Idiot proof output transformer switching?

[martyh]

I want to switch between output transformer taps and also between output jacks. It’s for a tube based headphone amp/preamp. Since I won’t be the only person to use this device, I would like to make the switching as idiot proof as possible. I’m envisioning someone playing with the switches at full gain and trying to imagine what might go wrong. The goal is to avoid shorting across the secondary winding or leaving the secondary unloaded. What do you all think of the attached schematic? Overkill or not? Is there a better way?

Thanks,
Marty

 

[rayma]

I would wire a secondary load resistor permanently across the 600R winding.
The value should be much lower than shown, around ten to twenty times the load, like 10k.

You could then use a two position break-before-make switch instead for selecting the tap.
Same for the headphone selector switch. Any DPDT toggle switch types would work for stereo.

Bear in mind that if the tap selector switch is operated while listening, you could get too much volume.

 

[audiowize]

What does the middle position of the left switch do? Mute?

If this is driven by a cathode follower you don't need any loading.

 

[PRR]

............to avoid shorting across the secondary winding or leaving the secondary unloaded...

Why?

Small (especially self-bias) tube amps do not explode when shorted. (Tubes are not transistors.)

Small (especially low-NFB triode) amps do not explode quickly when un-loaded. (You are thinking of Randy's 5W pentode amp which did not survive many playings of American Woman until Gar added a dummy-load.)

I'd put 2K across the 600r tap and be done with it.

 

[petertub]

I want to switch between output transformer taps and also between output jacks. It’s for a tube based headphone amp/preamp. Since I won’t be the only person to use this device, I would like to make the switching as idiot proof as possible. I’m envisioning someone playing with the switches at full gain and trying to imagine what might go wrong. The goal is to avoid shorting across the secondary winding or leaving the secondary unloaded. What do you all think of the attached schematic? Overkill or not? Is there a better way?

Thanks,
Marty

Don't be afraid of shorting, it does no harm !
Do solder a resistor across the transformer secondaries, a 2k-5k ohm, this
will protect the transformer during the time the switch is open ( or the
speaker/headset is disconnected).

 

[martyh]

Thanks for all of the replies. Not sure what I was thinking with center position of that first switch. Since the single load resistor works I'll just do that and be done.