Can a mute box damage a tube amp?

[PenPen]

Quick question. A while back I made a mute box for a friend. Basically it was a momentary switch that was normally connected input to output, when the switch is pressed it would shunt the output to ground. I instructed him to always put it last in the chain, closest to his amp, to avoid noise from any active pedals going to the amp. He was very pleased with the result, he wanted to do a kind of manual tremolo thing, his band does lots of off timing stuff and he wanted to drop out his guitar to this off time. He took it to record with Steve Albini for their LP, and it performed great in the studio. But he reported it did cause an odd effect on his amp. When he kept the switch pressed for a longer period of time, like 30 seconds, when he let off of it, the amp slowly faded back in, almost like it was warming the tubes back up. Now I'm a little concerned that it may be damaging his amp.

Can a tube amp go to no load on the INPUT like that safely? Should I add a small resistor to shunt through instead of straight wire?

[Mark Hammer]

Actually, many tube amps have included input jacks that grounded the input when nothing was plugged in.  Can't get much muter than that.

The risk is posed if there is any "hanging" capacitor at the output of the mute box which might result in a big transient pop, although that is really more a risk to the speakers than the amp itself.

I'd be interested to know what the amp was that behaved like that.  There may be some properties of it that are worth learning about.

[PenPen]

Thanks Mark. The circuit as is has no components, just two jacks, a switch, and some wire. Completely passive, and noiseless. That was a big concern when I made it, I knew they were going to record with Albini and I didn't want my box to cause any noise. When I built it I had figured that the design was safe, like you said some amp schems I've seen do that trick as well. I just got a little concerned when he told me that.

The amp is a Sovtek all tube head, I'm not sure about the model, I can find out though. From what he had been told it was a pretty rare head, something about it having higher output or something. And yes, I mean to say that it was made by Sovtek, that isn't just the tube brand. I never knew they made amps until I saw his.

[Paul Marossy]

I never knew they made amps until I saw his.

Oh yeah, they were popular in the early to mid 90s for a while. I almost bought a 50 watter about 10 years ago. Anyhow, the thing I would be concerned most about is whether or not the switch can actually switch quietly. In theory, any switch should work OK. A momentary switch might give you some trouble because it depends on the ability of the person pushing the actuator to do it efficiently.

[petemoore]

  I put a mute [ground shunt momentary] switch on my guitar.
  I don't know if using it 'broke it in or what, but it isn't clikking like it did when I first put it in, works great...for the 'cresendo' endings that solve to a sustained chord or note, thick and harmonic...then start hitting that switch at an increasing pace..catching up to the drummer who's just beginning to slow...

[Dave_B]

You might have a look at Buckethead's guitars.  On his jumbo Les Paul, he added a "kill switch" in place of one of his tone knobs and uses it extensively to do what you describe.  He uses it as a tremolo, but he also does some very effective "record scratching" stuff with the switch and his Whammy pedal.

[Mike Burgundy]

A switch to ground should not cause any problem at all. If the output of the last pedal can charge up enough voltage to effectively bias the amps first stage (which must have no input cap for this to work) into it's "off"region.
This should be solved by wiring the switch as a connection to ground for both in and out - by this I mean connect the in- and output jacks permanently, and wire the switch from one of the two to ground.
This grounds both the pedal's output, and the amps input. Both are now nicely pulled to ground when you switch back - no pops, and the way I see it, no fade-ins either.

[Mark Hammer]

Just as an aside, a great number of reverb units in amps are "cancelled" by simply grounding the input to the recovery section after the reverb pan.  That's why the footswitch connection can consist of a phone jack.

[PenPen]

If the output of the last pedal can charge up enough voltage to effectively bias the amps first stage (which must have no input cap for this to work) into it's "off"region.

This does make a lot of sense. The only other pedal he uses is a DOD BuzzBox, which is just for the octave effect he kicks in sometimes. Since this pedal isn't true bypass, its output cap is still always connected to the output. So this really does make a lot of sense to me. Thanks, I'll probably shunt both input and output in the mute box to ground then.