Channel Switching Implementation questions

[wizard333]

I'm going to put channel switching on a current build and was looking for some advice. I'd added switching to amps but mostly solo boosts or component value choices; I've never added channel switching.

Was going to shunt one or the other channel to ground right before the PI for the switching. In addition, I've seen some designs that add another relay on the front end, switching the input to the grid of one channel or the other; and other designs that just run both channels in parallel and only switch them at the end.

Q: Is there any advantage or disadvantage to running the 1st stages in parallel for both channels? i.e. input goes to the grid of both channels' first triode.

Also, any tips on keeping the switching as quiet as possible?

Thanks!

[txstrat]

I've tried several kinds of switching with relays over the years but I've come to the conclusion that building two channels with different inputs works better for me.
I can use a simple A/B box to switch the signal from one channel to the other, no switching noise, no hum or other issues. No compromise in sound (which coupling cap value to use). You'll need just one more triode stage using this way of switching and I find it's worth it.

Using a relay I always switched both input and output of the channels at the same time. That way no signal from the other channel can get through to the PI (I actually had that happening in a two channel amp, where the second channel had a cathode follower circuit).

I found sharing the first triode for both channels is the least noisy way, since the rather small input signal is already amplified by the first triode before it gets "through" the relay.
There are several designs out there using relays for channel switching (e.g. Fender ProTube TwinAmp, Fender Prosonic, Fender Tonemaster, just to name some).
Keep the grounds for the relay off the circuit grounds of the amp. Use a separate wire to the chassis ground. Use not too small filter caps for the relay supply. That should keep the switching a quiet as possible.

[wizard333]

The situation I have now is the A/B box situation, but I really want to go to internal switching. One issue you get there is any noise from your high gain channel is still present when using the clean channel, since its not switching in the amp and the noise is always on. Even if your high gain channel is quiet for a high gain channel, its probably still a lot noisier than your clean channel, so why listen to the noise while playing clean.

Also, I'm not going to be able to have them share the 1st stage due to the differing CKs on the 1st stage of either channel (.47uf vs 22uf).

I've heard about the separate ground issue before, seems like Enzo had a post on that a while back.

Am I right to assume that if I'm using a very clean supply to the relay that they wont induce much/any noise in the input signal? I've seen relays cause noise before but the ripple on the supply was very high.

If I'm switching before the PI but NOT at the input, and running the 1st stages in parallel, it doesn't seem like I'd get cross talk, or am I missing something there? Not using a cathode follower. Any other reason not to do that?

Thanks!

[brydon2]

I vote to ban all internal channel switching in guitar amps. I still can't fix my JCM 2000, its garbage now! Apparently a jfet issue thats only traceable by an engineer.

I vote for the KISS method, sounds better, less heartache!!

[txstrat]

any noise from your high gain channel is still present when using the clean channel, since its not switching in the amp and the noise is always on.

When you use a DPDT switch for your A/B box you could switch the unused channel to ground. That should stop the high gain channels noise while playing the clean channel.

[wizard333]

But only at the input. An external A/B can set the input to ground on the unused channel (mine does) but it cant ground the internally generated noise. The best you'll get is the noise you'd get with your guitar volume a 0; thats still more than I want to deal with.

[J M Fahey]

Hi Wizard.
You can, say, maybe not switch both channels *fully*, but use an extra switching contact to mute/ground some stage from the high gain preamp.
That should quiet it enough from bothersome to acceptable.

[wizard333]

What can I expect to happen to my 1st stage noise running parallel into the 1st grid of both channels, if anything? They'll be sharing the 1M grid resistor and 33k RF blocker.

[J M Fahey]

Not much change, if any.
In active elements (tubes or transistors) that are parallelled and used as a single one, there is a reduction of noise. There is an IC that is made out of 200 transistors in parallel, specially for that purpose.
If I remember well that trick was also used in tube equipment, in radar and radio-astronomy.
In your case, you are using your preamps separately, so I guess, no noise reduction and probably no noise increase.