Relays for 2-channel 2204

[ViperDoc]

What’s the best way to create a foot switchable 2204 with discreet preamp, EQ and Master volumes? I’m going to assume there is no need to add tubes, just switch between them? I’ve seen relay PCB switches on the web. How do you add both a footswitch relay and a manual switch for backup? I’m on a plane so I’ll try and draw up a schematic and post it. Also, where would you get a chassis deep enough? Think Fargen DBC arrangement. Thanks!

[ViperDoc]

All right, I copied the MOJO 800 schematic I had and sketched some changes. The 800-2204 amp I just built sounds really good (HUGE THANKS TO MARTIN MANNING, SLUCKEY, PDF64 AND ALL ELSE WHO HELPED). The low input bypasses the V1A gain stage and that makes it sound surprisingly more like a JTM45. With the pre gain and master dimed on the low input, it has great crunch tone but doesn't invade the high input's ground whatsoever. Rolled back, it cleans up surprisingly well. When I plug into the high input, there are worlds of gain added. Slapping V1A in front makes a huge difference. What is clear is that I have to dial the preamp gain and master quite differently for either input, so if I were to switch channels using a foot-switch-operated relay system, it would be ideal to have individual preamp gain and master volume pots.

Here's what I'd like to do:

1) Have a clean switching system with no noise--for that I'm willing to place an individual 6.3 V (or 5V) transformer to heat the relay system.
2) I want individual low gain and high gain preamp volumes and master volumes. I didn't draw the preamp changes on the schematic b/c I'm not sure how to handle the high gain preamp volume pot. Would/could this call for discreet first stage preamp tubes for each channel?
3) have individual tone stacks and master volumes--that's drawn in.
4) have adjustable NFB--drawn in
5) dare I say it, but I love effects loops. I've got a metro zero loop. Not drawn in. WANT.



I've never drawn circuits so I know the relays don't show everything. THIS IS PURELY CONCEPTUAL. And I'd like to know if I'm nuts or how I might pull this off.

THANK YOU!

[sluckey]

Hurts my eyes to try to see that schematic.

[ViperDoc]

Hurts my eyes to try to see that schematic.

I realized that after I posted it. It’s a PNG from DIYLC. I’ll see if I can fileshare it or something.

[sluckey]

Maybe print to pdf and attach the pdf.

[Colossal]

Hurts my eyes to try to see that schematic.

I realized that after I posted it. It’s a PNG from DIYLC. I’ll see if I can fileshare it or something.

Try exporting it at 300dpi, if that is an option in DIYLC.

[ViperDoc]

Let's try this:

JCM800-2204 2CHnfb SCH.png

[martin manning]

That will work, using two DPDT relays.

[ViperDoc]

That will work, using two DPDT relays.

I see that! Cool. Here's a rough idea about the individual channel preamp gain controls:

JCM800-2204 2CHprenfb SCH.pdf

I know you can daisy chain relays. How do you use a foot switch to control 2 relays at once? Just wire them both straight to the same foot switch jack?

[martin manning]

I think you will have to switch the cascaded stage in and out, and similarly for the separate gain pots to keep them from interacting. Multiple relays can be energized from one source and one switch.

[Xander8280]

How you have drawn the gain controls with the wipers always together will load on one another (if one full CCW that node is grounded regardless of channel selection there is no output). Think of a Les Paul with the volume wipers tied together in the middle position, if you mute the neck pickup you got a kill switch in the Neck and Middle selections of the toggle switch. *Que Ace Frehley*


You could just switch the wipers of the two gain pots (like you're doing with the masters) and double the 0.001uF cap that feeds both gain pots to 2.2nF. This will keep that RC corner frequency the same but, the 470kOhm||500pF preceding it will have a high frequency shelving frequency shifted higher by an octave. I personally wouldn't worry about that too much, you'll have separate tone stacks to tweak the low end to your liking for each.

[Xander8280]

I think you will have to switch the cascaded stage in and out, and similarly for the separate gain pots to keep them from interacting. Multiple relays can be energized from one source and one switch.

+1

[ViperDoc]

How you have drawn the gain controls with the wipers always together will load on one another (if one full CCW that node is grounded regardless of channel selection there is no output). Think of a Les Paul with the volume wipers tied together in the middle position, if you mute the neck pickup you got a kill switch in the Neck and Middle selections of the toggle switch. *Que Ace Frehley*


You could just switch the wipers of the two gain pots (like you're doing with the masters) and double the 0.001uF cap that feeds both gain pots to 2.2nF. This will keep that RC corner frequency the same but, the 470kOhm||500pF preceding it will have a high frequency shelving frequency shifted higher by an octave. I personally wouldn't worry about that too much, you'll have separate tone stacks to tweak the low end to your liking for each.

Indeed, Xander, much appreciated. You and Martin are spot on. Those gain knobs need to be separated at all times. I've never built an amp that uses relays, and thinking of all the ideals I'd prefer in a channel-switching amp makes my head spin. On top of what I've been investigating with the discreet channels, I am a huge fan of (good-sounding) effects loops. I'd prefer a pre-master volume loop position, so with individual channels, it seems it would be easier to design a loop point after the phase inverter and follow that with a PostPIMV. I've never built an amp with one of those, either! Different camps on that for sure. I'd likely be desecrating the 2204 in that case, but hey. I'm just here to learn more.

[Xander8280]

The phase inverter is differential (two complementary signals, like an XLR cable) so it's best to keep the effects loop before it.

Tube effects loops I've done that worked nice were a cathode follower with a volume control going to the SEND jack and then a gain stage fed from the RETURN jack. I set the attenuation of the SEND to be the same as the RETURN gain stage's but negative. So the phase inverter gets hit with the same signal level it would if there was no effects loop. There are much better loops out there but, this allows for a solid amount of gain on the RETURN signal, I preferred this so I could drive the power amp with anything.

[d95err]

Having the fx loop after the master volume is a bad idea, IMHO. It means the looo level is dependent on the current MV setting. In practice, this means you will get terrible signal-to-noise ratio if the MV is low and/or you’ll have way too high signal level when the MV is high. Move the MV to after the loop and you can trim the loop signal to a consistent level.