I'm looking for more info on large grid stoppers in guitar amps. I understand the high frequency rolloff that can occur. But, basically, I've noticed a lot of DIY designs are using huge grid stoppers on the output tubes, while a lot of commercial designs don't use any at all. I know that will stop blocking distortion, but I don't know how that effects the tone. Especially when we want distortion. (And I've changed mine from 1k to 100k on my 6v6 output tubes, and don't really hear a difference.. but I am close to 40 and starting to go deaf)
And, does anyone know how to calculate the frequency cutoff of a cap between the phases of a PP amp, like the high cut in a vox ac30? Because it's between phases, I don't think it's quite the same as a standard lowpass filter. (and if it is the same as a standard lpf, I don't know what to use as the R value.. because in these circuts the C between the phases is after the coupling caps from the phase splitter)
Thanks for any help.
And, does anyone know how to calculate the frequency cutoff of a cap between the phases of a PP amp, like the high cut in a vox ac30? Because it's between phases, I don't think it's quite the same as a standard lowpass filter. (and if it is the same as a standard lpf, I don't know what to use as the R value.. because in these circuts the C between the phases is after the coupling caps from the phase splitter)
Thanks for any help.
Forumla for R and input C frequency breakpoint is:
1/2 pi R C=F
I just recently designed a bipolar 500 watt amplifier and it oscillated.
I tried changing VAS capacitor and feedback capacitor but it still oscillated.
I put base stopper on the output transistors and that fixed it.
1/2 pi R C=F
I just recently designed a bipolar 500 watt amplifier and it oscillated.
I tried changing VAS capacitor and feedback capacitor but it still oscillated.
I put base stopper on the output transistors and that fixed it.
I think this works:
Imagine replacing that cap with two caps in series (they need to be double the capacitance, of course). Now, by symmetry, the junction between those two caps must stay at zero volts. That point could therefore be connected to ground with no effect. That 'grounded' cap on one side of the push-pull is effectively in parallel with the grid-leak resistor of the following stage.
In some amps (with global negative feedback) the cap in that position is just to prevent HF oscillation, and so its cut-off frequency is above the audio range.
If my suggestion above is correct, then the turn-over frequency for the high-cut would be given by Nigel's formula above (in post #2) with C = twice the phase-to-phase capacitance and R = grid leak (of following stage) in parallel with the PI output impedance.
Thanks guys. I knew the basic formula, and what Malcolm says about the capacitor makes sense.
I guess it's about time to model this thing in Spice anyway, so I'll see if the model matches the numbers.
I guess it's about time to model this thing in Spice anyway, so I'll see if the model matches the numbers.
???
The zero volts between the two caps only goes for DC. AC is passed along just fine. Connecting the point between the caps to ground would short the audio path.
The high frequency cutoff is calculated with the tubes' miller capacitance (stated on datasheet) and the resistance in series with the grid. This also includes the resistance from e.g. a guitar volume pot set halfway, so be carefull, because high frequency response can differ with a guitar's volume setting! Different tubes and topologies have different miller capacitance and can allow for more resistance.
There is no DC on that cap (which I hypothetically split into 2 caps in series) as it is after the coupling caps from the PI.
For AC: let's say there is +X volts to ground on the 'push' side, then there must be -X volts to ground on the 'pull' side (as they are out of phase). The junction of the two (hypothetical) caps will always be at zero volts.
The hypothetical connection of that junction point to ground, just completes the effective low pass filter. (Only high frequencies are shorted to ground.)
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