Best Practice for Maximum Voltage Directly Switched by User

[techie1234]

I am in the process of building a tube guitar amp and one of the features I’d like to add is to allow the player to select the amount of B+ power supply “sag” that is in the circuit (which can be a desirable thing).  I’m still experimenting with whether to do this by adding a resistor or switching a filtering cap in/out of circuit.  However, I’m currently contemplating how to make this selectability available at the amp’s panel in the final build.  I’m initially reluctant to have 300+ VDC directly switched by the user (and find switches capable of supporting this rare/expensive) so am thinking that a HV DC relay is probably the answer.  However, would be interested in others’ thoughts on what the general best practice is for the maximum amount of voltage that is considered “safe” to bring directly to the user to mechanically switch (esp. if the switch has a metal bat that the user touches).  All thoughts / comments / rules of thumb appreciated!

[Ian.M]

With an appropriately rated switch and a reliably grounded metal panel, 240V RMS mains supplies (downstream of an appropriate fuse or circuit breaker) can safely be switched with a metal lever toggle switch.   However if the front panel isn't grounded you'd be foolish to exceed 50V on such a switch unless the circuit is limited to a touch-safe current.

The resistor option doesn't cause a high current surge if its switched with the amp on.   Switching capacitors in parallel can get nasty if one is charged and the other isn't.   The easy option would be a DC rated, MOSFET based solid state relay, but it wont be cheap.   Rolling your own MOSFET based switching circuit is another possibility.

[techie1234]

With an appropriately rated switch and a reliably grounded metal panel, 240V RMS mains supplies (downstream of an appropriate fuse or circuit breaker) can safely be switched with a metal lever toggle switch.   However if the front panel isn't grounded you'd be foolish to exceed 50V on such a switch unless the circuit is limited to a touch-safe current.

The panel will be mechanically bonded to the chassis so it should be good in that aspect.  We certainly switch 240VAC mains in everything we (well, not here in the US...) have.  But, at what voltage (esp. DC - which is the case in this instance) is it "just not a good idea" to be presenting that potential to a user-accessible (even if suitable rated and grounded, as one would assume the metal switch body would be) part?  And if I cannot find a reasonably-priced switch perhaps it will be a non-issue.  Perhaps I'm over-thinking this a bit as well.

The resistor option doesn't cause a high current surge if its switched with the amp on.   Switching capacitors in parallel can get nasty if one is charged and the other isn't.   The easy option would be a DC rated, MOSFET based solid state relay, but it wont be cheap.   Rolling your own MOSFET based switching circuit is another possibility.

Yeah, I was thinking about that and figured I might need to account for inrush and bleeding of the cap in that approach, depending upon whether the cap is "coming in" or "going out" of circuit.  Still thinking and tinkering - thanks for the thoughts!

Something like this is what I was contemplating for the relay, although it would be even better if the coil voltage was a bit lower (and an AC coil) so I could power it directly off of the 12.6V heater line from the PT:

https://www.ebay.com/itm/201707321307

Here's one that has a 6V coil and is a bit cheaper:

https://www.mouser.com/ProductDetail/FTR-J2AK006W

[David Hess]

With a grounded front panel I think the rotary switches I am familiar with would be safe enough.

What about a rheostat though?  The current level for the B+ supply is not high and a insulating shaft can be used for the control.