OK just to clarify, I want to build this amp using some vintage Acrosound TO-310 transformers I picked up and want to use their vintage published schematic. I'm not asking for a review of the overall design or suggestions on changing to another "better" topography. And while I feel I have a good understand of single ended amps, I'm just starting to understand and work with push pull stuff. These bypass cap values don't make sense to me.
So again, I'm simply trying to understand their suggested bypass caps. They have a 1K resistor being bypassed with a 250uf cap which is good to 6.4hz, I get that one. But then on the output tubes, they have a 250R being bypassed by a 100uf, which is only down to 64hz. I am clearly missing something here, as that doesn't make any sense to me.
This design was used by many people and tested well with this iron, so I don't think there is a problem with the design, just my understanding of it. TIA
So again, I'm simply trying to understand their suggested bypass caps. They have a 1K resistor being bypassed with a 250uf cap which is good to 6.4hz, I get that one. But then on the output tubes, they have a 250R being bypassed by a 100uf, which is only down to 64hz. I am clearly missing something here, as that doesn't make any sense to me.
This design was used by many people and tested well with this iron, so I don't think there is a problem with the design, just my understanding of it. TIA
I think the formula you are using is for a high pass filter which is a cap and resistor in series, not a cap and resistor in parallel as is the bypass cap and Rk.
See this thread:
https://ampgarage.com/forum/viewtopic.php?t=26974
sbirkenstock, try thinking about it this way...
Cathode bypass caps are essentially a high pass filter for AC signals shorting to ground. The value of capacitor (100uF, 60uF, etc) is chosen based on the value of the resistor used and the frequency you set at a cutoff of -3dB.
the formula used for determining the half boost frequency is (providing a given resistor and capacitor value):
fg = 1/(2*Pi*R*C)
and the formula used for determining the bypass capacitor value given resistor value and chosen cutoff frequency is:
Cathode bypass capacitor value = 1/(2*Pi*Resistor Value*Frequency)
Ck = 1/(2*Pi*R*F)
for instance lets say you have a shared cathode resistor of 160R and you want to bypass all frequencies down to 10Hz.
Ck= 1/(2*3.14159*160*10)
Ck=99.47uF nearest standard value is 100uF (giving you a -3dB of 9.95Hz)
now lets say we need to choose a bypass cap giving the same 10Hz cutoff frequency, but we have a 350R cathode resistor.
Ck= 1/(2*3.14159*350*10)
Ck=45uF(essentially)
easy peasy, my man. You dig?
https://ampgarage.com/forum/viewtopic.php?t=26974
sbirkenstock, try thinking about it this way...
Cathode bypass caps are essentially a high pass filter for AC signals shorting to ground. The value of capacitor (100uF, 60uF, etc) is chosen based on the value of the resistor used and the frequency you set at a cutoff of -3dB.
the formula used for determining the half boost frequency is (providing a given resistor and capacitor value):
fg = 1/(2*Pi*R*C)
and the formula used for determining the bypass capacitor value given resistor value and chosen cutoff frequency is:
Cathode bypass capacitor value = 1/(2*Pi*Resistor Value*Frequency)
Ck = 1/(2*Pi*R*F)
for instance lets say you have a shared cathode resistor of 160R and you want to bypass all frequencies down to 10Hz.
Ck= 1/(2*3.14159*160*10)
Ck=99.47uF nearest standard value is 100uF (giving you a -3dB of 9.95Hz)
now lets say we need to choose a bypass cap giving the same 10Hz cutoff frequency, but we have a 350R cathode resistor.
Ck= 1/(2*3.14159*350*10)
Ck=45uF(essentially)
easy peasy, my man. You dig?
I believe that is the same formula this calculator is using
https://www.learningaboutelectronics.com/Articles/Bypass-capacitor-calculator.php#answer
My experience with an SE amp has been the lower the resistor value, the higher the cap needs to be to shunt the same AC hz to ground, this schematic is the opposite? Again there must be something going on other than the obvious.
Reading that thread maybe this is what is going on with a shared resistor/bypass cap in a PP amp:
"When both tubes are conducting for the full waveform, with linked cathodes, the balanced signals will largely cancel out.
In this condition a bypass cap has very little effect, as there's little Vac to send to ground.
As the signal level increases past the point at which the tubes are in cut off for some portion of the wave, for the time they're in cut off the cancellation will stop and the cap will begin to make a difference."
https://www.learningaboutelectronics.com/Articles/Bypass-capacitor-calculator.php#answer
My experience with an SE amp has been the lower the resistor value, the higher the cap needs to be to shunt the same AC hz to ground, this schematic is the opposite? Again there must be something going on other than the obvious.
Reading that thread maybe this is what is going on with a shared resistor/bypass cap in a PP amp:
"When both tubes are conducting for the full waveform, with linked cathodes, the balanced signals will largely cancel out.
In this condition a bypass cap has very little effect, as there's little Vac to send to ground.
As the signal level increases past the point at which the tubes are in cut off for some portion of the wave, for the time they're in cut off the cancellation will stop and the cap will begin to make a difference."
Note that the cathodes appear in parallel to the cathode resistor, and each looks like about 1/Gm.
At the almost class A bias of this amplifier, not much peak output power is lost by not using any bypass capacitor, and two advantages are gained. Overload of the output stage is more gradual (but sooner by several dB). And output stage bias no longer shifts around with signal level, bleeding down with the RC time constant.
All good fortune,
Chris
At the almost class A bias of this amplifier, not much peak output power is lost by not using any bypass capacitor, and two advantages are gained. Overload of the output stage is more gradual (but sooner by several dB). And output stage bias no longer shifts around with signal level, bleeding down with the RC time constant.
All good fortune,
Chris
Not sure what I would recommend for your Nostalgia project; only you can decide how far you'll allow yourself to stray from the 1950's original design. If you could contemplate using separate, bypassed, cathode resistors for the output valves you'd improve DC balance through the OPT over the life of the output valves, but that's a step away from the original, museum-quality stated intent.
Back in the Eisenhower era when these were designed, electrolytic capacitors were expensive and paper/oil capacitors were both big and expensive, so there was plenty of incentive to keep values as small as could be gotten away with. "Optimizing" wasn't really possible for cathode bypass caps because the "optimum" would have been impossibly big. Today we can do whatever we want, affordably, and today we would do time constants much larger than 1/4 second, to move the dynamic change of bias voltage with signal level to be much slower than the envelope of music waveforms (hopefully).
Or, just eliminate the bypass cap, as in the Williamson, by running near enough to class A. I bet you'll experiment and see for yourownself.
Always good fortune,
Chris
Back in the Eisenhower era when these were designed, electrolytic capacitors were expensive and paper/oil capacitors were both big and expensive, so there was plenty of incentive to keep values as small as could be gotten away with. "Optimizing" wasn't really possible for cathode bypass caps because the "optimum" would have been impossibly big. Today we can do whatever we want, affordably, and today we would do time constants much larger than 1/4 second, to move the dynamic change of bias voltage with signal level to be much slower than the envelope of music waveforms (hopefully).
Or, just eliminate the bypass cap, as in the Williamson, by running near enough to class A. I bet you'll experiment and see for yourownself.
Always good fortune,
Chris
If you want to bypass a self bias resistor, both for single ended (which is always Class A, except for guitar amplifiers), and for class AB push pull . . .
Consider the Ohms of the self bias resistor
Consider the 3 different cathode impedances:
Triode/Triode Wired Pentode/Beam Power;
Ultra Linear;
Pentode/Beam Power native mode
Given the same output tube, those cathode impedances vary widely according to the tube operating modes;
But also according to the plate load impedance (especially for Triodes/Triode wired, next for Ultra Linear; and very little for native pentode/beam power mode).
Consider the Ohms of the self bias resistor
Consider the 3 different cathode impedances:
Triode/Triode Wired Pentode/Beam Power;
Ultra Linear;
Pentode/Beam Power native mode
Given the same output tube, those cathode impedances vary widely according to the tube operating modes;
But also according to the plate load impedance (especially for Triodes/Triode wired, next for Ultra Linear; and very little for native pentode/beam power mode).