JCM 800 Preamp

[baribari]

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

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I am not sure if it is Teflon but I have worked with Mil pcbs and Teflon was troublesome with warping. If you not at Ghz frequencies there is no need. Looks nice though.

 

[Amled87]

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

View attachment 92442

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I'm actually really surprised to see all those orange drops in there. Looks like a nice layout though.

 

[Tubetec]

I havent seen any amps where the power tubes get DC heaters , you do see some amps with a seperate preamp Dc heater supply , it does bring the hum down a bit especially if your cranking the gain .

Bus baring the power tube heaters with two parralel lenghts of copper Ive seen done on several amps , Ampeg SVT and Mesa come to mind .
its a quick way to do it and its rock solid/bomb proof .

Even with DC heaters on preamps I prefer AC lead dress .

Teflon is worth using in certain audio applications ,the grid and capsule of tube mics are a good example where we want extra high insulation resistances.

 

[EthanSpaulding]

The Soldano built by BAD uses DC heaters with the new board. But all the straight wire builds that Mike did himself are AC heaters and quiet.

 

[CJ]

Nice clean channel circuit, probably a James Brown design, sounds great,

http://www.thetubestore.com/lib/thetubestore/schematics/Peavey/Peavey-6505PLUS-112-Schematic.pdf

 

[Fdieck56]

I think that could be white solder mask on a conventional PCB. Teflon boards are ungodly expensive and hard to fab.

 

[Amled87]

Just making sure I've got this correct, for this transformer I'd connect the primary input in series for 110v and then connect the 325v secondary in parallel for 325v 0.8A?

The specs are
Secondary :
#1 325v .4A
#2 325v .4A
#3 6.3v 4A
#4 6.3v 4A

It's an Antek 325v power transformer with a center tap.

That would yellow to yellow, and white to white? I'd ground the white then I imagine?

 

[EthanSpaulding]

Doing the full 100 watt build?

If doing the full 100Watt 800, you will want to create a center tap of White/Yellow. It will look like the original jcm800 schematic posted. Except that you don't have a lower voltage -Vbias tap, so the dropper resistor will be larger.

There are no circumstances, I can think of, where you would ground both white wires unless you are considering half wave rectification. I don't think you want to half wave rectify a toroidal transformer. Someone that knows more would have to answer that.

 

[Amled87]

Doing the full 100 watt build?

If doing the full 100Watt 800, you will want to create a center tap of White/Yellow. It will look like the original jcm800 schematic posted. Except that you don't have a lower voltage -Vbias tap, so the dropper resistor will be larger.

There are no circumstances, I can think of, where you would ground both white wires unless you are considering half wave rectification. I don't think you want to half wave rectify a toroidal transformer. Someone that knows more would have to answer that.

Well, my build is omitting the PI and EL-34 output stages. It's just the preamp, to be used with other amps send/return or feed into the PA of those amps.

My issue is just wrapping my head around hooking the transformer up.

If I'm not mistaken :

The primaries would be in parallel for 110v, then the 325v secondaries would be in parallel. The first filter cap would be between the transformer and the full wave diode bridge rectifier.

I'd then use a diode bridge full wave rectifier to convert to DC (after rectification I'd have roughly 450-470v) another filter stage, followed by a few series 10k 2-watt resistors to drop the voltage to 300v or so.

Does that sound correct?

The 100k load resistors on the 12AX7's will drop that 300v even lower and within the 12AX7's working voltage, does that seem to be right?

 

[EthanSpaulding]

That PT is overkill. Consider this one:
https://www.antekinc.com/as-05t240-50va-240v-transformer/
Still over spec'd for a preamp, but at least you won't have to drop 200+ volts at only a few milliamps of draw from your 12ax7's.

And yes. Parallel the 110Vs to Mains. And use a Bridge rectifier to derive B+. Earlier when I mentioned that you want 240V - 284V at the tones stack node, that is at the supply. Not at the plate. The above transformer will give you ~311V. That is plenty of headroom to eliminate ripple and experiment around with tones.

 

[Amled87]

That PT is overkill. Consider this one:
https://www.antekinc.com/as-05t240-50va-240v-transformer/
Still over spec'd for a preamp, but at least you won't have to drop 200+ volts at only a few milliamps of draw from your 12ax7's.

And yes. Parallel the 110Vs to Mains. And use a Bridge rectifier to derive B+. Earlier when I mentioned that you want 240V - 284V at the tones stack node, that is at the supply. Not at the plate. The above transformer will give you ~311V. That is plenty of headroom to eliminate ripple and experiment around with tones.

Well, if at all possible I'd like to possibly use what I have. A voltage divider before rectification would only drop 100v or so. I guess I could always buy a 3rd transformer if this one isn't suitable though. Like I said, I bought this one ona whim with the thought I could add a PI and output stage later.

 

[Amled87]

That PT is overkill. Consider this one:
https://www.antekinc.com/as-05t240-50va-240v-transformer/
Still over spec'd for a preamp, but at least you won't have to drop 200+ volts at only a few milliamps of draw from your 12ax7's.

And yes. Parallel the 110Vs to Mains. And use a Bridge rectifier to derive B+. Earlier when I mentioned that you want 240V - 284V at the tones stack node, that is at the supply. Not at the plate. The above transformer will give you ~311V. That is plenty of headroom to eliminate ripple and experiment around with tones.

Well, picked this up for 37$ shipped. It's smaller to mounting it either inside/outside would be easier in my enclosure. Thanks for the help Ethan!

 

[Rusan]

Oh wow, I just now saw that this is the same as the thread I'd been posting on earlier over in The Lab. I'd skimmed over this one a few days ago when it first started, and hadn't looked at it again. Sorry guys. Maybe a moderator can fix my mess.

 

[EthanSpaulding]

You can definitely use that AS3T325 for a sweet build. Great for Marshall type stuff. I just finished a build with the AS3T350. 4x6L6's and does more than 100 watts of power with a B+ of 480V idling each 6l6 at 30mA/each. Save the big guy and use it when you finish the pre!

 

[Amled87]

Little up date, still waiting on my power transformer to arrive, but got the turret board put together, the enclosure drilled and everything assembled.

Once my power transformer arrives, I'll be ready to build my power supply board and get it installed and then try it out through the return of my DSL 40.

 

[EthanSpaulding]

How come you put your tubes so far from the board? There is a 2204 layout out there that you should have copied. If you don't re-invent the wheel, you won't have to deal with the problems already solved.

All those wires can't be run together. The jcm800 has 450 degrees of phase shift in the preamp. Wire grouped together should not be more than 180 degrees apart. Does that makes sense? The grid of a triode setup as a common cathode amplifier is 180 degrees out of phase of the plate. The plate of the triode is 180 degrees of phase with the plate of the next stages plate. Good rule to live by. How you have it, you run the chance of creating an oscillator or a preamp that has an unstable transient response(finger nails on chalkboard pick attack).

 

[Amled87]

How come you put your tubes so far from the board? There is a 2204 layout out there that you should have copied. If you don't re-invent the wheel, you won't have to deal with the problems already solved.

All those wires can't be run together. The jcm800 has 450 degrees of phase shift in the preamp. Wire grouped together should not be more than 180 degrees apart. Does that makes sense? The grid of a triode setup as a common cathode amplifier is 180 degrees out of phase of the plate. The plate of the triode is 180 degrees of phase with the plate of the next stages plate. Good rule to live by. How you have it, you run the chance of creating an oscillator or a preamp that has an unstable transient response(finger nails on chalkboard pick attack).

Well, the tubes are only about 1.5" or so away from the board. I more a less copied the Ceriatone 2204 layout, which I thought I had everything ran the same way from that drawing. At the moment, none of the 100k resistors or B+ is hooked up.

Based on the what you're saying I need to move each side of the tube socket away from each other? I just have things bundled together currently as I needed room to work and my enclosure is a little small. I have roughly enough wire on both sides to move them.

So, if I understood correctly I can run side A of each tube together and then run side b of each tube together? These are all issues I didn't know would present a problem which is why I came here for guidance. Remember, first build so learning as I go.

 

[Amled87]

How come you put your tubes so far from the board? There is a 2204 layout out there that you should have copied. If you don't re-invent the wheel, you won't have to deal with the problems already solved.

All those wires can't be run together. The jcm800 has 450 degrees of phase shift in the preamp. Wire grouped together should not be more than 180 degrees apart. Does that makes sense? The grid of a triode setup as a common cathode amplifier is 180 degrees out of phase of the plate. The plate of the triode is 180 degrees of phase with the plate of the next stages plate. Good rule to live by. How you have it, you run the chance of creating an oscillator or a preamp that has an unstable transient response(finger nails on chalkboard pick attack).

Tomorrow I'll remove that turret board, use this layout for a second board and reposition the tube sockets right behind it that way I can run the wires as they're shown on this layout. Would that alleviate my issue Ethan?

 

[EthanSpaulding]

Yes. Routing side A only with side A and side B only with side B is good. Merlin Blencowe even makes a good case for twisting each triodes wires together. I've never done that, but the point is to minimize any stray coupling.

The Ceriatone layout looks good. Just abandon those holes and make new ones. I understand it's getting crowded to put the tube sockets between the board and the back panel jacks. Just test fit everything a couple times. You could even remount the board closer to the controls since it looks like everything is wired up over there. The jcm800 is not as high-gain as a lot of amps, so you can get away with a lot. You can series string the heaters together as well.

One more thing: You will want your output level to be "stomp box level". Simplest way to do it would be to replace the 1M master-volume with a 470k dropping resistor to 50k potentiometer. Technically, that is not a low output impedance, but it is as good or better than a lot of stompboxes and simpler than building a buffered output/effects send circuit. It will drive a 10 foot cable without too much top end roll-off. Likely you will have this close to the power amp, so there will be no issues.

 

[Amled87]

Yes. Routing side A only with side A and side B only with side B is good. Merlin Blencowe even makes a good case for twisting each triodes wires together. I've never done that, but the point is to minimize any stray coupling.

The Ceriatone layout looks good. Just abandon those holes and make new ones. I understand it's getting crowded to put the tube sockets between the board and the back panel jacks. Just test fit everything a couple times. You could even remount the board closer to the controls since it looks like everything is wired up over there. The jcm800 is not as high-gain as a lot of amps, so you can get away with a lot. You can series string the heaters together as well.

One more thing: You will want your output level to be "stomp box level". Simplest way to do it would be to replace the 1M master-volume with a 470k dropping resistor to 50k potentiometer. Technically, that is not a low output impedance, but it is as good or better than a lot of stompboxes and simpler than building a buffered output/effects send circuit. It will drive a 10 foot cable without too much top end roll-off. Likely you will have this close to the power amp, so there will be no issues.

I've moved the tube sockets behind where I'll mount my turret board. That way I can follow the Ceriatone layout (for only the preamp section) exact.

My question though, since the DSL 40C has a send/return that uses what I believe to be a buffered effects send I would be ok plugging the output of my 1Meg master volume into the return of the DSL. Effectively bypassing the DSL preamp (V1-V3) and be ok? It seems I wouldn't need the 470k dropping resistor into a 50k pot if that was the case, that seem correct?

I'll provide sections of the DSL schematic if you wouldn't mind checking that.