change in current, which will lead to a change in voltage across the resistor. To answer your other question, you could certainly make only a portion of the cathode bypass cap switchable. In this case it would act like a kind of "thick" control. Ken

12/26/1999 3:02 AM

Ken Gilbert

Switching a bypass cap in and out of the cathode circuit will alter gain IF there is appreciable signal across the shared cathode resistance. If there is no signal voltage across the cathode resistor, then there is no AC to short by the cap.

There would be no signal across that resistor IF the output stage is running in class A. Switch over to class AB though and you're gonna get some change in current, which will lead to a change in voltage across the resistor.

To answer your other question, you could certainly make only a portion of the cathode bypass cap switchable. In this case it would act like a kind of "thick" control.

Ken

12/26/1999 7:17 PM

Bruce

quote:
"There would be no signal across that resistor IF the output stage is running in class A. Switch over to class AB though and you're gonna get some change in current, which will lead to a change in voltage across the resistor."

Huh!?
Well Ken you just baffled the heck out of me with that reply!
Please restate this so mere mortals can understand what you just said. Ha ha

Bruce

12/26/1999 8:46 PM
Ken Gilbert	Sorry Bruce... T'was late in the day and my mind was fading fast. The gin didn't help. In any event, if the output stage is push pull, and running in class a, then it's essentially a well balanced differential amp. As one tube turns on, the other one turns off just as much. The cathodes (which are joined together) sit at pretty much the same DC potential throughout the waveform. Therefore the same voltage is developed across the shared cathode resistance. If the voltage across a fixed resistance does not change, then the circuit could be called "constant current." And due to the fact that there is fixed DC potential across that shared cathode resistance, then shorting out the ac transients (which is one way to look at the function of the bypass cap) is not needed, since there AREN'T any. Adding and removing the cap will not stabilize the cathode voltage--it's already stable. As a result, it won't increase the gain either, since there is no cathode degeneration going on--the cathode's voltage is fixed, and is not decreasing the gain of the stage. All of this gets neatly thrown out the window as soon as one tube starts cutting off. Then, there is not a smooth transistion of plate current from one tube to the other--the nice complimentary handoff of current is no longer so complimentary. As a result, you get a fluctuation of current being pulled through the shared cathode resistor. That fluctuation of current leads to a fluctuation in effective bias voltage which is developed across that resistor. Degeneration starts to take place as one tube cuts off, and the other turns on really hard, since the "on" tube cannot steal any more current from the "off" tube. Once it starts to do this, the cathode will be "pulled" up in voltage, causing the well known degeneration and subsequent loss of gain. At THIS point, adding a bypass cap would obviously reduce the problem, since the tube would be able to pull current out of the cap instead of relying on the other tube. The cap also tends to store the charge across the cathode resistor, keeping the "bias voltage" much more stable. Like any other stage, you can tailor the LF response of this diff-amp by adjusting the value of C. Lower C, faster time constant, higher break frequency, less LF. There was a very interesting paper that I printed out from the golden days that outlined the differences in THD and TID of differential output stages WRT bypass caps. I'll see if I can get my paws on it. I hope that explanation is a little less hard to understand! Ken

12/26/1999 8:46 PM

Ken Gilbert

Sorry Bruce... T'was late in the day and my mind was fading fast. The gin didn't help.

In any event, if the output stage is push pull, and running in class a, then it's essentially a well balanced differential amp. As one tube turns on, the other one turns off just as much. The cathodes (which are joined together) sit at pretty much the same DC potential throughout the waveform. Therefore the same voltage is developed across the shared cathode resistance.

If the voltage across a fixed resistance does not change, then the circuit could be called "constant current." And due to the fact that there is fixed DC potential across that shared cathode resistance, then shorting out the ac transients (which is one way to look at the function of the bypass cap) is not needed, since there AREN'T any. Adding and removing the cap will not stabilize the cathode voltage--it's already stable.

As a result, it won't increase the gain either, since there is no cathode degeneration going on--the cathode's voltage is fixed, and is not decreasing the gain of the stage.

All of this gets neatly thrown out the window as soon as one tube starts cutting off. Then, there is not a smooth transistion of plate current from one tube to the other--the nice complimentary handoff of current is no longer so complimentary. As a result, you get a fluctuation of current being pulled through the shared cathode resistor. That fluctuation of current leads to a fluctuation in effective bias voltage which is developed across that resistor. Degeneration starts to take place as one tube cuts off, and the other turns on really hard, since the "on" tube cannot steal any more current from the "off" tube. Once it starts to do this, the cathode will be "pulled" up in voltage, causing the well known degeneration and subsequent loss of gain.

At THIS point, adding a bypass cap would obviously reduce the problem, since the tube would be able to pull current out of the cap instead of relying on the other tube. The cap also tends to store the charge across the cathode resistor, keeping the "bias voltage" much more stable.

Like any other stage, you can tailor the LF response of this diff-amp by adjusting the value of C. Lower C, faster time constant, higher break frequency, less LF.

There was a very interesting paper that I printed out from the golden days that outlined the differences in THD and TID of differential output stages WRT bypass caps. I'll see if I can get my paws on it.

I hope that explanation is a little less hard to understand!

Ken

12/26/1999 9:38 PM
Ricky	Is there some sort of "rule of thumb" for this cap's value? Fender always used the 25uf cap but I assume there's some room to go either way. I guess what I'm asking is, what do the different values sound like? and, how much is too large a value (what is the window of usable values for this posistion?) thanks for all your help, Ricky

12/27/1999 4:16 PM
Benjamin Fargen	Ricky, I'll give the generic low tech answer...... As you increase the value of the cathode bypass cap you allow more of the low and low/mid characteristics of the circuit to become present. How drastic the effect is depends on the circuit you are dealing with. Lowering the cathode bypass cap from 25uf to say 15uf or 10uf can really clean up the mud in the lower freq when tweaking a power amp section........but sometimes you can start to lose that fatness your looking for as well. Do some tests and decide what sounds best to your ears. A perfect example is the "Clubman" circuit....I ended up using a 10uf cap in stead of the "suggested' 250uf cap and felt the tone was much more pleasant to MY ears. Experiment and have fun with it. Benjamin

12/27/1999 4:16 PM

Benjamin Fargen

Ricky,

I'll give the generic low tech answer......

As you increase the value of the cathode bypass cap you allow more of the low and low/mid characteristics of the circuit to become present. How drastic the effect is depends on the circuit you are dealing with.
Lowering the cathode bypass cap from 25uf to say 15uf or 10uf can really clean up the mud in the lower freq when tweaking a power amp section........but sometimes you can start to lose that fatness your looking for as well.

Do some tests and decide what sounds best to your ears.

A perfect example is the "Clubman" circuit....I ended up using a 10uf cap in stead of the "suggested' 250uf cap and felt the tone was much more pleasant to MY ears.

Experiment and have fun with it.
Benjamin

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