Using a spdt switch to select between triode mode and pentode mode on a guitar amp.

[G]

Hi all. I'm building a little guitar amp and would like to incorporate a switch to select either triode or pentode operation of the output tube (EL34). Has anyone done this personally? I'm wondering about the consequences of switching modes while powered up and playing.

[SHiFTY]

I did this with 807s. As long as I turned down the input to zero, and the switchover was fast, there was only a small pop from the speaker. However try it with an old speaker first, YMMV...

[G]

Thank you sir.

[Sherman]

I have also done this, both on a guitar amp and on a hifi amp. One thing to be aware of though- most switches are not rated for the voltage they will see in this application.

When switching with the power on there will be arcing across the switch. That arcing will cause the switch to fail sooner or later. You can avoid that by only switching with the power off. If there is no power and the PS caps are mostly discharged it won't arc and the switch should last indefinitely. Or you can just replace the switch occasionally when it fails.

(If your OPT has the taps for it you can also use a rotary switch to switch between triode, ultralinear and pentode. The same cautions about voltage and arcing apply.)

[G]

Originally posted by Sherman
I have also done this, both on a guitar amp and on a hifi amp. One thing to be aware of though- most switches are not rated for the voltage they will see in this application.

When switching with the power on there will be arcing across the switch. That arcing will cause the switch to fail sooner or later. You can avoid that by only switching with the power off. If there is no power and the PS caps are mostly discharged it won't arc and the switch should last indefinitely. Or you can just replace the switch occasionally when it fails.

(If your OPT has the taps for it you can also use a rotary switch to switch between triode, ultralinear and pentode. The same cautions about voltage and arcing apply.)

A very solid point Sherman. I had just considered that conundrum while considering what switch to use. I will have to consider this problem for a while. Thank you for your reply.

[G]

I just considered something. Since there is essentially no load connected to the contacts would the switch arc? Have you seen them arc? If you have then conversation over but it would seem to me that with almost no current flow there would be minimal arcing. Maybe I'm looking at it wrong.

[ErikdeBest]

This has been asked recently. I remember SY's post. He stated that although the switching happens at high voltages, maybe in excess of the swith rating, the difference between the terminals of the switch are not of a high value. An example, to explain it better. with a swith you connect one side to the b+ before the tap, the other end to the b+ at the plate of the output tube. The middle connection goes to the grid. (logical). Now the difference between the B+ before the tap and the B+ at the anode isn't much (just the DCR ot the trafo*the quiescent current, some volts, no more). when the difference is not that high, there will not be arcing! Bu be aware that the swich is well isolated, because it can be there at a high B+, working fine. But when YOU put your fingers there, than you will form the path to ground...

[tubelab.com]

I have used both a 2 position and 3 position switches for the "mode" switch on a single ended champ style guitar amp. The switches that I used were rated for 300 volts, and the B+ could go up to almost 400 volts (yet another switch). I personally saw a 6V6 sacrificed to the fire gods by flipping the screen grid switch while tha amp was operating at full clip trying to squeeze out far more power than it was made to do. The switch was toast too. This may sound exterme, but it is a common occurrence in the life of a guitar amp. The voltage at the plate is the B+ voltage PLUS the instanteous peak AC signal voltage. There can be up to 700 volts at the plate in this situation. I verified this voltage with a scope in this amp. This is only a 5 watt amp. More power = more voltage.

[G]

Originally posted by tubelab.com
I have used both a 2 position and 3 position switches for the "mode" switch on a single ended champ style guitar amp. The switches that I used were rated for 300 volts, and the B+ could go up to almost 400 volts (yet another switch). I personally saw a 6V6 sacrificed to the fire gods by flipping the screen grid switch while tha amp was operating at full clip trying to squeeze out far more power than it was made to do. The switch was toast too. This may sound exterme, but it is a common occurrence in the life of a guitar amp. The voltage at the plate is the B+ voltage PLUS the instanteous peak AC signal voltage. There can be up to 700 volts at the plate in this situation. I verified this voltage with a scope in this amp. This is only a 5 watt amp. More power = more voltage.

So I could use a "mute" switch to ground the signal at the input jack and safely switch from triode to pentode operation? I think that would work. I just plink around at home. No gigging for me. Too much electrical work to be done.

[G]

Originally posted by ErikdeBest
This has been asked recently. I remember SY's post. He stated that although the switching happens at high voltages, maybe in excess of the swith rating, the difference between the terminals of the switch are not of a high value. An example, to explain it better. with a swith you connect one side to the b+ before the tap, the other end to the b+ at the plate of the output tube. The middle connection goes to the grid. (logical). Now the difference between the B+ before the tap and the B+ at the anode isn't much (just the DCR ot the trafo*the quiescent current, some volts, no more). when the difference is not that high, there will not be arcing! Bu be aware that the swich is well isolated, because it can be there at a high B+, working fine. But when YOU put your fingers there, than you will form the path to ground...

That would make sense. No difference in potential (or very little) means no current flow (or very little). Hence no arcing. My view may be simplistic though.