JCM 800 Preamp

[EthanSpaulding]

The 1M master volume of the 2204 preamp is high level. It will drive the 50W power section beyond clipping. Notice in the DSL40C schematic you have two separate voltage dividers that drops the signal to FX level. First you have the R46 560k dropper into the 500k (two 1M VR1 and VR2 in parallel) and then the 330k 100k voltage divider (R40 R44) just before the IC1b FX Send buffer. IC1A is your Return make-up gain, and you can't bypass that. So, your preamp will need to be padded down. That will be cool anyway as you may want to stick a reverb or delay between your preamp and the return of the DSL40c.
Doing a separate 1M master volume without a dropper may be cool as well. Some power amps need a higher level output- the VHT/Fryette power amps for instance don't make full power at FX send levels, but that is actually pretty rare to not take FX level input to a power amp.

470k into a 50k master volume is about right to get a 2204 preamp to FX level output. It will depend slightly on your B+, but those are tested and proven values. Ideally you want an output impedance of less than 4.7k so you can drive a 40 foot cable of 2nF cable capacitance to and from your pedal board. The 50k master volume will let you down in that case

 

[Amled87]

The 1M master volume of the 2204 preamp is high level. It will drive the 50W power section beyond clipping. Notice in the DSL40C schematic you have two separate voltage dividers that drops the signal to FX level. First you have the R46 560k dropper into the 500k (two 1M VR1 and VR2 in parallel) and then the 330k 100k voltage divider (R40 R44) just before the IC1b FX Send buffer. IC1A is your Return make-up gain, and you can't bypass that. So, your preamp will need to be padded down. That will be cool anyway as you may want to stick a reverb or delay between your preamp and the return of the DSL40c.
Doing a separate 1M master volume without a dropper may be cool as well. Some power amps need a higher level output- the VHT/Fryette power amps for instance don't make full power at FX send levels, but that is actually pretty rare to not take FX level input to a power amp.

470k into a 50k master volume is about right to get a 2204 preamp to FX level output. It will depend slightly on your B+, but those are tested and proven values. Ideally you want an output impedance of less than 4.7k so you can drive a 40 foot cable of 2nF cable capacitance to and from your pedal board. The 50k master volume will let you down in that case

So, if I understand correctly I'd go from my treble pot output to a 470k series resistor and then into a 50k potentiometer. Lug 3 of the 50k would be ground and my preamp out would come off the middle of the 50k as I was doing previously with the 1meg master?

 

[EthanSpaulding]

Yes exactly!

 

[Amled87]

Yes exactly!

So, i forgot to put the 470k resistor in series with the 50k pot. However, when ran into the return of another amp I have no output at all. I used another switchcraft 12A (shorting jack) for the output as well. I soldered the ground to the shunt on the output.

I can't for the life of me figure it out. The tube filaments are heating up and my voltage is about correct at each plate. Could it be the output jack? Should I disconnect the ground from the shunt?

Edit: only checked voltage at the power supply first cap. Had 400v+ there so figured it was good. After rechecking the following 40uF caps in my power supply (it's a 100uf, 40uF and 40uF) I've not got any voltage at cap #2 & #3. Upon inspection, there's no connection between the first 10k dropping resistor and the cap #1. Thus voltage wasn't being passed to the second and third cap in my power supply.

*Facepalm* it's always the simplest things.. Now to see if that fixes the issue.

Also, I didn't expect a 240v secondary to produce well over 400vdc? Should be closer to 336vdc, no?

 

[Brian Roth]

Since the transformer is "very lightly loaded", it may be providing higher AC voltage than the 240 V spec, which is under full load. Measure the AC voltage from that "240" Volt secondary to verify.

Bri

 

[Amled87]

Since the transformer is "very lightly loaded", it may be providing higher AC voltage than the 240 V spec, which is under full load. Measure the AC voltage from that "240" Volt secondary to verify.

Bri

Yeah I'm showing closer to 280v. That's ok though, as that'll still fall Into spec for my few 12ax7's.

I've got to make a few tweaks as soon as the filter caps discharge (well just the first one since I didn't connect the first one to the 10k resistor connecting it two the second and third)

*Facepalm* but, after rectification I'm still pretty close to the specs from the actual 2204 schematic, so all should be ok! Thanks for tip Bri!

 

[Brian Roth]

"Back of the napkin" calculation for 280V AC comes up with 396 VDC.

AC voltage * 1.414 = DCV

I don't know if you have any bleeder resistors across the B+ rail. That will gracefully discharge the rail when power is removed.

Bri

 

[Amled87]

"Back of the napkin" calculation for 280V AC comes up with 396 VDC.

AC voltage * 1.414 = DCV

I don't know if you have any bleeder resistors across the B+ rail. That will gracefully discharge the rail when power is removed.

Bri

Yeah, I'm running close to 400v so that sounds about right. I do have bleeder resistors in place, I'm just giving plenty of time for discharge because even residual voltage from the cap is lethal. Once my multimeter reads 0v I'll get everything corrected as I like I said, I didn't connect the first filter cap to the second and third. There's 10k resistors in-between on the + side that I should of connected between the caps, and I overlooked the first one so my B+ stops at the first filter cap!

Another lesson learned. Always double check!

 

[Amled87]

"Back of the napkin" calculation for 280V AC comes up with 396 VDC.

AC voltage * 1.414 = DCV

I don't know if you have any bleeder resistors across the B+ rail. That will gracefully discharge the rail when power is removed.

Bri

Another question as I'm tidying everything up. Should I ground pin 3 of my master volume (it goes to ground) to the back of the pot as well? As I have it, it just goes to ground, but the back of the case isn't grounded.

 

[Brian Roth]

Someone else who does these types of builds will need to comment .... I work mainly with studio equipment with a random Fender amp or Leslie 122 speaker thrown in...lol.

I think the backshell of many/most/all? pots are tied to the mounting bushing which would in turn connect the backshell to chassis when installed. But this is not my area of expertise.....

Bri

 

[EthanSpaulding]

Another question as I'm tidying everything up. Should I ground pin 3 of my master volume (it goes to ground) to the back of the pot as well? As I have it, it just goes to ground, but the back of the case isn't grounded.

Short answer: NO, do not ground to the back of the pot.

There are several theories of grounding. For example "Star Grounding" and "Buss Grounding". Marshall, Fender and Ceriatone don't follow a grounding theory. They just randomly ground to Chassis ground or Signal ground at their whim. Imagine a whimsical pony frolicking in a meadow... never mind. Just follow how the Ceriatone diagram does it, get it rocking. Enjoy your hard work and perceverance! Everyone has to start somewhere and grounding gets subtle.

In the future, for High-Gain amps: 1) Buss Ground only. 1.5) Signal Ground and Chassis Ground connect at 1 point only 2) Elevate your heaters and use hum-dinger if AC. 3) no signals more than 180 degrees apart near each other.

 

[Amled87]

Just wanted to give a big THANK YOU to everyone who participated or helped me in this thread! Everything is wired up and working and imo it sounds awesome going through the return of a few of my amps. Being a JCM 800 there's not alot of distortion, but when boosted with my tube screamer I'm more then happy with it!

Now for the next build lol!

 

[CJ]

i just built something like this to plug directly into the PA so the git freak doesn't have to lug an orange and 2 x 12 cab, sounds great, has the 5150 clean channel with no crunch into a Reddi DI output transformer into a 1 K pot which drops the level back down by about 4:1. 12AY7 into a 12AU7 to drive the xfmr. very quiet with or without gnd lift sw on output, torroid pwr xfmr mounted below chassis.

 

[Amled87]

i just built something like this to plug directly into the PA so the git freak doesn't have to lug an orange and 2 x 12 cab, sounds great, has the 5150 clean channel with no crunch into a Reddi DI output transformer into a 1 K pot which which drops the level back down by about 4:1. 12AY7 into a 12AU7 to drive the xfmr. very quiet with or without gnd lift sw on output, torroid pwr xfmr mounted below chassis.

I believe my next build is going to be another Marshall style circuit with a 3 way on off switch for the cold clipper (39k in middle ala Soldano, 15k in parallel on the up position for 10k ala Marshall and a 5.6k in parallel on the down position for 4.9k grid clipping. The correct way to do this would be a relay and a vactrol as it'll pop when switching the toggle, but that's alleviated by turning the volume off then switching.

Then bypass the 3rd stage 820-ohm resistor with a 0.47uf cap, change the cap in the 470k/470pf network to 2200pf to cut some highs and prevent oscillation, bypass the first stage 100k plate resistor with a 220pf cap, change out the 25k mid potentiometer for a 50k linear to get a little more sweep from the pot and then maybe add a switchable LED/Diode clipping circuit before the preamp volume pot.

I really like how simple the JCM circuit is, but there is so much you can do to it.