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Need Help With Screen Switch

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I have an amp I built using a OPT w/ UL tap, and I want to use a switch to switch between UL and Triode mode. However, the first couple switches I used caused horrible noises to come out of the speakers.
I tried again without a switch, with both UL and Triode mode hardwired and there was no noise whatsoever, so I am assuming the noise was created from the switch (maybe it was not insulated well enough from the high voltages).
Is there any switches (either rotary or ON/ON rocker switches) you would recommend that any of you have used with success at voltages of 300VDC and up?
 
DON'T switch it with power on! Problem solved!

I should have clarified. I have never switched it with the power on, and I built the original switch into the back of the chassis near the OPTs so that no one would do that. I know better than that ;)
Just having the screen run through the switch was causing noise (thumping, etc.), which is why I was asking for switch suggestion (probably ones with better voltage insulation than the one I used, which was a cheap $0.50 switch from china with no real specs)
 
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A zener is not a stopper. Put a stopper resistor on the screen.
I thought that was just to keep the screen at a lower potential than the plate, which a zener would do. I'm not having any issues with it being a zener at all (plus I have a bunch of 20V 3W zeners). The noise issue I had was when using the switch, but once I removed it (I wired and tried both triode and UL mode, bot noiseless), all noises went away. If I'm wrong on this, please correct me.
 
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Hi Nailbunny7,

I have used a switch for triode / U/L etc in all my amps..

I have use rotary switches with a plastic shaft..from Maplin:
http://www.maplin.co.uk/p/sub-miniature-toggle-switch-e-on-on-dpdt-fh04e

I have used miniature toggles as well where the toggle is grounded.
Its the insulation resistance of the switch that's important, the voltage rating of the contacts is not a real issue because what potential is across the switch contacts..

Look at the insulation resistance..
http://www.maplin.co.uk/p/sub-miniature-toggle-switch-e-on-on-dpdt-fh04e

I have used this on EL34 X8 PPP at 400V..as long as there is a stopper on the grid in my case 1K. I keep the wires to and from the grid twisted as pairs and go all the way to the tube socket /or as twisted as far as possible to the tube base. I have switched zeners in and out as well..
so selector is Triode/U/L/ES/RH and RH with Zener. I try to make the power step up so I have no sudden changes from something like Triode >>pentode.
In which case I would do triode/U/L/pentode in that order...

Remember to watch the bias in each setting..<<<CCS can be useful.
The pic was a quick mod to an existing SE..rotary switch plastic shaft..in this case each tube wire loom is tied together instead of twisting (I was constantly modifying) This switch was also switching the RH/schade FB resistors at the same time.. use a separate loom for each tube.

Regards
M. Gregg
 

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Hi Nailbunny7,

I have used a switch for triode / U/L etc in all my amps..

I have use rotary switches with a plastic shaft..from Maplin:
Sub-Miniature Toggle Switch E On-On DPDT | Maplin

I have used miniature toggles as well where the toggle is grounded.
Its the insulation resistance of the switch that's important, the voltage rating of the contacts is not a real issue because what potential is across the switch contacts..

Look at the insulation resistance..
Sub-Miniature Toggle Switch E On-On DPDT | Maplin

I have used this on EL34 X8 PPP at 400V..as long as there is a stopper on the grid in my case 1K. I keep the wires to and from the grid twisted as pairs and go all the way to the tube socket /or as twisted as far as possible to the tube base. I have switched zeners in and out as well..
so selector is Triode/U/L/ES/RH and RH with Zener. I try to make the power step up so I have no sudden changes from something like Triode >>pentode.
In which case I would do triode/U/L/pentode in that order...

Remember to watch the bias in each setting..<<<CCS can be useful.
The pic was a quick mod to an existing SE..rotary switch plastic shaft..in this case each tube wire loom is tied together instead of twisting (I was constantly modifying) This switch was also switching the RH/schade FB resistors at the same time.. use a separate loom for each tube.

Regards
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 ;)
 
I used to own a production hi fi amp that had selectable screen mode switches. They always emitted a short crackling noise during the actual transition. There was no advisory in the manual that I know of that ever advised non use of that switch during operation. I think that sound is normal but can probably be minimized using some of the advice given here previously.

To me the advice to not use the switch while the power is on is a bit disingenuous so do NOT be defensive about using it then. To me it would be the same as saying "my car engine makes noise after I start the engine" and the person replies "Well, you shouldn't start the engine."

In other words, work to minimize the switching noise but do not be overly concerned that it makes some kind of noise with switching while power is on.
 
Disingenuous, no.

Cautionary, yes.

In switching with a break before make switch the screen will be subjected to open circuit during switching.

In switching with a make before break switch, you may short the OPT anode and screen taps when switching from Triode to UL.

Why risk it? Relax and spend a minute that it takes to shut down the amp and let the caps discharge before switching.
 
I used to own a production hi fi amp that had selectable screen mode switches. They always emitted a short crackling noise during the actual transition. There was no advisory in the manual that I know of that ever advised non use of that switch during operation. I think that sound is normal but can probably be minimized using some of the advice given here previously.

To me the advice to not use the switch while the power is on is a bit disingenuous so do NOT be defensive about using it then. To me it would be the same as saying "my car engine makes noise after I start the engine" and the person replies "Well, you shouldn't start the engine."

In other words, work to minimize the switching noise but do not be overly concerned that it makes some kind of noise with switching while power is on.

I know I've said it already, but I need to emphasize that I was NOT talking about switching noise (that would be expected, plus I haven't ever switched it while the amp was running because I was worried it may damage the OPTs).
The issue was noise created by the switch itself because it (allegedly) had too low of insulation resistance (probably because I used a very low quality switch), so the screen voltage was getting into both the plate and UL tap (possibly oscillating, would explain the thumps, as suggested in this thread already).
I will post my experiences when the new switches come in :)
 
In switching with a break before make switch the screen will be subjected to open circuit during switching..

and...? It seems like the microsecond in which that occurs is unlikely to cause any permanent damage.

In switching with a make before break switch, you may short the OPT anode and screen taps when switching from Triode to UL...

It doesn't seem to me that anyone with any knowledge of electronics would put a make before break switch in that position. Just because I do not post here often does not give allowance for you to assume I do not understand electronics. You assume too much and invent a straw man in the attempt.
 
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