Marshall/Fender mash up layout and schematic feedback

[Mason]

I have a Marshall 2205 that I bought not too long ago. It was pretty heavily modded when I got it and I brought it back to stock. Not a bad sounding amp, but because of the way the pcb has been hacked up for the mods that were done to it I have concerns about its longevity. So instead of chancing it I thought it could be a good platform for a custom turret board build.

Since the front panel has two channels, I was thinking I could use the boost channel section to make a 2204. It maps out pretty perfectly. The only changes would be the 2204 preamp volume control would now be called gain, and what would be the master volume would just be called volume. And then for the normal channel section, I was thinking of just putting in a fender style preamp, like the super reverb normal channel. So I’d have a footswitchable relay that would switch the lone input jack to either go to the 2204 preamp, or the fender preamp. Whichever preamp is not engaged would be grounded out.

The master section of the panel is also pretty straightforward. The master volume will be a ppimv, and the presence will be the Marshall style. Since I’ve got the reverb knob, I was going to try to add the 1 tube reverb to the build as well.

Attached are a schematic and layout for the amp. I started with a 2204 schematic and edited it from there, same with the layout. The schematic doesn’t show relays for the switching, but I’ll be using the relay board that’s sold here and daisy-chaining two together to handle switching the channels and switching the negative feedback resistor. Right now the reverb is only set up for the fender style channel. I’m not sure I’ll want it for the 2204 side yet.

The layout isn’t 100% complete, I left out the power transformer, impedance selector, fuses, power and standby switches, the power board for the relays, and the connection for the footswitch. I’m still figuring out placement for the relay power board. I think I have enough room on the input side of the chassis near v1, but I’ll have to measure to make sure. I’m also still finalizing placement for the relays, but they’re in the ball park of where I want them. The other stuff is just going to be following the standard JCM 800 layout.

So the things I’m unsure of are the point in the phase inverter where I’m connecting the two different preamps and the way I’ve tied the reverb circuit in. Any advice around those areas will be much appreciated. And of course any general critiques of the layout as well. I’m sure there’s bound to be some errors in the layout and/or schematic. If you see any please point them out, I’m still learning!

[vampwizzard]

Id imagine youd want to come back in near the master volume, as its the last point before the PI on the existing JCM800 preamp channel. interesting amp hope you get it built!

[Mason]

Id imagine youd want to come back in near the master volume, as its the last point before the PI on the existing JCM800 preamp channel. interesting amp hope you get it built!

Yeah, that's what I thought. The marshall circuit has a .022uf coupling cap after the volume pot, and the fender has a .001uf there. I'm not sure if the value matters much or not, right now I have both values, and each preamp goes to their respective cap before hitting the grid of the PI.

[sluckey]

You need a couple mixing resistors to prevent the two channels from interacting with each other.

[vampwizzard]

looks like he has one off preamp 2.. but with a MV they'd have to follow your extra instructions on the dual lite?

[Mason]

You need a couple mixing resistors to prevent the two channels from interacting with each other.

I have a 220k after the fender preamp, so would I just need to add another one after the volume pot in the Marshall circuit?  I was looking at your dual preamp builds for some inspiration. You have one where you have the switch on the output of the preamp rather than the input, which I think eliminates the need for mixing resistors. In your opinion which one would be the better option? Switching the input, or switching the output of the preamps?

[sluckey]

I'd probably do it like this...

[Mason]

Got it, looks good. I see you took out the .001uf cap after the fender preamp. Do you think it's just unnecessary to have each preamp going to a different cap before the PI?

[sluckey]

Yes. The purpose of that cap is to prevent the big dc voltage on the grid of the PI from bleeding down through the previous stages, tone stacks, etc. Only need one.

[Mason]

Makes sense. I wasn’t sure how the value might effect the tone so I put values for each circuit in. I’ll change it to be like how you suggested. Thank you!

Aside from that, are the any other potential issues y’all see?

[pdf64]

The reverb is going to be somewhat weak, not a real Fender effect, is it really going to be useful?
Having it on one channel but not the other may seem weird.
And as is, it’s eating up signal level.

The negative feedback switching idea is wrong headed. Have a read of http://aikenamps.com/index.php/designing-for-global-negative-feedback

[Mason]

So because of the presence control and using the marshall value of 4.7k ohm to ground after the negative feedback resistor, I'll need to rethink the 820ohm value for the fender style negative feedback resistor? I'll read through that article and hopefully come away with a better idea.

For the reverb, I know it's not gonna be quite as nice as the traditional fender-style reverb, but I still want to try it. I took the circuit directly from the 1 tube reverb thread linked here, https://el34world.com/Forum/index.php?topic=7957.0. Is it the reverb circuit itself that's causing signal suck, or the way I'm implementing it?

[pdf64]

Passive mixers will cut signal level, that’s an inevitable consequence of how they work. The 2M2 pot value is an attempt to try to mitigate for that though.
A Fender normal channel can seem somewhat weak but perhaps that can be made up here with a bit more power amp gain.

With only one valve to play with, I’d prefer an fx loop to a built in reverb, as there’s a lot more flexibility and it can be available to both channels.
But that’s just me, and it’s your baby 

[Mason]

With only one valve to play with, I’d prefer an fx loop to a built in reverb, as there’s a lot more flexibility and it can be available to both channels.
But that’s just me, and it’s your baby 

Haha, there are already chassis holes for an fx loop if I want to try that. Adding the reverb to this build is more of a, "Can I make this work and sound decent?" kind of thing rather than an attempt at vintage fender quality spring reverb. Also gives me the ability to use the knob labeled reverb for an actual reverb circuit instead of repurposing it. This is pretty minor, but it always bugs me to have a control labeled as something that it's not doing.

[pdf64]

Ok then, though I’d consider a 12DW7 for the reverb valve, so that the 12AU7 section can push more current into the tank.
Or an ECL86 or 82.