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Switch between tubes with relay

emretelci

Member
2015-10-23 12:17 pm
Hi,

There is situation that I need help about switching tubes. The situation is I am making a guitar pedal (overdrive) with a tube... The original design has one tube, but I want to use three tubes and switch between them using relays. I know this sounds interesting but I am trying to figure out how I can do that...

Only one tube should be active and by using relays, I need to switch between tubes. My question is, where should I install the relays on this drawing.

Thanks...

PS: Pin 3 is connected to pin 4 on the schematics. And all the tubes are identical.

[IMGDEAD]https://www.emre.co.uk/wp-content/uploads/2016/05/tube_layout-1.jpg[/IMGDEAD]


And here is the schematics:
[IMGDEAD]https://www.emre.co.uk/wp-content/uploads/2016/05/2.png[/IMGDEAD]
 
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turk 182

Member
2012-10-26 3:03 pm
are you thinking of switching all the connections from tube to tube?
heaters would need to see a constant supply otherwise warm up time will be a problem.
in looking at the schematic i'm not sure that plate voltages are adequate for a 12at7 nor does there seem to be any sort of voltage multiplier in the power supply.
interesting concept though.
 

emretelci

Member
2015-10-23 12:17 pm
These are have equivalent PIN order, but not equivalent tubes.

Maximum anode current:
5751 has about 1mA, the other two capable about 10mA.
12AT7 and 12AU7 not equivalent, but mostly interchangeable (depending of the circuit).

Thank you. That's ok actually for me. I have tried and all gives a different tone so all are working for me in same circuit without anything...
 
The easiest way to do that with the three (actually two) single-pole relays you showed, is just to have three circuits and switch the outputs. Dunno if you're trying to switch between tubes in the middle of playing though, in which case you'll need cap coupling to avoid pops.
 

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emretelci

Member
2015-10-23 12:17 pm
are you thinking of switching all the connections from tube to tube?
heaters would need to see a constant supply otherwise warm up time will be a problem.
in looking at the schematic i'm not sure that plate voltages are adequate for a 12at7 nor does there seem to be any sort of voltage multiplier in the power supply.
interesting concept though.

Thank you very much. I don't think I need to switch all the connections. Actually I don't know which ones to switch to make one tube work only... That's the reason I am asking here to figure out which pins I should switch...

You are right about heaters, I was thinking that. So heaters may stay connected I think, without relays...
 
merlins suggestion to switch the outputs is spot on. It also gives the possibility to have individual and independent adjustments for every tube.
I would however have an output cap + ground reference for every tube, this to prevent "click" when switching.
Turk182: the tubes will draw 300mA each (or 150mA if using 12V supply) a 9V battery won't be useful.
 
euro21 said:
These are have equivalent PIN order, but not equivalent tubes.
To a guitarist, anything with the same pinout is an equivalent!

Be aware that if you run ordinary valves for long periods with the heater on but no anode current then you risk getting a cathode interface layer which will drop the gain. Two solutions to this:
1. don't run the heaters on the valves not being used
2. use only 'computer service' valves, which have special cathodes

This thread should be in the Instruments & Amps forum.
 
I was commenting on euro21's comment. It was intended to be a little derogatory, with the emphasis on 'little'. I was agreeing with euro21.

The OP said that the tubes were "identical", then he changed that to "equivalent, but different" before making it clear that he actually meant 'different but same pinout'. As guitar people obviously speak a different language from us it makes sense to put them in a different forum section, so people know which dialect is in use. Their aim is the production of music; our aim is the reproduction of music.

Just for the sake of clarity, here:
"Equivalent" means same pinout, identical characteristics (within the usual tolerances), possible variation in other matters such as microphony or service life (some people might call this a 'near equivalent', and only a completely identical valve would be called 'equivalent'). Example: 12AX7 and ECC83 are equivalent.
"Substitute" means same pinout (or minor difference, of no consequence in most circuits), similar characteristics such that in most circuits one can work nearly as well as the other and give similar performance or sufficient to get by as a temporary repair. Example: 5751 and 12AX7 are substitutes, as their gain is different but they are otherwise similar.
"Same pinout" means same electrodes attached to same pins, but could be huge difference in characteristics - if so not a substitute in most circuits. Example: 12AU7 and 12AX7 have the same pinout but they are so different that they could not usually be swapped except in the most uncritical situations, or where the large change in circuit behaviour is actually desired.
 
Another nice thing with the relays is :
the tubes and associated circuits are located in the amp, filament and B+ can be
taken from the amp, the guitar is also connected to the same location.
The pedal is totally passive, it only contains switches for the relays, 4 leads is
all that is needed, DC ( for relay control) + 3 leads for the relays.
No need for a 9V battery !!
 
but then that means that it's no longer a stand alone device that can be used ahead of any amp.
Unless it's build in 2 parts : the pedal, connected with 4 wires to : the box
that is placed on top or close to the amp, the box is mains connected and
contains all tubes and circuits .
The point is that the pedal needs no active electronics, it's just a maneuver device,
everything else is on top of ( or built inside ) an amp.