JCM 800 Preamp

[Amled87]

I hope I don't sound too pedantic, but you actually have a drawing of the preamp layout, not the schematic. A preamp schematic looks like the picture you linked in post #2, and for someone who is used to following signal flow the schematic is much easier to follow, and easier to spot things like the capacitor that TheJames was worried about in post #30. I would recommend making both to get used to thinking in terms of function, not just position (i.e. not "this resistor goes to pin 7" or "this resistor connects to pin 1" but "this resistor goes to the grid of the second section" or "this resistor connects to the plate of the first triode").

One thing that may make your journey from schematic to a layout easier is to use "highlighter" pens to mark on the schematic and then make corresponding marks on your layout drawing. That way, you can be sure all of the schematic elements are correctly translated to the layout.

One of my good friends in Florida has been making Marshall "work-alikes" for many years, and he does gorgeous work with turret boards.

http://royalamps.com/photos.html#!prettyPhoto
For the power supply, maybe consider using a small piece of perfboard to hold the rectifiers and filter cap(s).

Bri

Brian, if I understand you correctly about removing the low input I'd just delete the jack but keep the .022uF cap to the next stage of the 12ax7 like this?

I'll redo my schematic and then do a layout drawing on turret and also look at Royal Amps for ideas hopefully.

 

[TheJames]

I really dislike thinking in terms of layouts...But here are some pointers.

Draw your layout with say tubes at the top, inputs and controls (volume, bass/mid/treble, etc) across the bottom. Like a Fender layout.

Put your B+ along the top side of the layout, put grounds on the bottom.

I wouldn't really worry about the A/B side of the tubes. Work your way left to right, input to output. Input will be at the lower left and the first gain stage will be the left side of the first tube. Move along the schematic to the right in the layout.

This will make your layout as easy to follow as the actual schematic. Incidentally, it will also help things like grounding doing it this way.

 

[Brian Roth]

@Amled87: That last schemo you posted looks logical. I would still use a "shorting jack" for the "High" input jack to ground the tip of the jack to ground when no plug is inserted.

Bri

 

[Amled87]

I really dislike thinking in terms of layouts...But here are some pointers.

Draw your layout with say tubes at the top, inputs and controls (volume, bass/mid/treble, etc) across the bottom. Like a Fender layout.

Put your B+ along the top side of the layout, put grounds on the bottom.

I wouldn't really worry about the A/B side of the tubes. Work your way left to right, input to output. Input will be at the lower left and the first gain stage will be the left side of the first tube. Move along the schematic to the right in the layout.

This will make your layout as easy to follow as the actual schematic. Incidentally, it will also help things like grounding doing it this way.

Thank you, I'll definitely give this a shot as anything that makes it easier is a big help!

 

[Amled87]

@Amled87: That last schemo you posted looks logical. I would still use a "shorting jack" for the "High" input jack to ground the tip of the jack to ground when no plug is inserted.

Bri

Thanks Brian, I'll be sure to get a Switchcraft 12A 1/4" shorting jack for the input. Again, thanks everyone for all your help. Now to redraw my schematic and then use that to draw my turret layout.

 

[Amled87]

After following the advice of theJames and the pointers from you Brian, this is the layout I came up with. I used a highlighter to mark off each section as I went. If someone wouldn't mind could they check it with the schematic to make sure I have things correct?

 

[TheJames]

I think that's better myself. You might want to consider rotating the tube. You might find that the layout is easier with the tube rotated a little bit.

I didn't go through the whole layout, but right at the beginning with the 22nF between the stages, it looks like it ties directly to the grid of the second gain stage instead of at the junction of the 470k//470p network.

Maybe look at this:

https://github.com/bancika/diy-layout-creator/releases
I've never used that tool, but it looks potentially really cool. If it works well, that will be much easier to use (and share) than hand-drawing this on a piece of paper.

 

[Amled87]

I think that's better myself. You might want to consider rotating the tube. You might find that the layout is easier with the tube rotated a little bit.

I didn't go through the whole layout, but right at the beginning with the 22nF between the stages, it looks like it ties directly to the grid of the second gain stage instead of at the junction of the 470k//470p network.

Maybe look at this:

https://github.com/bancika/diy-layout-creator/releases
I've never used that tool, but it looks potentially really cool. If it works well, that will be much easier to use (and share) than hand-drawing this on a piece of paper.

I'll check it out! Quick question about the 22nF Between the stages, In the schematic it's in-between the hi & low input. Since I deleted the low input should it go directly from the output of of the first stage to the input of the second with the 470pf/470k junction attached to it? Instead of at the junction of course.

 

[EthanSpaulding]

I would just copy the layout from a 2204. They did build a certain amount of these turret construction.

Also, skip twisting the heaters, that is a waste of time. Soldano runs his heater wires straight and those builds are astoundingly quiet. The extra wire to create the the twists and how hard it is to do twisted heaters cleanly may actually give you more hum. You will want to create a voltage divider to elevate the heaters at 50V-85V and reference to that spot with some 100 ohm artificial center tap resistors. The elevated heaters will make the amp quieter and help protect the cathode follower from exceeding its maximum Cathode to Heater voltage.

 

[TheJames]

This is where redrawing the schematic might help you out. Honestly I always have to take some extra time sorting out switches and such so it's not a big deal that you didn't quite follow this.

In the JCM800 schematic, the 22nF goes left, runs up to the "switch" side of the low-gain input jack. Since there's nothing plugged in, the switch connection shorts to "input" contact of the 1/4" jack which runs over to the parallel 470k//470pF junction. You could re-draw the schematic with 22nF point right, and draw the right side of the 22nF connected to the 470k//470p junction.

 

[EthanSpaulding]

I'll check it out! Quick question about the 22nF Between the stages, In the schematic it's in-between the hi & low input. Since I deleted the low input should it go directly from the output of of the first stage to the input of the second with the 470pf/470k junction attached to it? Instead of at the junction of course.

The 22nF is a coupling cap and is needed to keep DC voltage of the grid of the next stage.

 

[Amled87]

This is where redrawing the schematic might help you out. Honestly I always have to take some extra time sorting out switches and such so it's not a big deal that you didn't quite follow this.

In the JCM800 schematic, the 22nF goes left, runs up to the "switch" side of the low-gain input jack. Since there's nothing plugged in, the switch connection shorts to "input" contact of the 1/4" jack which runs over to the parallel 470k//470pF junction. You could re-draw the schematic with 22nF point right, and draw the right side of the 22nF connected to the 470k//470p junction.

Excuse my misunderstanding James, but isn't that what I've drawn here? By deleting the low input the 22nf cap goes right and connects to the 470k/470pf junction.

 

[TheJames]

I would just copy the layout from a 2204. They did build a certain amount of these turret construction.

Also, skip twisting the heaters, that is a waste of time. Soldano runs his heater wires straight and those builds are astoundingly quiet. The extra wire to create the the twists and how hard it is to do twisted heaters cleanly may actually give you more hum. You will want to create a voltage divider to elevate the heaters at 50V-85V and reference to that spot with some 100 ohm artificial center tap resistors. The elevated heaters will make the amp quieter and help protect the cathode follower from exceeding its maximum Cathode to Heater voltage.

I'm pretty sure Soldano gets away with straight wire (really like bus bar) for the heaters because he's running them off of DC instead of AC. I think it would be ill-advised to use straight-wire with AC heaters. I can support the elevated heaters, but seeing as how this is a first build, I'd maybe suggest keeping it simple and offer up the tricks to make it better later.

No offense to Amled87, but he's made a couple of circuit moves that are potentially dangerous. I would suggest simple and safe first and foremost.

 

[Amled87]

The 22nF is a coupling cap and is needed to keep DC voltage of the grid of the next stage.

I understand that it's a coupling cap and is needed between the first and second gain stage. I'm not going to remove it, just confused on the correct location for it because I plan to delete the low input.

 

[Amled87]

I'm pretty sure Soldano gets away with straight wire (really like bus bar) for the heaters because he's running them off of DC instead of AC. I think it would be ill-advised to use straight-wire with AC heaters. I can support the elevated heaters, but seeing as how this is a first build, I'd maybe suggest keeping it simple and offer up the tricks to make it better later.

No offense to Amled87, but he's made a couple of circuit moves that are potentially dangerous. I would suggest simple and safe first and foremost.

Completely agree, first build so not looking to have all the tricks and etc yet. Just want something that ends up working and with safe building practices.

 

[TheJames]

Excuse my misunderstanding James, but isn't that what I've drawn here? By deleting the low input the 22nf cap goes right and connects to the 470k/470pf junction.

The problem is that the 22nF goes straight to pin7 of the tube (grid) when it should go to the parallel 470k//470p network, then to the 1M "volume" pot first, then the output of the pot feeds pin 7.

 

[Amled87]

The problem is that the 22nF goes straight to pin7 of the tube (grid) when it should go to the parallel 470k//470p network, then to the 1M "volume" pot first, then the output of the pot feeds pin 7.

Ah, so the cap should go to the 470k/470pf network first, then the pot and the output of the pot connect to the following gain stage. Does that look correct on this layout drawing?

 

[TheJames]

Ah, so the cap should go to the 470k/470pf network first, then the pot and the output of the pot connect to the following gain stage. Does that look correct on this layout drawing?

Yep yep. That's better.

First note...You don't necessarily need it and you might want to try it with and without, but there's a 1nF cap the sits across the input and wiper of the 1M volume pot. That would typically be mounted on the pot itself.

Second note...You probably want to think about the grounds. The layout as drawn has the ground of the second stage happening at the same point as the input and the first stage. I would suggest that you ground the input to the chassis, then ground the first stage to a point further away from the input, the second stage ground happens downstream from the first stage, etc. The idea is that you want to keep the ground currents moving towards the ground at the power inlet.

This is why I suggest that rotating the tube might help with the layout.

 

[Amled87]

Yep yep. That's better.

First note...You don't necessarily need it and you might want to try it with and without, but there's a 1nF cap the sits across the input and wiper of the 1M volume pot. That would typically be mounted on the pot itself.

Second note...You probably want to think about the grounds. The layout as drawn has the ground of the second stage happening at the same point as the input and the first stage. I would suggest that you ground the input to the chassis, then ground the first stage to a point further away from the input, the second stage ground happens downstream from the first stage, etc. The idea is that you want to keep the ground currents moving towards the ground at the power inlet.

This is why I suggest that rotating the tube might help with the layout.

I chose to eliminate the 1nF cap as it's not on some of the JCM 800 drawings. For the grounds, I drew it that way because it made it easier. However, I planned to ground the input to the chassis closest to the input and the grounds from the 2nd stage be connected to the chassis ground coming from IEC inlet in the rear. That seem logical?

For example, the input and first stage would be ground nearest the input to the chassis. The 2nd stage and pots would be ground to the chassis closer to the rear where my mains enter through the IEC power entry.

 

[CJ]

OT: got to work on a Soldano the other day, pretty impressive, i believe this is a teflon circuit board, no?