JCM 800 Preamp

[Amled87]

I'm in the process of acquiring the components to hand wire a JCM 800 style preamp to use with the Return input (phase inverter/power amp) of my DSL 40C.

A couple notes :
1. I am new to circuit design and layout.
2. I have basic knowledge and the understanding of schematics, adequate soldering skill.
3. This is in no way meant to replace a 3000$ amp, if I wanted a JCM 800, I'd just buy one. The end goal here, is to learn the ins and outs of the circuit, proper layout and schematic design.

Now, to the point. I am planning on using just the preamp section of a JCM 800 which includes the V1b&V1a gain stage, V2a gain stage and the cathode follower driven tone stack on V2b, which would be sent out to another amps phase inverter/power amp.

However, since I'm only using the preamp section, I don't see much need for a power transformer. So, would there be any upside to using one versus using a voltage doubler to provide 230/240vdc to the 12AX7 stages and using ohm's law to calculate a voltage divider to provide the heaters with 3.13v wired in parallel?

With proper filtering and smoothing caps, I don't see having a stable 200-240vdc for the tube stages being an issue nor the 3.13vdc parallel heater voltages either. Besides, price is definitely questionable as I could build the voltage doubler/divider for alot less then a quality power transformer.

I'd love to hear any insight, or ideas. This will be mounted in a 15"x7"x3" Hammond enclosure.

 

[Amled87]

 

[Tubetec]

Galvanic isolation and limiting from the low source impedence of the mains vs a situation where chassis (hopefully) ends up at more or less ground potential ?
In the event of a mains wiring fault or unpolarised two pin mains connection , how much is yours or someone elses life worth vs paying for a proper transformer . Directly powered AC mains radios always had chassis insulated from touch by the user , how do you manage that when a guitar has its strings connected to chassis ?

 

[Amled87]

Galvanic isolation and limiting from the low source impedence of the mains vs a situation where chassis (hopefully) ends up at more or less ground potential ?
In the event of a mains wiring fault or unpolarised two pin mains connection , how much is yours or someone elses life worth vs paying for a proper transformer . Directly powered AC mains radios always had chassis insulated from touch by the user , how do you manage that when a guitar has its strings connected to chassis ?

Well, I planned to use a wall 20vdc 2A power supply to start my build and with proper grounding. After prototyping and making sure my circuit works, then invest in the proper transformer with the proper secondaries to provide the correct voltages I need. Not an issue of skimping on the correct components, but a reason to not buy twice. However, I guess if I'm going to prototype I can shell out the 70-80$ for the transformer as well.

 

[TheJames]

$70-80 for the transformer is more like a full power transformer for a full amp. I posted a link to a toroidal transformer that should do what you need in your other thread. $20 with $12 in shipping or so.

Again...I haven't dug through the schematic of your DSL40C, but it would not surprise me if you couldn't find a path that closely resembled the actual JCM800 signal path and maybe tweaked a couple of values to make it much closer to your desired goal.

 

[Brian Roth]

You cannot use a voltage multiplier with a DC power source input.

Bri

 

[Amled87]

$70-80 for the transformer is more like a full power transformer for a full amp. I posted a link to a toroidal transformer that should do what you need in your other thread. $20 with $12 in shipping or so.

Again...I haven't dug through the schematic of your DSL40C, but it would not surprise me if you couldn't find a path that closely resembled the actual JCM800 signal path and maybe tweaked a couple of values to make it much closer to your desired goal.

I found a Hammond transformer that'll provide 120v @ 140ma & 6.3v @ 1.5A that I had. I'll be using that, the next step is getting that figured out, It's a Hammond 262F6.

As for the DSL, the crunch channel is essentially a JCM 800. However, I'd prefer not to lug around 70lb amp when I can build a preamp to move around alot easier location to location, especially being able to use it with an audio interface and impulse responses.

 

[Amled87]

You cannot use a voltage multiplier with a DC power source input.

Bri

The goal wasn't to use a multiplier with a DC source. I had planned to use a 20vdc power supply to prototype, then once the circuit works, remove that and replace with an adequate transformer and a voltage multiplier to convert the AC to DC and up the voltage if I needed it.

However, I am using a Hammond 262f6 now which is a 120 primary and 120v @ 140ma & 6.3v @ 1.5A secondary transformer.

 

[Brian Roth]

This is getting complicated with two different threads covering the same project. :-(

Bri

 

[Amled87]

This is getting complicated with two different threads covering the same project. :-(

Bri

I apologize. I'll only reply here and delete the other. I've got the Hammond 262f6, but I'll also pick up the toroidal transformer that was suggested to me as it provides 216v @ 0.12A & 12.6v @ 0.8A.

 

[Brian Roth]

The other thread had an Ebay link for an inexpensive transformer which could be a good choice. Antek is another vendor I'm aware of. I never used their products but they seem to have a good reputation.

This link is a bit mind boggling, but worth some time to investigate. Most (all?) of the higher voltage xfmrs also have filament voltage windings and the prices are good.

https://www.antekinc.com/grids/
Bri

 

[Amled87]

The other thread had an Ebay link for an inexpensive transformer which could be a good choice. Antek is another vendor I'm aware of. I never used their products but they seem to have a good reputation.

This link is a bit mind boggling, but worth some time to investigate. Most (all?) of the higher voltage xfmrs also have filament voltage windings and the prices are good.

https://www.antekinc.com/grids/
Bri

The toroidal transformer I'm referencing is the one from the eBay link that got provided.

As the plan has changed, I'll be using the eBay toroidal with 1N4007 diodes to convert the 216v secondary #1 to DC for the B+.

The 12.6v secondary #2 I'd use to wire the the heaters and that would be 1 wire each to pin 4&5 and pin 9 will be left open? I'll also need to recalculate my filter caps for the B+ as well unless the 220uF 315v electrolytics I have would still be sufficient?

 

[Brian Roth]

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

 

[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

Thank you! Ill definitely give it a read and save it for future refernce. I'm now going to start with my enclosure, the toroidal power transformer, rocker switch and fuse for on/off power. Once I have that all set, I'll provide a schematic of what I have at that stage before I begin wiring the preamp via terminal strips/turret board.

Again, thanks for your help as this really streamlined and made it faster getting to my goal.

 

[Brian Roth]

I jumped back to the other thread to check the specs on the Ebay xfmr and then back up the first of this thread to look at the DC voltages in the schemo. I think you will need a voltage doubler to get the required DC rail. But, run it through the PSU Designer I linked above. I'll "what if" it here in awhile just because I'm curious! I have the PSU Designer app here on my hard drive

Bri (off to take a shower now....lol!)

PS I relinked the Ebay listing to try and keep this thread intact:

https://www.ebay.com/itm/2932592739...=1&mkrid=711-53200-19255-0&ufes_redirect=true

 

[Amled87]

I jumped back to the other thread to check the specs on the Ebay xfmr and then back up the first of this thread to look at the DC voltages in the schemo. I think you will need a voltage doubler to get the required DC rail. But, run it through the PSU Designer I linked above. I'll "what if" it here in awhile just because I'm curious! I have the PSU Designer app here on my hard drive

Bri (off to take a shower now....lol!)

PS I relinked the Ebay listing to try and keep this thread intact:

https://www.ebay.com/itm/2932592739...=1&mkrid=711-53200-19255-0&ufes_redirect=true

12ax7's can potentially operate anywhere from 12v to 300v. I'm not setting out to hit the voltages in the schematic exact. They're ran higher in the schematic because the output section and etc needs more voltage and the voltage is there from the power transformer so they used it.

I'd imagine after rectifying the 216v secondary of toroidal transformer that was provided and converting the AC to DC using 1N4007's and adding smoothing/filter caps I'd be in the ball park of 200-220v which will work for me and my build.

 

[EthanSpaulding]

That transformer should be well suited for a jcm800 build. Different jcm800's had different B+. The ones that are sought after, are the ones with the B+ around the 442V range. The schematic shows the higher voltage version with B+ around 470V. To sweeten the high voltage ones up, guys will increase the value of the B+ dropper resistor between the phase inverter and the tone-stack driver stage. The sweet spot seems to be between 240V for a very compressed liquid sound to 284v for a ruckus rock tone with a lot of attack. My point is to not try to hit 340V cause you'll end up having to drop it to get in the sweet spot. 216V bridge rectified is around 300V btw. About perfect. You can then add a stage or two of R-C filtering and adjust the R to dial in your tone.

 

[Amled87]

That transformer should be well suited for a jcm800 build. Different jcm800's had different B+. The ones that are sought after, are the ones with the B+ around the 442V range. The schematic shows the higher voltage version with B+ around 470V. To sweeten the high voltage ones up, guys will increase the value of the B+ dropper resistor between the phase inverter and the tone-stack driver stage. The sweet spot seems to be between 240V for a very compressed liquid sound to 284v for a ruckus rock tone with a lot of attack. My point is to not try to hit 340V cause you'll end up having to drop it to get in the sweet spot. 216V bridge rectified is around 300V btw. About perfect. You can then add a stage or two of R-C filtering and adjust the R to dial in your tone.

I plan to keep the B+ around 240-280v. I knew I'd have more voltage after a bridge rectifier. So, I'm picking up a few different 2watt resistors to dial the voltage where I want it. I'd imagine a few 10k 2-watt resistors would get me close enough.

I'd imagine a simple bride rectifier would be ok here? Something simple like the schematic I just provided with filter caps? The 216v secondary connected to this bridge rectifier?

Oddly, in the JCM 800 schematic it shows the diodes for the + side all going the same direction and the - side going the same direction as well before they tie together? But its different in this?

 

[EthanSpaulding]

The jcm800 in the posted scheme has a center tap. You cannot wire the Ebay transformer like that because it has no center tap. The bridge rectifier circuit is what you will need to do. Your drawing is perfect. Nice work.
Get some 4.7k and 2.2k 2 watt resistors as well. It's very sensitive if you are picky. The PR02 series is very cheap for B+ droppers and once you install and remove them several times you sent feel so bad about throwing them away for being all bent up and ugly looking:
https://www.mouser.com/c/passive-co...-resistors-through-hole/?m=Vishay&series=PR02
If you have a windows machine, install the PSUD2 software like Brian mentioned. Having some basic electronics books around is also great.

 

[Brian Roth]

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