Multi preamp tube switching box

[runmikeyrun]

Sort-of stompbox related...

I was thinking about what a pain it is to swap tubes in an amp or valvecaster when trying to find "that" tone.  By the time you pull the tube and insert a new one it's hard to remember exactly what the last one sounded like. 

I (crudely) drew an idea-  a box that holds 3 preamp tubes connected to a wire and a 9 pin plug.  The plug goes into your amp, into the socket of the tube you've removed.  You place three tubes in the box, and can freely switch between them via a rotary switch on the box, which instantly gives you the different sounds based on the tubes.

A couple things I've realized as I've quickly thought this through half way. 

First, to be able to switch instantly between tubes with no break in signal means the tube heaters in all 3 must be on at all times.  The amp might not be able to handle supplying heater current to all of them at once, but It would be easy to add a small power supply to do this inside the box.

Second, same issue but with plate voltage.  Ideally the preamp tubes would get their voltage from the amp so it is exactly the same as if it's in the socket. 

Third, the possibility of popping, arcing, or damaging the tube while switching with voltages hitting the tube while it's already hot.

Questions:

if I'm only applying voltages to one tube at a time (as soon as that tube is switched on) will that cause any spikes to the amp or damage the tube? 

Will having a separate heater current supply introduce unwanted noise?

Could I perform this with relative ease?  A multiple pole rotary switch that simultaneously switches the plates, cathodes and grids seems too easy to just work.  I'm guessing there has to be something in there to suppress voltage and current spikes.
Thanks in advance!






Ham fisting my way through one build to the next since 2003.

[thermionix]

I'd say you would want all three tubes to have heater and plate voltages going, then just double pole switch between the inputs/outputs.  That additional current wouldn't phase most amps, but for pedals or small preamps could be a real concern.  Popping would be loud, but maybe that could be mitigated somewhat with resistors.  You'd probably want a MBB switch.

[runmikeyrun]

Good call, I guess the additional current and voltage to run two more preamp tubes really isn't a whole lot for most standard amps. 


Ham fisting my way through one build to the next since 2003. 

[thermionix]

There are other possible concerns, depending on the circuit.  You don't want a grid to lose its ground reference when switching the input away, for example.

Back to the popping, the earlier a noise happens in an amplifier circuit, the more it gets amplified.  Could get really bad trying to switch the first preamp stage.

[printer2]

Tubes are tough, I would not worry about the popping. A heater supply in the box is best. Lead length out of the amp into the box might be a problem if they are long, causing ultrasonic oscillations. Otherwise doable.

[runmikeyrun]

Tubes are tough, I would not worry about the popping. A heater supply in the box is best. Lead length out of the amp into the box might be a problem if they are long, causing ultrasonic oscillations. Otherwise doable.

So you don't think there will be any popping as the switch is turned?  I think I will give this a go. 

I'm honestly surprised no one else has built something like this.  About the only thing I found was mention of a device built by a company to test tubes in their amp's preamp, but seems like no one else has even attempted this.  Not sure why.




Ham fisting my way through one build to the next since 2003. 

[printer2]

Oh there will be popping but this is only for test purposes, right? To help with the popping put a high value resistor, 1M would be fine, on the switched side of the coupling caps so they do not float out in space while the switch changes position. You could build the actual circuit around each tube and then switch them in and out, might be nicer to them. Not sure if it is needed though. With the heater blasting the cathode you will get a cloud of electrons hovering around but without the grid or plate having voltage on them to attract or repell them not much will happen. With the grid and plate each being open circuits not much will happen.

And then you flick the switch. (What? no smoke billowing out of the ears emoticon? I think we need one with the next upgrade of the forum software.)

Yeah things will go thump, should not be enough time for the mechanical bits heat up and melt. Careful about your downstream equipment. Hat to have a 50W amp gain cranked and give the speaker a big jolt. Maybe build in a mute switch shorting out the signal leaving the circuit. That would take care of any thumping from the rotary switch. Just a smaller thump releasing the mute switch.

[runmikeyrun]

The mute switch is a great idea!  Definitely going to do that.  A simple single button momentary remote footswitch. 


Ham fisting my way through one build to the next since 2003. 

[merlinb]

The valvecaster is so simple I would just build three of them side by side in a junk enclosure. All voltages applied, with three footswitches (or use cheap toggle switches since this is just for testing).

[Ambugaton]

I know when sizing out a power transformer for an amp you take into account the mA draw of each tube. I really don't think a 12a*7 draws that much current but 3 additional tubes might be reason to make sure you aren't gonna burn through your PT over time.

EDIT: I think this was brought up in every post including the op... Maybe I should read

[runmikeyrun]

Thanks for the advice guys.  I'm hoping to order parts for this in the near future.  I will let you know how it turns out.


Ham fisting my way through one build to the next since 2003. 

[printer2]

12AX7's triode usually consume in the 1 mA range. If a power transformer blows up, well...