JCM800 Build

[RadioComm]

Good Morning,

About 15 years I purchased on ebay a modified JCM800 4010. All underneath the chassis was stock. The previous owner though has ripped out the original board, and build a new amp. It sounded good, but not the Marshall tone I wanted, which to me is heavy on the attack and very sensitive to one's fingering. I implemented Robert Souza's (dreamtone.org) (schematic/layout attached), and it nailed what I expected from a Marshall. Also attached are some pictures of my build.

Since the previous owner had an effects loop installed, I decided to put one in and take the punched/drilled holes (switch/jacks/pots/tube), but let it go and just enjoy that Marshall tone. Decided to resurrect this project (15 years later). I'll post about it in the next post.

[RadioComm]

The effect loop was a kit, and included the board with board components and a schematic. I've attached pictures and the schematic for wiring the loop into the amp.

This project though was sold as an unfinished effort, and it was up to the buyer to finish. I installed the board, but never wired it up. So from the attached schematic, the undefined components values are:
    * R44 / R45 / R46 (x2) / R48 / R49
    * C52
    * Pots (will get current values)

Have no clue as to what values to start with. How should I proceed figuring those values out? Also what are the thoughts on the validity of this schematic?

Thanks so much in advance for the responses. I'm now running out for a couple of hours, but will be back later.

[RadioComm]

Found a picture of the previous owner's work.

[pdf64]

The labelling for the send and return level controls seems swapped over to me?
The pot values and valve type are required to come up with reasonable values for the other components.

[joesatch]

Building something similar now. why do these boards have the grid stopper mounted on them? Isn't it better to solder it directly at V1 grid?

[PRR]

The labelling for the send and return level controls seems swapped over to me?..

+1. I was like "???" and then saw your post.

While we could over-think the pot values, we could just guess an assumption and crack on with it. FX loop level should be very low so it is not a critical design.

But we could also wait (it's been 15 years so far) and see if someone yells "Hey! That's a Pete-Paul Piccolo design and the docs are here___!"

[sluckey]

Building something similar now. why do these boards have the grid stopper mounted on them? Isn't it better to solder it directly at V1 grid?

A long time ago those "grid stopper" resistors were known as input mixing resistors. You could put them anywhere you wanted to. Fender liked to mount them directly on the input jacks. Other manufacturers put them on the board. Still others had the preamp tube located near the input jacks and simply connected the resistors directly between the jacks and the tube socket grid pin. I don't recall ever working on a '60s amp that wired the input "grid stoppers" as we do today.

[RadioComm]

The labelling for the send and return level controls seems swapped over to me?
The pot values and valve type are required to come up with reasonable values for the other components.

Thanks @pdf4 and @PRR. Noted for the swapped labeling. And yes, oops... Forgot to include that I need the pot values on top of the resistors and cap. Updated in original post. The pots that are in there now are just left over from the previous owner. I was just going to re-use them if I could, otherwise just buy new. I'll get the current pot values tomorrow morning and post them.

As far as the placement of the grid stoppers, I just implemented Robert Souza's layout. Did absolutely no thinking. Thanks @sluckey for the knowledgeable response.

I'm AFKB for the rest of the evening, but will be back tomorrow.

[RadioComm]

[But we could also wait (it's been 15 years so far) and see if someone yells "Hey! That's a Pete-Paul Piccolo design and the docs are here___!"

lol. I could wait another 15 years I guess, effect loop not on top of my priority list... At all. Very happy the way it sounds now. The effects loop kit was purchased on ebay. The schematic the seller provided was a hard copy, there was no web site. The seller made it clear that the board and schematic was sold as is with absolutely no support.

[RadioComm]

The effect loop pots that are in there are 250K each.

[pdf64]

A long time ago those "grid stopper" resistors were known as input mixing resistors. You could put them anywhere you wanted to. Fender liked to mount them directly on the input jacks. Other manufacturers put them on the board. Still others had the preamp tube located near the input jacks and simply connected the resistors directly between the jacks and the tube socket grid pin. I don't recall ever working on a '60s amp that wired the input "grid stoppers" as we do today.

In this case, there’s only one input per stage, so the input mixer term would seem to be inappropriate, as its only feasible purpose is a grid stopper.
So that’s a significant change compared to 50s/60s amp designs.
And due to the extra cascading CC stage, there’s maybe 20dB more gain than those amps.
So calling it a grid stopper seems fitting.
The 50s Fenders which Marshall based their amps on tended to use board mounted input mixers.

[RadioComm]

Hello everyone,

Went over the 'unfinished/unverified' JCM800 effects loop schematic and made the following updates:

• Swapped the send/return labels for the level pots
• Swapped/corrected the send/return wires in the DPDT switch so that the send goes into V6a and returns from V6b
• Measured voltage from amp board into fx loop
• Recorded component values on purchased fx loop board

Notes are attached in the PDF schematic.

Searched the internet for an effects loop schematic that was 'somewhat similar' to that of the JCM800 fx loop layout, and found a schematic based off of the Vox V941 (atttached).  From many other schematics, I'm going to go with a 12AU7 for V6, and 100K for R48 to drop the voltage level on V6a to a lower one on V6b.

Since I'm not too sure where to go from here, I'm going to continue searching the web from 'similar' schematics as to make the best guess for the unknown values.

Any suggestions or precautions would be greatly appreciated.

Thanks.

[RadioComm]

Here's my approach on top of the changes in the schematic of the previous post (notes_fxloop_schematic.pdf).

1) Looks like C52 / R45 are tone adjustment components and add brightness to the audio signal:
 - I'm going to remove entirely the connection from the tip to the ground of the return input jack, thus removing R45.
 - I'm removing the C52 connection, and directly hooking up the tip of the return input to pin 2 of the return level pot.
 - After testing, if more brightness is needed then I'll figure out what C52/R45 combo to use.

2) Looks like the R46 resistors would only affect input levels. From what I've seen on other effects loops, the values are around 3K. Will start there and will fine tune.

3) Now for connecting the two send/return input jacks (R44/R49/direct send tip to return shunt connection). That's where I still need to figure it out. Ultimately with both male jacks in, I'd want no connection what so ever between the two input jacks. The entire audio signal would be routed to the effects and back. Then with both of them out, I'd want the shunt to be a dummy load approximating the load of the effect pedals. I have shunts on my input send jack.
- This is where the Vox and this circuit  differ from other effect loops that I looked at.
---- Most of them have the input jacks on the outside of the circuit and the level pots within them. When the two input jacks are shunted, the circuit is bypassed.
---- This circuit has the level pots on the outside and the input jacks on the inside, so that when the two  input jacks are shunted, the level pots still affects the audio signal.

Any thoughts or precautions?

Thanks again.

Added jpg of the notes on schematic.

[RadioComm]

Went searching though my archives (a bunch of old boxes) to see if I could find some previous notes on this project, and stumbled across a complete effects loop schematic specifically for the  JCM800 (attached). This was one that I had found on Steve Aloha's site https://blueguitar.net. RIP Steve, and many thanks for your awesome web site.

I'm going to scrap the previous effort, and implement a layout for that schematic and build it. I'll post pics when I'm done.

Happy Holidays Everyone!

ttfn

[RadioComm]

For the moderators: The previous schematic has a copyright. Don't know if it is still valid... Please note it is still available on Steve's site, and has been for at least 15 years. The copyright is from 1986/1997.

[tubenit]

I added this FX loop to the lengthy thread with many other schematics of FX loops here in ARCHIVES:   tube effects loop on board (el34world.com)


With respect, Tubenit

[pdf64]

...I'm going to go with a 12AU7 ...

If using instrument level pedal type fx (rather than rack fx intended for pro line level), a 12AU7 triode may not have enough voltage gain to recovery the signal back to its previous level.

[RadioComm]

...I'm going to go with a 12AU7 ...

If using instrument level pedal type fx (rather than rack fx intended for pro line level), a 12AU7 triode may not have enough voltage gain to recovery the signal back to its previous level.

Thanks for the response @pdf64. But as stated in the previous post I'm dropping the first unverified effects loop, and working on a layout to implement Kurt Stewart's JCM800 effect loop (the one @tubenit put in the archives). This schematic specifies a 12AU7. Once built and tested, if it needs tweaking, I certainly will start with the triode.

Also to implement the layout I'll be using for the first time DIY Layout Creator. I use to do this by hand, pencil and paper. This is a *fun* endeavor. Thanks Doug for all the resources.
ttfn

[sluckey]

You may consider using a 12DW7. Pins 1,2,3 are like a 12AU7 and pins 6,7,8 are like a 12AX7. Use low gain (pins 1,2,3) for send and high gain (pins 6,7,8) for return. No circuit changes needed.

[tubenit]

You may consider using a 12DW7. Pins 1,2,3 are like a 12AU7 and pins 6,7,8 are like a 12AX7. Use low gain (pins 1,2,3) for send and high gain (pins 6,7,8) for return. No circuit changes needed.

I've used this approach with "one tube" reverbs and with active FX loops and liked the results.


With respect, Tubenit

[RadioComm]

You may consider using a 12DW7. Pins 1,2,3 are like a 12AU7 and pins 6,7,8 are like a 12AX7. Use low gain (pins 1,2,3) for send and high gain (pins 6,7,8) for return. No circuit changes needed.

I've used this approach with "one tube" reverbs and with active FX loops and liked the results.


With respect, Tubenit

Awesome! Great advice. I'll go through my tube boxes (collected over the years at HamFests) to see if I can find any 12DW7. If not I'll order some.

Thanks @sluckey.