I understand a build out resistor is necessary to keep a cathode follower from high frequency resonance or oscillation when driving capacitive loads (like long runs of cable).
However, I've seen a couple different ways of placing that build out resistor in circuit, so I wanted to ask what happens when you move that build out resistor to different places in the circuit. I drew up an example showing the three I've seen:
I usually place Rbo (the build out resistor) as in Ex. A.
I understand that placing Rbo as in Ex. B will add slight additional cathode load, but in use it's pretty much the same as Ex. A.
Will Rbo placed as in Ex. C also act to dampen high frequency resonance with high Cload, as in Ex's A and B? Or will it not work at all that way?
However, I've seen a couple different ways of placing that build out resistor in circuit, so I wanted to ask what happens when you move that build out resistor to different places in the circuit. I drew up an example showing the three I've seen:
I usually place Rbo (the build out resistor) as in Ex. A.
I understand that placing Rbo as in Ex. B will add slight additional cathode load, but in use it's pretty much the same as Ex. A.
Will Rbo placed as in Ex. C also act to dampen high frequency resonance with high Cload, as in Ex's A and B? Or will it not work at all that way?
Schema #3 is the proper way, to a degree isolate the transmission line capacity from the driving tube & its circuitry.🙂
Thanks.
I was under the impression the build-out resistor acts in a similar fashion to how a grid stopper works on the triode's control grid input, and so should be located as physically close to the actual cathode pin as possible. Or does the physical placement of the build-out resistor not matter as much?
Of course I simulated the three example circuits and found the build-out resistor works well to suppress high frequency resonance in the presence of a 1nF parallel load, in all three examples. The difference I could see was that the gain was reduced by Ex. B, a bit less so by Ex. A, and not at all by Ex. C.
I was under the impression the build-out resistor acts in a similar fashion to how a grid stopper works on the triode's control grid input, and so should be located as physically close to the actual cathode pin as possible. Or does the physical placement of the build-out resistor not matter as much?
Of course I simulated the three example circuits and found the build-out resistor works well to suppress high frequency resonance in the presence of a 1nF parallel load, in all three examples. The difference I could see was that the gain was reduced by Ex. B, a bit less so by Ex. A, and not at all by Ex. C.
Placement doesn't really matter since 99.99% of the shunt capacitance you're trying to protect against will be in the cable outside the box.
That makes sense.
That build out resistor will still make a voltage divider with the shunt resistance at the input of the driven device (amplifier volume control, grid leak resistor, etc.), no? I suppose that means it's not a good idea to make the build out resistor 1k ohms if it's going to drive an ICEpower amp module with 8k ohm input impedance. Correct?
Come to think of it... Installing that build out resistor is like adding a lot of ESR to the output capacitor, is it not?
That build out resistor will still make a voltage divider with the shunt resistance at the input of the driven device (amplifier volume control, grid leak resistor, etc.), no? I suppose that means it's not a good idea to make the build out resistor 1k ohms if it's going to drive an ICEpower amp module with 8k ohm input impedance. Correct?
Come to think of it... Installing that build out resistor is like adding a lot of ESR to the output capacitor, is it not?
Using circuit #3 may result in oscillations if you are using very high transconductance triodes or pentodes as followers. I've always found placing these and grid stoppers as close as possible to the socket was worthwhile, hence I always use circuit #1 if using a tube based follower. The loss with 100R in series with the output and before the pull down resistor R2 (example #1) is about 0.01dB excluding the effect of the load - still likely less than 0.1dB or so for 100K load.