"Revoicing" a Peavey JSX to Marshall

[JustMike]

 I have a donor Peavey JSX head that I'm going to convert to more of a Marshally sound. I realize this is a substantial mod, but I'm a hobbyist and want to enjoy the learning process.
 So, it looks like the JSX has the clean channel tone stack after V1b only which then goes thru V1a, the FX loop and then to V3a, Master vol. and out to the PI.
 It looks like the CRUNCH and ULTRA channels gain levels are only dealt with in V1. The only differences between the two are the gain pot values-CRUNCH is 50k and ULTRA is 1M. There is some diode clipping between the 2 halves of V2 and some minor tone stack value differences between CRUNCH & ULTRA.
 That's about all I see at a glance.


 I'd like to keep the 3 channel functionality but Marshallize the voicing and tame the gain on this. Where should I start?
 Thanks again!

[PRR]

I'm usually in favor of minimal mods.

But this maze of features really makes me want to chisel off everything except the chassis, PT, OT, and line cord, and start over. Even the power tube sockets because they are on PCB.

And that only makes sense *after* the PCB and ribbon-cables (yuck) have gone bad.

While PCB/ribbon amps have a bad name generally, once in a while Peavey really got the technique done right. I say if it aint broke, don't fix it.

Obvious "Marshall" differences are first tonestack values and lack of a clipping stage with 10K cathode resistor. And of course WAY too many add-on frills like 3-band EQ inside a three stage booster so loud it has deliberate crossover distortion to reduce idle hiss.

[JustMike]

I'm not against your "Chisel" approach, but then I'd be left with basically building a 3 channel channel switcher from the ground up which I don't know if I'm up for. Can we compromise? It looks like the biggest challenge would be dealing with the active EQ. I did find a couple of threads about this amp http://el34world.com/Forum/index.php?topic=15708.msg151979#msg151979
 Maybe I'll try Plexi50's mods and see how that works out.

[PRR]

> I don't know if I'm up for

I stared at that plan for quite some time and I know I am not "up for" it.

> a 3 channel channel switcher from the ground up

So build three channels and put a 3-way switch after it. Maybe not as parts-thrifty as Peavey; OTOH that Peavey is so over-complicated that maybe simpler is also thriftier.

[tubenit]

But this maze of features really makes me want to chisel off everything except the chassis, PT, OT, and line cord, and start over. Even the power tube sockets because they are on PCB.

I'm with PRR on this one and personally would not attempt to mod the PCB board.  I am confident that I am not capable of doing that extensive of a mod on a PCB board and not end up ruining the amp.  So, I would gut the amp and rebuild it into something I wanted.

with respect, Tubenit

[kagliostro]

+1 for PRR & Tubenit

Franco

[JustMike]

"So, I would gut the amp and rebuild it into something I wanted"


OK, I want a Plexi voiced channel switcher with an active FX loop.
My biggest concern with this type of build is the channel switching and noise. I like to build from layouts and I just haven't seen any for a channel switcher.

[tubenit]

You have several options.  I think Sluckey has a channel switcher Marshall that is a Plexi50w & JCM800 preamp.  And I think he has both a schematic and a Visio layout.

OR ............ you can simply use an A,B, & Y pedal which is simple and works great.

Adding an FX loop is pretty easy on these.  There is a great deal of information on FX loops in ARCHIVES. 

The metro amp FX loop or Iron Sounds FX loop would work:

https://store.metropoulos.net/products/zero-loss-fx-loop-kit

http://www.ironsounds.com/

With respect, Tubenit

[tubenit]

From ARCHIVES:      relay switching

http://el34world.com/Forum/index.php?topic=14348.0

also ARCHIVES:   active FX loop

http://el34world.com/Forum/index.php?topic=10208.0

With respect, Tubenit

[tubenit]

Ah, found Sluckey's Plexi50w and JCM800 schematic and layout all done in Visio.

Sluckey's stuff & drawings is absolutely stellar!

OK, I want a Plexi voiced channel switcher with an active FX loop

Now with this layout and the FX & relay information,  you should have enough information to pick it up from here IF you want to pursue this.

With respect, Tubenit

[JustMike]

That kind of sheds a new light on everything since I have a Plexi/JCM (it alternates occasionally) I built from scratch that would be an easier mod. A couple of parts and a tube...Voila!   I'll just try Plexi50's mods to the JSX.


 Thank you Sir(s)!

[JustMike]

 In following Plexi50's mods, they refer to the XXX which is a similar ckt. He says that when taking out the clipping diodes he was considering replacing them with a resistor to eliminate poppong. The XXX schematic appears to already have the resistor (R3-1M) in parallel with the diodes and R96 (470k) to gnd. The JSX has a 1M pot here (Noise gate). Although it doesn't reference ground, will the noise gate suffice for this resistor?


 OK. I'll answer my own question- Apparently so, there's no noise when switching.


 Below is  the excerpt from Plexi50's post. I've commented in Green. Can someone answer my questions?


This is what i have done to my XXX. Sounds 85% better. Still the EQ rc network sorta sucks but good tone can be found after fiddling for a while. The EQ response is like all or nothing and the sweet spot is some where in between. I cant bring myself to dig into the rc network and reconfigure to a normal tone stack. Maybe someday if i still have it. I have too much else going on right now to devote to this amp. Still much better after making the following preamp and power board changes:


Change R11 & R7 150K Plates to 100K Doesn't this affect the ratio? as designed, it's a 10:1 ratio, chgng these would make it something like 75:1...lower gain?


 Remove C19 & C20 Cathode Caps Less gain at whatever the RC freq. is?


Change R23 68K Slope to 100K This is standard on the JSX-Already done!


Remove V2 Diodes D1 & D2 / Removes Early Clipping / Smoother / May use resistor to subdue slight channel pop between Ultra & Crunch As above, no resistor needed.


Change LTPI Resistors R108 & R111 to 1Meg / Buss R106 I don't understand this. Appears to be a grid resistor on the PI. What does it do?


Change Screen Resistors R101-R102 R117-R118 to 1K 5 WattsRemove CR-100 & CR-103 What do these do?

[JustMike]

Change R11 & R7 150K Plates to 100K Doesn't this affect the ratio? as designed, it's a 10:1 ratio, chgng these would make it something like 75:1...lower gain?


 

Typo, I meant as designed, it's a 100:1 ratio.

[sluckey]

Change R11 & R7 150K Plates to 100K  Doesn't this affect the ratio? as designed, it's a 10:1 ratio, chgng these would make it something like 75:1...lower gain?

Don't overthink it. Lowering the value of the plate load resistor will lower the gain of the stage.

 

Remove C19 & C20 Cathode Caps Less gain at whatever the RC freq. is?

Less gain at all frequencies due to degenerative feedback.

Change LTPI Resistors R108 & R111 to 1Meg / Buss R106 I don't understand this. Appears to be a grid resistor on the PI. What does it do?

It's called a grid stopper. But you will buss it, which means replace it with a piece of wire, so it disappears and does nothing.

Change Screen Resistors R101-R102 R117-R118 to 1K 5 WattsRemove CR-100 & CR-103 What do these do?

Those diodes are flyback protection diodes. They provide some protection to your OT, output tubes, and sockets in the event you play the amp with no speaker load connected.

[PRR]

> 150K Plates to 100K Doesn't this affect the ratio?

Look at the *whole* circuit.

The 150K/100K is in parallel with tube plate resistance (say 60K) and whatever this thing drives (say 220K). 36K, 32K, not a big difference.

Also the smaller resistor leads to more tube current, which increases Gm. (Also reduces Rp.)

Gain will drop, but not a lot. Overload action may change (if this is a stage that overloads).

IMHO, 150K or 100K is about all the same. But as long as the iron is hot, just try it. To avoid yanking the 150K, just tack a 330k across it (150K||330K= 103K which is == 100K for all practical purpose.)

> Don't overthink it.

+1

[JustMike]

 Thanks for following (and leading) me through this. You guys are an invaluable resource and I really appreciate it!


So as I'm going thru this schematic, it looks like the only difference between the Crunch and Ultra channels is the gain pot values and a couple of resistors in the tone stack. Could this be all there is? If so, I'm going to make both channels identical as I think the Ultra channel is a little over the top.


 Also, the noise gate may not be necessary now that I've eliminated the diode clippers, yes? If this is the case, I'd like to do the following; put some diodes or LED's in the Ultra channel only (where? is the location in the ckt critical?) and repurpose the noise gate pot R22 to be able to dial in or out the level of diode clipping.


 

[JustMike]

 I don't know enough to understand how the tone stack in this amp works and don't want to mess with it anyway. But I do want to know if I can place a more traditional passive tone ckt where it is. It's seems like it's in the same place as it is in the JCM circuit except the JSX has another tube following it. Will this work?
Does anybody know of a Hot rodded Marshall type circuit that has a 3rd preamp tube after the tonestack?


Thanks again.

[JustMike]

The Mesa Stiletto Deuce 2 has the passive tone stack after V3 so I assume I can follow this schematic.
 I think I'm going to use this thread to document my findings as I work thru this project. Please feel free to drop in and interject if anyone sees me straying from good protocol. My plan is to keep the 3 channels, leave ch.1 as is and rewire the tonestacks for ch's 2&3 to passive.


I do have another question; Can someone tell me if in the JSX, does K4 bypass diodes D1 & D2? Or does it bypass C63?

[JustMike]

 Also, the noise gate may not be necessary now that I've eliminated the diode clippers, yes? If this is the case, I'd like to do the following; put some diodes or LED's in the Ultra channel only (where? is the location in the ckt critical?) and repurpose the noise gate pot R22 to be able to dial in or out the level of diode clipping.

 OK, this is wrong. Those aren't clipping diodes, they're part of the noise gate circuit. But since I've taken them out this effectively eliminates the noise gate ckt. so my original question about repurposing R22 to dial in & out some diode clipping remains.


 And, I'm forging ahead with putting in a Marshall passive tone stack. I think I can if I make them post C2 &R9 and it will work. I still don't grasp how the existing tone ckt is active...Is it because it's on the plate of V2 rather than the cathode? I've ordered some pots from Peavey and when they come in I'm going to change them out and start experimenting with C2 (between plate & cathode).