Channel switching for a AX84 Single Ended Lead Tube Guitar Amplifier project?

[Rod]

Hi!

How can i add channel switching in the ront panel of the amp for a clean and a distortion channel in this project?, i also want to make the channels switchable via a footswitch?

Where can i find also some projects with this kind of channel switching?

In amps with 3 or more channels, this channels share the same preamp stages right?


Thanks.

[jmaf]

If you don't mind the price tag, "The Ultimate Tone", a book by Kevin O'Connor has a whole section dedicated just to switching methods.

RG Keen(forum member RG) also has a neat solution for two channel switching, mostly used in effects pedals but usable in amps as well: The Millenium Bypass

Other than that, you can dive into existing schematics and see how they do it. There are tens of types of channel switching solutions, maybe start off a current production amp like the HotRod Deluxe or similar.

Hope this helps.

[Merlinb]

Switching between two channels is not too hard since you only need one footswitch. The choice is then, what switching device do you want to use? The main options are:

1. Relays - simple to use but hungry on power.
2. Solid state relays (SSR) / opto MOSFET - easy to use and low power consumption
3. JFETs - Tricky to use but even lower power consumption.

There are also LDRs, but they make pretty lame switches compared to the above. Best left, unless you know what you're doing.

If you use relays or SSRs then all you need to do is give them some sort of power supply, and then hook up the coil/LED to your footswitch. Hey presto!

Switching more than two channels gets a lot more complicated because you need one footswitch for every channel, and a decoder. Here's an example:

[jazbo8]

The SEL is a single channel amp, so you can't do channel switching with it... but here is a simple rhythm/lead (clean/OD) switch arrangement, you will need two SPDT switches, wire them in parallel with a normally closed stereo 1/4" jack, so when you plug in the footswitch, it bypasses the front panel switch. The 1/4" jack should be insulated from the chassis so you can use all three contacts.

Jaz

[Tage]

The SEL is a single channel amp, so you can't do channel switching with it... but here is a simple rhythm/lead (clean/OD) switch arrangement, you will need two SPDT switches, wire them in parallel with a normally closed stereo 1/4" jack, so when you plug in the footswitch, it bypasses the front panel switch. The 1/4" jack should be insulated from the chassis so you can use all three contacts.

Jaz

Mybe a coupling cap before the switch would be a good idea. but otherwise I see this being an economical, method, though I think I would try to ground the grid of the lead tube grid when it's not in use. But that would require a very different method

[tubeswell]

What Tage said - better to isolate the stages after the coupling caps

[jmaf]

Mybe a coupling cap before the switch would be a good idea. but otherwise I see this being an economical, method, though I think I would try to ground the grid of the lead tube grid when it's not in use. But that would require a very different method

Agree with coupling cap too. But it seems to me that if you ground the grid on stage 3 you'll create a frequency trap 22nF -> 220K -> ground.

[jazbo8]

It works with or without a coupling cap, since the CF of the SEL was designed to be direct-coupled anyway.

Jaz

[Amp Kat]

It works with or without a coupling cap, since the CF of the SEL was designed to be direct-coupled anyway.

Jaz

It's a bit more difficult with a single SE tube because both signals are tied to the grid. It's easy to isolate the channels whether you use relays,optos or IC's and as Merlin says I opted away from optos as they are very sporadic from piece to piece and just too inconsistent. There is a major problem however no matter how you isolate the channels and it is dealing with loss of signal when the other channel is on especially on the clean side because the grounded pot creates a low impedance to the coupling cap and it drags the volume way down much lower than if the ground wasn't there. If you use two grid caps it helps but then the signal bleeds through to the other cap since they are tied together at the tubes grid input. The other problem is if you don't ground the pots they interact with each other so it is almost required to isolate them from each other. I'm working on a diode option right now and hopefully will have it done soon.

Would like to hear some other options and to clarify I am talking about a single tube SE only not two tubes.

Also some AX84's I've seen are 2X power tubes and some single tubes so on circuits with 2 output tubes the switching is not difficult at all.

[jazbo8]

You are right about the level matching with a simple switch like I suggested, I basically just leave it in the Lead position all the time, so kinda defeat having the switch in the first place... How about isolating the coupling caps with grid block/mix resistors, like in some of the 18W designs? You still need to pre-level the volumes of each channel but there should be no bleed-through...

Jaz

[Merlinb]

If you intend to do switching 'on the fly' in the middle of performances then you certainly do need coupling caps, otherwise you'll get almighty pops as the DC voltages across the switch equalise.