Need Help With Screen Switch

[M Gregg]

Thanks for the info on your experiences. My output stage does use a CCS for the biasing, so I'm not worried about it going out of bias.
I also just ordered some heavy duty DPDT switches with over 10M of insulation resistance, so hopefully that does the trick

Miniature toggle:

Regards
M. Gregg

 

[TheGimp]

Both the Maplin switch and the C&K that I use are break before make. As long as one is cognizant of this there is no reason not to use them, given appropriate measures are taken before switching.

Also note that switch contact bounce time of toggle switches is on the order of 3-5ms, not microseconds.

 

[exeric]

I don't like the noise during switching either but the convenience overwhelms the need to turn off B+ and wait for the caps to run down. That's my case. And I don't think it necessarily does any damage to the amp provided one doesn't use make before break switches.

One thing I've thought of doing on my next project is to put 1 meg ohm resistors across those switch contacts. Even at 300 volts it would only draw 0.1 ma. Not much in the scheme of things. If there is a tendency to arc across the contacts during the interval of switching the resistor would allow another path for a "controlled" arc. At least in my own mind that should quiet it down considerably during the transition and perhaps even lessen contact and insulation erosion that might otherwise occur over time. But only experiment will tell.

 

[TheGimp]

I wonder if a 1nF ceramic disc (1KV) across the contacts would introduce too much signal from the open contact position. It certainly would suppress arcing better than the resistor.

 

[exeric]

Maybe the 1nF will work. I'm always a little more reluctant to use something with capacitance or inductance in situations like that. Not because I know it won't work. But more because sometimes one doesn't know what one doesn't know. Strange interactions sometimes creep in where an ac audio signal is involved.

I actually think the 1 meg resistor might work if you use a switch with > 100 Meg ohms insulation resistance like Gregg was recommending. A 1 meg ohm resistance is obviously at least 100 times less resistance so if electricity has a choice that's where its going to go.
The thing about arcs is that it takes a lot to create one in the absence of a carbon track created by a previous arc. You basically have to overcome the resistance of air. Maybe you know what the resistance of air is. I don't. But my guess, should I hazard to venture one, is that the 1 meg resistor will be the preferred path for that electricity.

The main thing is not to create a carbon track from a pattern of previous arcs in air. Once a carbon track is created then all bets are off and a 1 meg ohm resistor may not be the preferred path. I'm speculating here.

 

[TheGimp]

I think the 1nF may be problematic. It only represents a 3979 Ohm impedance at 40KHz. That is awfully low to go from anode to screen.

Might be worth a try on a breadboard to see how it does effect it.