How to briefly mute audio when flipping a switch

ok, so my ignorant experimentation continues in the world of preamps and I have come to yet another problem area.
I have been goofing around with using pentodes as triodes in a tube preamp and have a switch wired up to switch between the two modes.  However, the switch gives a big pop when i do it and I'm curious to know if anyone has suggestions or places to look for a way to briefly mute the signal, or just stop the blasted popping when switching back and forth, but without affecting the quality of the signal.

The switch does two things -
1.  Wires the screen on an EF86 from pentode to triode
2.  Adds an extra resistor to ground on the grid of the EF86, in creasing the total resistance to ground in order to compensate for the lower volume from the triode mode.  Right now when in triode or pentode mode the volume is about the same for A/B for my comparison.

Thanks for any help you can provide!

You could have the switch control a set of relays, where one relay mutes the audio momentarily, and the other switches the tube from triode to pentode. A logic circuit would have the mute happen first, then activate the second relay, then release the mute.

A "logic circuit" could be something as simple as a 555 monostable driving your "mute" relay.

... perhaps even powered by your heater supply if you don't have anything better at hand

There is a mesa boogie patent on that subject, how to use the relay kick back to switch FETs used to mute when switching, look for the schematic of mark V and search those patents, one of which is on that, I can't remember now, later I can help you with the search.

JS

Thanks for the input! 
Now for my next questions -
1.  Which relays would you suggest?
2.  What is a logic circuit and how do I make such a thing work?

It POPs because there is a big difference in static DC plate voltage.

This may take some time (many milli-seconds) to re-equalize in all the caps.

Before you load-up on relays and logic.... would it be practical to put in a second tube and switch just the audio (after coupling caps) instead of one tube working two different DC conditions?

figuringstuffout said:

2.  Adds an extra resistor to ground on the grid of the EF86,

Click to expand...

I suspect this leaves the grid floating, which then causes transient noise too. This should be rearranged for always retaining galvanic connection of the grid to ground.

How about fitting a momentary action mute button. Press it, flip the switch then release it.

Cheers

Ian

You can google "555 monostable relay driver" for example.

Plenty of examples to choose from.

One advantage of such "low tech" approach is that a 555 can directly drive a relay and there's hundreds of different circuits for applications on the internet.

One disadvantage is that a 555 polutes the rails when it chages its output state, so you must decouple it thoroughly.

Take a look here for example and search for "relay":
http://www.bowdenshobbycircuits.info/page9.htm

Is this a hifi preamp or are you more flexible? It should not be difficult to arrange a JFET or optoMOSFET to provide a soft mute when switching (using another pole of the switch to operate the JFET). RC to the gate, probably no special timing IC's required.

Hmm... a DPDT switch, a 555 and a couple of passive components would be all that's needed for a mute circuit (with a relay).

A 556, a DPDT, a couple of passives and 2 relays could yield a "mute" and a slightly delayed triode-pentode switching.

But hey...

You could do the "mute" thing possibly with a DPDT, two 'lytic caps, a couple resistors and diodes and a relay ... passively.

see below.

If you are doing this to compare triode/pentode and not for a commercial item, just have a speaker switch nearby so you can mute the speaker while you swap.

All the other methods are either too complex and/or put extra stuff in the signal chain.  IMHO, none of them are worth the trouble or the slightly less sound quality.

I don't really see a reason to be so dismissive to "loss in sound quality" - a relay that shorts an audio path to ground for a moment wouldn't really be able to "spoil" the signal path quality to a significant measure - no matter how miniscule such a degradation would be.

Pesonally I would also add a RC constant to the "triode-pentode" relay so it would delay the action a little, meaning the "mute" would switch slightly before the "triode" switch, thus avoiding any stray weirdness in the signal, making it safer for switching "in the heat of the moment".