JCM 800 Preamp

[Amled87]

The JCM schemo uses a fullwave rectifier with a center tapped power transformer:

https://byjus.com/physics/full-wave-rectifier/
Marshall used two diodes in series for each "branch" to deal with the voltages involved.

A fullwave BRIDGE is wired:

https://www.circuitstoday.com/full-wave-bridge-rectifier
This (on paper) will provide 1.414 x the transformer secondary voltage. So (on paper) a 216 VAC xfmr will provide 216 x 1.414 = 305.4 VDC across the filter capacitor following the fullwave bridge.

In the real world, the actual voltage will be higher because the transformer is VERY lightly loaded by just those two 12AX7's. I'd guesstimate 325 VDC.

Amazing what a shower and a snack can do for me...I had a bad Sunday with my best friend in the hospital in a near coma.

Bri

The JCM schemo uses a fullwave rectifier with a center tapped power transformer:

https://byjus.com/physics/full-wave-rectifier/
Marshall used two diodes in series for each "branch" to deal with the voltages involved.

A fullwave BRIDGE is wired:

https://www.circuitstoday.com/full-wave-bridge-rectifier
This (on paper) will provide 1.414 x the transformer secondary voltage. So (on paper) a 216 VAC xfmr will provide 216 x 1.414 = 305.4 VDC across the filter capacitor following the fullwave bridge.

In the real world, the actual voltage will be higher because the transformer is VERY lightly loaded by just those two 12AX7's. I'd guesstimate 325 VDC.

Amazing what a shower and a snack can do for me...I had a bad Sunday with my best friend in the hospital in a near coma.

Bri

So I've ordered my mains entry connector, rocker switch for power, inline fuse, Toroidal transformer, 1N4007 diodes to create a rectifier bridge, 10k, 4.7k and 2.2k 2-watt resistors to drop the voltage, 100uf-450v Panasonic UPW filter caps did I leave anything off for the power supply?

As I see it, the mains voltage would come in (125v) and connect to the transformer primary. The first secondary (216v) of the transformer would then go through a diode bridge rectifier to get my DC (304v on paper) then I'd use the 2w resistors in series to get my voltage down to where I'd want it?

The second primary (6.3v) would connect to pins 4&5 of my 9-pin sockets. Do I have that all correct?

 

[Amled87]

For the filaments to run at 12.6V....yes one wire to pin 4 the other to pin 5 and ignore 9.

Here is a very decent tool to do "what if" power supply designs:

https://www.duncanamps.com/psud2/
Bri

Also Brian, from the look of the datasheet for that toroidal transformer provided, if I were to connect the 12.6v secondary to pins 4&5 that would be 12.6v to pin 4 and Ov to pin 5 and since it's in series that would be ok? Am I understanding that correctly?

 

[Brian Roth]

I would suggest connecting one end of the 12.6V secondary to pin 4 of each of the 12AX7's and the other end of the secondary to pin 5 of each 12AX7. IOW, the filaments would be wired in parallel across the secondary.

Bri

 

[Amled87]

I would suggest connecting one end of the 12.6V secondary to pin 4 of each of the 12AX7's and the other end of the secondary to pin 5 of each 12AX7. IOW, the filaments would be wired in parallel across the secondary.

Bri

Based on the datasheet for the toroidal that would provide 12.6v to one pin and 0v to the other? As one side of the secondary is 12.6v and the other is 0v. That correct?

 

[Brian Roth]

It's a slightly odd way they described the xfmr, but yes, that is correct. The point is there is 12.6 VAC provided between the two brown wires.

Bri

 

[Amled87]

It's a slightly odd way they described the xfmr, but yes, that is correct. The point is there is 12.6 VAC provided between the two brown wires.

Bri

Ok, I feel like I have the heater wiring squared away now, thank you. Any recommendation on layout or wiring for the full wave bridge rectifier that's connected to the 216v secondary? I can read and understand the schematic no problem. But, layout and or wiring is where I hope to gain the most knowledge/experience from this.

 

[TheJames]

How are you planning on building this? Turret board? PCB?

If you're going to use AC heaters, then you twist some wire together and jump across from tube to tube for the heaters. The twist doesn't have to be super tight. Look at a Fender or Marshall build pic and you'll see.

You might consider both PCB and turret board. PCB for the PSU (especially if using radial or snap-in caps) and a small turret board for the preamp itself.

The problem with a PCB is once it's made, it's hard to make changes. However you could lay it out with some generic points so you could try different things. For instance, you could make 2-3 different cathode lands to try out different cathode schemes (0.68uF//2.7k, 22uF//1.5k, 0.68uF//820r) and maybe put a jumper to select the different setups.

With any high-gain tube preamp, you want to watch out for stray signals and pay particular attention to grounds. You don't want squeeling oscillation (even if you can't hear it) nor any hum from bad grounding. You'll hear people talk about "lead dress" and what they mean is keeping all the signals clear of each other and crossing wires carefully to make sure something doesn't get induced where it shouldn't be.

That said...Some of the most killer sounding amps I've heard where an absolute nightmare inside and literally sat on the verge of exploding!

 

[Amled87]

How are you planning on building this? Turret board? PCB?

If you're going to use AC heaters, then you twist some wire together and jump across from tube to tube for the heaters. The twist doesn't have to be super tight. Look at a Fender or Marshall build pic and you'll see.

You might consider both PCB and turret board. PCB for the PSU (especially if using radial or snap-in caps) and a small turret board for the preamp itself.

The problem with a PCB is once it's made, it's hard to make changes. However you could lay it out with some generic points so you could try different things. For instance, you could make 2-3 different cathode lands to try out different cathode schemes (0.68uF//2.7k, 22uF//1.5k, 0.68uF//820r) and maybe put a jumper to select the different setups.

With any high-gain tube preamp, you want to watch out for stray signals and pay particular attention to grounds. You don't want squeeling oscillation (even if you can't hear it) nor any hum from bad grounding. You'll hear people talk about "lead dress" and what they mean is keeping all the signals clear of each other and crossing wires carefully to make sure something doesn't get induced where it shouldn't be.

That said...Some of the most killer sounding amps I've heard where an absolute nightmare inside and literally sat on the verge of exploding!

I'll be building the preamp for sure with terminal strips using all axial components. For the PSU, I haven't decided on using a PCB or not yet. It'll mostly consistent of a few diodes and 2-3 100uf 400v radial elco caps so I may just using a breadboard or do it on a turret as well.

 

[Amled87]

Here's what I have drew up for the preamp section. The output will be taken from middle wiper of the master volume pot. If somebody doesn't mind, could you look over this and make sure I've got my layout right? I deleted the low input side and the coupling capacitor between the the two inputs.

 

[TheJames]

That picture is pretty washed out. I can't make heads or tails of what's going on myself.

I would just follow the schematic. A note...If you're saying that you've eliminated the 22nF cap from the high input to low input, you've just put B+ across the gain pot and also high DC into the grid of the next gain stage. I wouldn't recommend that as it's not going to work out well.

Whenever you take signal off of the tube, you need to remove the DC and that's handled with a cap. Generally speaking, the bigger the cap, the more bass response. In the early stages of a high-gain preamp, you want to dump the bass to you can get some "tight" gain. However, with single coil pickups, it might get a bit too bright and thin.

Anyway...Follow the schematic. Always use a cap between stages (there's even caps blocking DC in the tone stack). Watch out for the B+, it could put a hurting on you.

 

[TheJames]

Might I suggest you watch some build videos on the YouTubes:

 

[Amled87]

That picture is pretty washed out. I can't make heads or tails of what's going on myself.

I would just follow the schematic. A note...If you're saying that you've eliminated the 22nF cap from the high input to low input, you've just put B+ across the gain pot and also high DC into the grid of the next gain stage. I wouldn't recommend that as it's not going to work out well.

Whenever you take signal off of the tube, you need to remove the DC and that's handled with a cap. Generally speaking, the bigger the cap, the more bass response. In the early stages of a high-gain preamp, you want to dump the bass to you can get some "tight" gain. However, with single coil pickups, it might get a bit too bright and thin.

Anyway...Follow the schematic. Always use a cap between stages (there's even caps blocking DC in the tone stack). Watch out for the B+, it could put a hurting on you.

So, if I delete the low input and connect V1b output to input pin 2 V1a that'll be ok as long as I leave the 22nF cap between the two stages?

 

[Amled87]

Might I suggest you watch some build videos on the YouTubes:

I'll watch and read anything that gets thrown my way, thanks!

 

[TheJames]

If you don't want the option of "low-gain" you could drop that input and just cascade the high-gain stage to the subsequent low-gain stage. All you would be eliminating is the low-gain input jack, the rest of the schematic would stay the same.

While I know you're after the JCM800...It wouldn't be a bad idea for you to look through as many classic Fender/Marshall schematics as possible. I know some people like to look at layouts...But I find the layout to be more confusing than the schematic. Anyway...The key here is looking at how the gain stages are put together. Figuring out the difference between say a Deluxe and a Twin. Or in the Marshall world, what's the difference between a JTM45, a Superlead, and a JCM800.

There are two utilities from Duncan that are really helpful. The PSU Designer that's been linked already, and the tone-stack designer. You should load up both of those and play around with them. The tone stack designer is particularly helpful in understanding what that portion of the amp does and how it affects the tone and gain of an amplifier.

I would also suggest that there's another "EQ" and "gain" aspect to guitar amps. Speaking specifically for Fenders and Marshalls...The phase inverter gets into the "gain" action as it will distort at a certain point. That's part of the sound of the older Marshalls (pre-master volume). Tonally speaking, the harder you drive the amp, the more midrange comes forward. I don't think you'll experience this so much with just a preamp though.

There's perhaps an add-on project to what you're looking at and that would be the "amperizer" or some-such nonsense to emulate the interaction of amp/cab impedance curve. You'll find that preamp into impulse response will be pretty dull sounding.

 

[Amled87]

If you don't want the option of "low-gain" you could drop that input and just cascade the high-gain stage to the subsequent low-gain stage. All you would be eliminating is the low-gain input jack, the rest of the schematic would stay the same.

While I know you're after the JCM800...It wouldn't be a bad idea for you to look through as many classic Fender/Marshall schematics as possible. I know some people like to look at layouts...But I find the layout to be more confusing than the schematic. Anyway...The key here is looking at how the gain stages are put together. Figuring out the difference between say a Deluxe and a Twin. Or in the Marshall world, what's the difference between a JTM45, a Superlead, and a JCM800.

There are two utilities from Duncan that are really helpful. The PSU Designer that's been linked already, and the tone-stack designer. You should load up both of those and play around with them. The tone stack designer is particularly helpful in understanding what that portion of the amp does and how it affects the tone and gain of an amplifier.

I would also suggest that there's another "EQ" and "gain" aspect to guitar amps. Speaking specifically for Fenders and Marshalls...The phase inverter gets into the "gain" action as it will distort at a certain point. That's part of the sound of the older Marshalls (pre-master volume). Tonally speaking, the harder you drive the amp, the more midrange comes forward. I don't think you'll experience this so much with just a preamp though.

There's perhaps an add-on project to what you're looking at and that would be the "amperizer" or some-such nonsense to emulate the interaction of amp/cab impedance curve. You'll find that preamp into impulse response will be pretty dull sounding.

Well, I also have the PI/Power amp sections of my other amps to use with it, as well as some load boxes. Again, I know I'm not building a full fledge JCM here so I don't expect the results to be so. However, I do expect to have a working preamp I can use with my other send/return amps as well as a few different cab/sim load boxes.

I'm am thankful for all the information you guys have shared thus far and I've learned a lot. I have reworked my paper drawing of the preamp schematic, care to look over it? I know it's a little messy, that'll be cleaned up during the actual build phase.

 

[Amled87]

Well, I also have the PI/Power amp sections of my other amps to use with it, as well as some load boxes. Again, I know I'm not building a full fledge JCM here so I don't expect the results to be so. However, I do expect to have a working preamp I can use with my other send/return amps as well as a few different cab/sim load boxes.

I'm am thankful for all the information you guys have shared thus far and I've learned a lot. I have reworked my paper drawing of the preamp schematic, care to look over it? I know it's a little messy, that'll be cleaned up during the actual build phase.

Ah, noticed an issue. I don't have the 0.022uF cap that tied the hi and low inputs between the output of V1A and the volume pot grounded.

Per the schematic, the cap should go to ground as well as the volume pot correct?

 

[ccaudle]

drawing of the preamp schematic

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").

 

[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").

Thank you for your reply, yes it's a layout. I actually used the schematic I posted to draw that out. As I have the schematic and it's provided by marshall themselves, I don't see no need for me to change that, as I can read it and anyone who looks over the thread could as well. As I mentioned the help I'm requesting is making sure the layout looks correct, help with the layout and all the learning experiences I can pick up along the way.

But, thank you again!

 

[Brian Roth]

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

 

[Tubetec]

A universal Local voltage:110/230 20W shaver transformer makes a fine HT transformer for tube preamps ,
available off the shelf at any electrical retailer ,