Here are the mods I did to my JCA20

I've almost completed the schematic

But I'm stumped on r36. Something should be there (a jumper, or resistor) or the amp doesn't appear to work. That's where the junction of the 470k/470k voltage divider feeds signal into the grid of v2a (pin 7)

Did you take these photos while you were tinkering with that spot?, or am I missing something?



Thanks!
 

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CNutz":11rsl6gf said:
I've almost completed the schematic

But I'm stumped on r36. Something should be there (a jumper, or resistor) or the amp doesn't appear to work. That's where the junction of the 470k/470k voltage divider feeds signal into the grid of v2a (pin 7)

Did you take these photos while you were tinkering with that spot?, or am I missing something?



Thanks!
Click to expand...

That’s removed. Did you see the trace cuts?
 
psychodave":2mr4gjqd said:
That’s removed. Did you see the trace cuts?
Click to expand...

Yes. The traces were cut to move the gain pot from between stage 1, and 2, to between gain stages 2, and 3

I've traced out the signal path on your bottom image, and the signal needs to get to gain stage 3 grid, but it's clearly broken by the missing r36.

Signal path is in yellow, and dies at r36 MIA


Unless I'm missing something, but I've looked it over many times :confused:
 

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CNutz":2ta64lun said:
psychodave":2ta64lun said:
That’s removed. Did you see the trace cuts?
Click to expand...

Yes. The traces were cut to move the gain pot from between stage 1, and 2, to between gain stages 2, and 3

I've traced out the signal path on your bottom image, and the signal needs to get to gain stage 3 grid, but it's clearly broken by the missing r36.

Signal path is in yellow, and dies at r36 MIA


Unless I'm missing something, but I've looked it over many times :confused:
Click to expand...

Maybe I missed something or took something for granted? I’ve been tinkering with this amp for 5 years. Keep in mind it started life as a Friedman modded amp and was disappointed with how it sounded, so I started tinkering from there with the help of Cameron.
 
I found three different Friedman modded jca20 gut shots. Two of them are just like yours, with r36 missing. However one has a jumper soldered there (this is what I would expect).

I'm baffled :confused:
 
Alright here's the preamp schemo, its missing the bright switch, but that easy to figure out. If anyone can solve the r36 mystery let us know.

UPDATED
 

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psychodave":2rtq4owk said:
CNutz":2rtq4owk said:
Alright here's the preamp schemo, its missing the bright switch, but that easy to figure out. If anyone can solve the r36 mystery let us know.
Click to expand...

R36 is connected under the board. I ran a meter across it for continuity. Apologies for the oversight. Some stuff was changed and never though about again. :LOL: :LOL:

The schematic looks correct. Just keep in mind that I used certain parts in certain areas for a tonal impact. For example, the vintage Hi Q 560pf cap made an audible difference for the better. Also, the power section enhances the total sound of the amp. It’s the sum of all the parts.

I also use these EL84’s. I bought 12 from this seller and they were closely matched. They pissed all of the Chinese and JJ EL84’s.
https://www.ebay.com/itm/231574926020
Click to expand...

Sweet, thanks for confirming that. I thought I was losing it :LOL: :LOL:

Yup, I'm trying to mark your choice of components on it.

I'll have another schem coming for the power amp.
 
CNutz":afxh7d5c said:
Alright here's the preamp schemo, its missing the bright switch, but that easy to figure out. If anyone can solve the r36 mystery let us know.
Click to expand...

Nice. From the looks of it, a great sounding hot-rodded scheme that's been adapted from typical 800 builds, to fit EL84s.
 
Dave,

Are you running a 1m resistor in parallel with the diodes to compensate For the master volume not being pre tone stack?
 
CNutz":3tb29vtt said:
UPDATED preamp schematic
Click to expand...

I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
 
FourT6and2":2zwy0bu8 said:
CNutz":2zwy0bu8 said:
UPDATED preamp schematic
Click to expand...

I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
Click to expand...

Yes, it's used as a treble bleed, not as a peaker. This is common on these style amps. Might see it on the voltage div, or right after the first coupling cap.
 

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CNutz":eh36m7cu said:
Here's the power amp section
Click to expand...

Looks right.

The depth circuit is really important and can make or break the amp. I suggest ceramic caps for the depth pot. Keep in mind this is an EL84 amp, and it will have nice bottom end, but don’t expect it to be massive like an Uber. Although there are tricks to make it sound absolutely massive, it tends to ruin the spirit of what it’s about.
 
FourT6and2":2rsipbhi said:
CNutz":2rsipbhi said:
UPDATED preamp schematic
Click to expand...

I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
Click to expand...
No that would be an incorrectly implemented voltage divider and equate to and and act as a grid stopper.



 
scottosan":urdblexd said:
FourT6and2":urdblexd said:
CNutz":urdblexd said:
UPDATED preamp schematic
Click to expand...

I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
Click to expand...
No that would be an incorrectly implemented voltage divider and equate to and and act as a grid stopper.



Click to expand...

Yeah I realize that. I was curious because in other Jose-style mods I typically see it done the other way (Cameron Jose, Fortin KK, Meshuggah, Ceriatone, etc.).
 
FourT6and2":17koxqme said:
scottosan":17koxqme said:
FourT6and2":17koxqme said:
CNutz":17koxqme said:
UPDATED preamp schematic
Click to expand...

I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
Click to expand...
No that would be an incorrectly implemented voltage divider and equate to and and act as a grid stopper.



Click to expand...

Yeah I realize that. I was curious because in other Jose-style mods I typically see it done the other way (Cameron Jose, Fortin KK, Meshuggah, Ceriatone, etc.).
Click to expand...
In those style amps, i think there is usually a gain pot here with the pull down resistor ( and sometimes a cap ) off the wiper to ground. This is the first CCV type amp I've seen with only one gain control. Nice solution to fit the mod into the existing JCA amp.
Anyway, this thread definitely delivers :rock:
Thanks to Dave and Cnutz for all the info :thumbsup:
 
FourT6and2":et7zoaxx said:
scottosan":et7zoaxx said:
FourT6and2":et7zoaxx said:
CNutz":et7zoaxx said:
UPDATED preamp schematic
Click to expand...

I havne't double-checked the photos, but are you sure that 68k/68k divider is in the right order? Shouldn't the 68k and cap to ground be before the 68k going to the grid and not after it?
Click to expand...
No that would be an incorrectly implemented voltage divider and equate to and and act as a grid stopper.



Click to expand...

Yeah I realize that. I was curious because in other Jose-style mods I typically see it done the other way (Cameron Jose, Fortin KK, Meshuggah, Ceriatone, etc.).
Click to expand...
the Cameron’s, real Jose’s and Friedman’s are almost always before. As for Ceriatone he copies Fortin and.....Fortin, I’ll leave that alone
 
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