ef86 pentode/triode switch: voltage rating?

[erikx]

I'm rebuilding the amps in an Akai/Roberts tube R2R for mic/line/guitar use. The signal hits an EF86 first (originally used to amplify signals off the tape head) before going to a 12AD7 for further amplification.

I'd like to use a pentode/triode switch for the EF86. What I'm really hoping to do, for layout/interface reasons, is wire it as a pull switch on the gain knob. Ideally, that means a 100K pot with a DPDT pull switch. My schematic (for intended pentode operation) says 195V B+, with 47V on the plate, and 65V on the screen at the quiescent point.

That says to me that I need a pot/switch rated for at least 200V. Is this correct? I'm having a really hard time finding this component---the closest I'm coming are some Bourns pots for guitars that are rated to 150V. (Like this---they also only go down to 250K, but I figured I could use a parallel resistor to ground to keep the input impedance down around 100K?)

Thanks. Most of the info I've come across involves switching P/T mode on output tubes, rather than preamps.
-E

 

[rayma]

I'd like to use a pentode/triode switch for the EF86. What I'm really hoping to do, for layout/interface reasons, is wire it as a pull switch on the gain knob. Ideally, that means a 100K pot with a DPDT pull switch. My schematic (for intended pentode operation) says 195V B+, with 47V on the plate, and 65V on the screen at the quiescent point.

I'd use the switch to control a 120VAC relay of the proper rating.

 

[artosalo]

I can't see any reason why that pot/swithc could not be used to switch between triode/pentode at the voltages you told.
To use a relay here is quite extravagant.

 

[erikx]

Artosalo, is that because the relevant voltage is the quiescent voltage? I'm thinking that when the tube is in cutoff, the switch will at least momentarily see all 195V. Not a constant condition though.

 

[erikx]

As it turns out, these pots aren't going to work at least. I just heard back from Bourns tech support: the 150V rating is across the pot, not the switch. The switches themselves are only good for 30VDC.

 

[rayma]

As it turns out, these pots aren't going to work at least. I just heard back from Bourns tech support: the 150V rating is across the pot,
not the switch. The switches themselves are only good for 30VDC.

That's typical for most small AC rated switches, and is why I recommended that you use the switch to control a 120VAC relay.
That will work just fine.

 

[erikx]

Indeed. Though I also thought it was a somewhat extreme solution, given that the payoff is to save the pull-switch design element.

The only reason this is of interest is that I don't want it to be easy to accidentally switch the tube into pentode mode. After a little reading, I'm toying with the idea of just using a Merlin- morph control as a sort of gain knob instead. There's another stage after this one, and each one gets a Big Knob on the front---putting this one on tone-shaping duty seems like an interesting idea.

 

[DF96]

Surely the DC rating of a switch assumes a relatively low impedance supply so an arc might continue? The anode circuit of an EF86 is going to be high impedance so any arc is going to be short-lived. In this situation I would use a switch up to its AC rating. However, make sure you use a new switch which has never been used to switch power so no arcing has yet occured to damage the contact surfaces.

 

[teemumm]

Surely the DC rating of a switch assumes a relatively low impedance supply so an arc might continue?

Arcing is a thing mainly related to the current, not voltage (for example in arc welding you have just about 40 volts, but over 100 amps). I don't think you see such currents with tube amps where this would be an issue.