Haunted by the switchable channel monster

[pjd3]

Please feel free to skip this paragraph to get right to the point. I should have know that this would happen but, I didn't get into trying to build tube amps to play it safe and have an amp that just works well, I also got into this to challenge myself to do things I didn't know I could do, even pushing my own limits, to use some of the skill and experience from my day jobs as an electro-mechanical R@D/Test and engineering tech, and to be part of an awesome community that build really cool and useful shit. And since I make 10 to 20 grand extra per year gigging, I get to use these things with the band. So, with that said.

At first this second project was going to be a sluckey plexi, period. But now, I can't fool myself. I know much of the repertoire I play calls often for that 80's hair band extra gain sound that may not happen with just the wonderful earlier plexi preamp, that is also called on very often. The board is all built and mounted and for the most part soldered to its accompanying preamp tubes and potentiometers.

I'm very seriously considering building another board with the two series gain stages, drilling one more 12ax7 hole for the two gain stages, and building or buying the little power supply board (5v or 6.3v) to power the relays, and create what I really wanted to do at the very beginning, which was Sluckeys Dual channel Marshall that appears to be a plexi channel and that 2 gain channel that are switchable. Except I want mine to be remote footswitch to control relays.

I took some interest in the switchable "one-wire" mod which seemed like it could have been a solution but, read too many reports of it not being satisfactory, that it was compromising something and didn't sound like a traditional series gain stage as in the later Marshalls. I personally wouldn't know if the one-wire would work or not for what I'm going for. So, I figure why worry about. I'll just see about going for the full topologies of each preamp type and make them switchable with as little compromise as possible.

Do you think I'm traversing up a realistic road here? It doesn't seem like something that should be too problematic if certain things are considered and done in a prudent manner. And I under stand there may be things that need to be tended to like switching noise, pops.

Whats ya think?
I would love this amp if I can do it.
Thank you all,
PJD3

[sluckey]

Should be very easy to convert the plexi preamp to a JCM-800 2204 preamp. No extra tubes required. You just need to study the 2204 preamp schematic. Better yet, study my dual 50 schematic since the plexi and 2204 schematics are on the same page.

[pjd3]

Hey Thanks Sluckey,

I don't want to simply convert it though, I want to have both preamps available and switchable. I guess I wan't to have pretty much your Dual Marshall but see about relay switching.

What do you think I need to consider for that?

Thank you,
PJD3

[sluckey]

What do you think I need to consider for that?

Just a relay.

However, you may want to look at Hoffman's Plexi50. It's a standard plexi but he has an optional "Hot Switch" mod that uses one of the plexi preamp triodes as a switchable cascade gain stage similar to the 2204. Worth a look even if the idea doesn't appeal to you. Hoffman's hot switch mod can also be applied to my plexi6V6 without adding another tube or board. Just requires a few wiring changes and a couple component changes. Pretty easy stuff. Just follow Hoffman's mod.

[pjd3]

Thank you Steve, I'm going to take a look at Hoffmans Hot Switch. Simpler appeals to me as long as it doesn't compromise anything useful or tonally. Hell, its only one amp and Christmas is coming. I think I'm good for a couple more snake oil components.

Heading over to Hoffmans,

PJD3

[pjd3]

Hi, would sure like to hear your outlook on channel switching approach, potential benefits and deficits of each (or something else!).

Although I've alot more to learn on this topic, I feel like I'm looking at two ways of going at this (if I am infact looking at this with a reasonable level of understanding)

In switching between a plexi preamp and a JCM800 2204 preamp with a 6V6 output section I see;

1. Passive A/B switching utilizing separate input jacks for each preamp, with the outputs of the preamps mixed with the 470K resistors.

2. A single input connected to both preamps through their own 33K to 68K resistors, and using a (remote footswitch contolled) DPDT relay to select which preamp ouput moves forward to the next gain stage.

My kneejerk concern was if there were any compromising loading issues regarding inputs or outputs "looking" at each other. Seeing as these preamp stages are primarily voltage amplification stages I thought maybe not something to worry about but, then again, at my stage of the game.... My next concern was which approach is likely to present the greatest opportunity for switching noise or pops. As I can't speak from experience I can only rely on reports from those that have invested time with both or experimenting. Designing a remote relay switch sounds kind of fun and challenging, and certainly would be new to me but, thats the thing. It would be new to me and I'm not versed on the type of new problems that could arise from integrating that into a high voltage gainy device.

As you can imagine, any and all feedback is very appreciated. I'll need to decide on which way to go and that will be where I spend my efforts devising.
Thank you all very much!
PJD3

[pompeiisneaks]

I've built two dumbles that have relay switching, and I built the Sluckey Dual Marshall with an added relay for the switching. I've loved both. The general idea for switching, that I've heard, is to add the switch somewhere early enough in the circuit that loading isn't an issue, and if you need it after where it is, add extremely large resistors to ground or between the legs to avoid the popping that happens. This gives any excess current a path to use while the switch isn't engaged at either side. (22M or the like). Other than that, usually proven builds will work as expected and are used for a reason I put hte switch on my dual marshall exactly where sluckey's is, with a relay, and then put a front panel switch in as well, so that either the footswitch OR the front panel could trigger the channel switching. Worked a treat.

~Phil

[tristanc]

1. Passive A/B switching utilizing separate input jacks for each preamp, with the outputs of the preamps mixed with the 470K resistors

This is what I've done in the two builds where I've combined preamps (plexi/october + 2204 + 6V6s, plexi/october + Merlin's High Gain + 6SN7). Kept it simple. Things to think about:

- I need to tweak the controls quite a bit moving from plexi <-> 2204. There's a volume jump (or a perceived one) and the 2204 being bright means the plexi might be muddy for the same settings. YMMV.

- If you're being really picky, the 470k mixers mean you've less signal making it through to the 3rd gain stage. So it's not exactly the same from a circuit point of view. I doubt anyone would notice apart from having to turn the gains up a bit. I'm looking at adding an active mixer to mine in the future, but that's a longer term project if it happens at all.

[norburybrook]

what about one of these two switchable high gain amps?

AH50-01Mayl2018.jpg

AFD-50-1-June2017.jpg

[romberg]

Here is a relay switched amp I built some years back:

https://ampgarage.com/forum/viewtopic.php?f=9&t=25084

The relays switch in either a 1959, 2204 or SIR #39 preamp (2, 3 or 4 gain stages) and are controlled via a front panel mounted switch and/or a two button foot switch.

I don't know if I would attempt to convert an existing amp to this. But it might give you some ideas.

Mike