cathode Switch on 12ax7

[angelodp]

Hi, I have a cathode switch on a gain stage. 10k cold clipper un-bypassed on one leg of switch, 2.4k/4.7uf on the other leg. Center leg of switch to the cathode pin on tube. Grounds tied together. Works but pops. is it a 1M across from the center leg of with to ground to stop the pop?

Thanks A

[pdf64]

Don’t leave the cathode momentarily open circuit. Leave the 10k permanently connected, and switch a 3k3 // 4u7 in parallel with it.
There will still be some thump on switching, because the DC operating point is being changed.

[angelodp]

Appreciate the heads up. Might you have a quick schematic for that?

Thanks A

[LOUDthud]

It's still going to pop because of the change in plate Voltage. With a 300V rail, a 10K cathode resistor will give a high plate Voltage, probably 270V. With a 2.4K cathode resistor the plate Voltage will be something like 200V (just guessing). The only way I see around it is to use two tubes and switch the inputs and outputs.

[martin manning]

No easy way out. Showing Va and Vo (22n coupling cap and 500k load) traces in the plot below. A 10k is always connected from k to ground. The big negative voltage spike at the output occurs when the 3k3//4u7 is switched in, momentarily making it look like the cathode is shorted to ground. The smaller spike is switching it out, when the bias shifts from the 2k4 back to the 10 Rk.

[sluckey]

Try this. Cathode R = 9.8K with switch open. Cathode R = 2.5K with switch closed.

[angelodp]

Ok, that looks like a solution. Thanks for taking time.

So it's. SPST - on/off

[martin manning]

I think that's going t have the same issues, but you can certainly try it.

[R.G.]

pdf64, loudthud and martin are right - it's still going to at least thump, maybe still pop.
Their reasoning is right, and the culprit is that newly paralleled cathode resistor. This changes the bias point, and that changes the plate voltage. The plate voltage change is high-pass filtered out of the plate as a pop or thump, depending on how fast the plate voltage changes.

Do you HAVE to change the cathode resistor down from 10K? If not, the scheme would be similar to sluckey's but without the 3.3k. This would be non-popping, as the cathode voltage would not change when the switch changed, as the cap is constantly charged to the same voltage as across the 10k.

If you HAVE to change the 10K to 2.5k you have a couple of options for doing it without popping. One is to make the change so slowly that the plate output cap filters out the change in voltage. This is going to require some kind of change-slowly circuit. The only really simple one I can think of is to make the 470K resistor in sluckey's suggestion be a pot and manually turn the shaft, not using a switch at all. There is a ream of other possibilities, but they get more complex.
The other is to mute the plate signal for a fraction of a second while switching is going on. A couple of cheap ICs would do this, but you'd have to be open to going with an IC circuit. I have done this for pedals with both CMOS hard logic ICs and with a $0.75 microcontroller.

[Reeltarded]

Why do switch changes on things we do while not playing make some us crazy about silenced switches?

How I Learned To Love The Pop

Dr Strangeclick

[angelodp]

Thud is better than the pop, and really all I want is to guard against damage to the circuit.

thanks A

[angelodp]

sluckey, yes does work well, with a more minor pop. All I am concerned about is having the ability to traverse from cold Clipper to the more standard K -RC, with no damage to the tube or circuit. So done deal for me and thanks for the support gentlemen.

Ange

[Reeltarded]

MBB switch. No harm to anything. It just thumps and there you go! The connection needs to be made before the previous one is broken, that's all.

There would be little harm unless you held a break switch open between settings, plus, that switching is THE MOST POP you can get.

Break before make is POP-U-LAR

[Ten Over]

Running with what R.G. said about using a pot instead of a switch, here are a couple of circuits:

[angelodp]

Very cool, so with the pot one can dial over with no appreciable pop. Thanks

A