Switch for B+ to switch off a gain stage

[Hambamble]

I'm building a clone of a Matchless lightning. The preamp has two 12ax7 for gain stages, and a third for the phase splitter. I want to be able to switch out the second tube which will almost make it into a matchless spitfire apart from about 3 cap values (which is pretty cool i thought!).

I came across a 4pdt switch rated at 2A 240v and 5A 120v. I was going to use it to:

i) cut the input to the 2nd tube
ii) cut the output from the second tube to the phase splitter
iii) cut the B+ to the second tube
iv) Maybe change the tone cap, but not really sure at this point. Think it will be one of those try it and see sorts of things. Probably better running a few LED's so I can see which 'channel' is actually in use.

i and ii should bypass this tube, and i thought iii would save the tube from working when its not needed.

My question is weather i can use the switch for the ~300V B+ lead, or if it will be dangerous (its rated 2A at 240v). I figured as long as the current is low, which it is, there shouldn't be a problem. If there's a better way to do this sort of thing though, Please let me know!

[Reeltarded]

Yes. A better way. Do not switch off the B+.

It's that simple. So much trouble for nothing but trouble. I could list the problems but instead I'll just say switching modes is going to drive you insane.

[alvarezh]

+1

You bypass the signal, switch or relay, your choice.

Cheers.

[Structo]

Switch the grid of the tube you want to turn off to ground.

In other words use a DPDT (On-On) to switch the the input where you want it to go and the other side to ground the grid of the tube you want to shut off.

Better yet, use a DPDT relay and switch it with a foot switch.

[Colossal]

Switch the grid of the tube you want to turn off to.

Make sure you retain a ground reference on the grid when switching that stage out.

[alvarezh]

How do you get the signal from the first stage to the P.I. grounding the cathode or the grid imput signal of the stage in between?

Thanks in advance.

[Colossal]

How do you get the signal from the first stage to the P.I. grounding the cathode or the grid imput signal of the stage in between?

Thanks in advance.

Use a DPDT relay to take the signal out from V1 (past the coupling cap, not before) and into the coupling cap in front of the PI. One side of the relay is a jumper, bypassing the stage in between. Grid leaks must stay in place to maintain ground reference on the stage(s) not receiving signal. Use a 2M2 resistor to ground on either side of the relay to eliminate switch pop.

[alvarezh]

Sorry, I misunderstood you Dave, what you meant to say in your original procedure was, to bypass the stage and additionally ground it's input grid. Fine.

My apologies for making you write the extra explanation.

Have a good week end!

[Colossal]

Sorry, I misunderstood you Dave, what you meant to say in your original procedure was, to bypass the stage and additionally ground it's input grid. Fine.

My apologies for making you write the extra explanation.

Have a good week end!

No worries! Yes, I just added to Structo's reply for the OP's benefit, so that if he jumpered the signal over one stage, he did not to leave the grid floating on the stage that gets bypassed.

[Hambamble]

Ok, so i think i understand it now... ground the grid when not in use and bypass the stage completely.

Just a question on the 2M2 resistors to reduce Chanel pop goes to the 'dead' channel, so it stays grounded relative to the active stage... that way when you switch them, it should be at the same level. You said to use one either side of the relay... not really sure where the second one would go any help would be great!

Sorry about the newb questions, this is my fourth amp build (3rd tube) and i really want to try something a bit new and adventurous. I think i'll draw up the schematic and see if you all approve. Last thing i want to do is make a mess of this. Thanks for your help!

[Colossal]

Ok, so i think i understand it now... ground the grid when not in use and bypass the stage completely.

Hi Hambamble,

Not quite. The grid of each triode usually has some ground reference already, you'll see that on the schematic. So it wil be 1M for the first gain stage, and maybe say 220k (or whatever) down the line where there is some interstage attenuation. Each stage must have and retain a grid-to-ground reference at all times so that the grid does not collect charge. When switching out your middle tube, it's just a matter of where you shunt the signal from. I was trying to point out that you don't want to arbitrarily take the signal, for instance, after a grid leak, but before...this way the grid(s) of the idling tube that is not in use do not rise and run away thermally resulting in Fukushima style failure.

Just a question on the 2M2 resistors to reduce Chanel pop goes to the 'dead' channel, so it stays grounded relative to the active stage... that way when you switch them, it should be at the same level. You said to use one either side of the relay... not really sure where the second one would go any help would be great!

I would use a DPDT (On-none-ON) switch or relay. You have two switches in one with path A or B on both sides. One throw switches both paths at once. So you have two commons. You would put a 2M2 to ground on each common. These commons will be your signal In and Out. To start, signal will first come out of your traditional Matchless input stage since you want that at all times.After the coupling cap (so DC is not on the switch or relay), the signal will go to the first common on your channel switch or relay (sharing that common is a 2M2 to ground). From there you will either jumper with a single wire across to the other switch to A, around the next tube completely for Spitfire mode, or you will go from B which will be the next stage, maintaining the Matchless Lightning format. So the other common of the switch or relay becomes the output of the decision. You would mount a 2M2 on that common as well. The 2M2s are there to shunt AC signal to ground at the moment the switch is fired, damping the pop.

Sorry about the newb questions, this is my fourth amp build (3rd tube) and i really want to try something a bit new and adventurous. I think i'll draw up the schematic and see if you all approve. Last thing i want to do is make a mess of this. Thanks for your help!

Sure, best to draw it up and see if you can figure out the switch logic. It might take a few beers and some tries. I can draw it up if you get stuck.

[xtian]

Here is an example schematic for triode switching from AX84:

http://www.ax84.com/static/sel/AX84_SEL ... 101004.pdf

See the lead/rhythm switch on p.3.

[Hambamble]

Ok, So here is the schematic as planned (without the power supply). I am using the switch to use the different tone stacks between the lightning and spitfire, so I think that the master volume on each section should provide that 1M grid ground. I have added the 2M2 ground to common to prevent pop when switching channels. Hope it's legible and makes sense! Thanks again for your help everyone, much appreciated!

[Reeltarded]

Cool. It looks like it works.

[Hambamble]

Awesome! Now all i need to do is build it...