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PostPosted: Mon, Apr 27, 2020 2:03pm 
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Supah Stah

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So here are the mods I did to my JCA20. Have fun modding and don’t be a bozo by saying the mods are your own or selling them. Tone stack is 560pf, V1 snubbers are 47pf. Diodes are 5 volt and MPSA20. Caps on the gain pot are 2000pf and 4700 pf. Depth caps are .0433 (two caps .0033 and .001. I mixed caps on purpose since they have a very audible effect here). Choke is a Mojo 778. Presence is .1uf. Everything you need to know is in the pictures. :thumbsup:

https://imgur.com/a/1uBmo9q


Here is the amp:


Last edited by psychodave on Mon, Apr 27, 2020 3:20pm, edited 1 time in total.

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PostPosted: Mon, Apr 27, 2020 2:19pm 
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Thanks for sharing, Dave :thumbsup:


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PostPosted: Mon, Apr 27, 2020 3:12pm 
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Very cool. Is this a Jose inspired Mod?

I plan on modding my JCA100HDM one day. I am first gonna fix that crap of a loop. Not sure about actual tone mods. I thought I might go small to start, like reducing some coupling caps to tighten up the low end, and a snubber cap to reduce the fizz. Go from there. I keep putting it off because I am not looking forward to pulling the PCB out, lol.


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PostPosted: Mon, Apr 27, 2020 3:23pm 
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:thumbsup:
Thanks Dave.
What is plate voltage please? and maybe also the V1 pin 1 voltage?
Nico


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PostPosted: Mon, Apr 27, 2020 3:24pm 
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Sounds great :thumbsup:


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PostPosted: Mon, Apr 27, 2020 3:30pm 
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:rock: Fuck yeah when you turned the master up! :rock:

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PostPosted: Mon, Apr 27, 2020 3:42pm 
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nico22 wrote:
:thumbsup:
Thanks Dave.
What is plate voltage please? and maybe also the V1 pin 1 voltage?
Nico


I have no idea what the voltages are.... I don’t remember from biasing. I honestly never measured V1 (or any other) I guess I could at some point. You could probably get an idea based on what the metro loop uses and the dropping resistor I needed. It should get you in the area.


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PostPosted: Mon, Apr 27, 2020 3:48pm 
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Cool :)

I don't know much about these amps. There are a few different versions?

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PostPosted: Mon, Apr 27, 2020 3:48pm 
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ok thanks Dave.
I noticed the 4,7k screens grids resistances. Pretty unusual value. how does it affect the sound?


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PostPosted: Mon, Apr 27, 2020 3:52pm 
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I have the 22h and did the depth mod to it, bought a sat mod off ebay didnt install yet but these are some killer little amps, cheap as hell too, sounds great


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PostPosted: Mon, Apr 27, 2020 4:13pm 
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nico22 wrote:
ok thanks Dave.
I noticed the 4,7k screens grids resistances. Pretty unusual value. how does it affect the sound?


It darkens it up a bit, but I made up for it by changing R48 and R6.


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PostPosted: Mon, Apr 27, 2020 4:22pm 
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Did you just use an exacto knife to cut and then the cut segment of trace just lifts up and away?

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PostPosted: Mon, Apr 27, 2020 4:26pm 
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SpiderWars wrote:
Did you just use an exacto knife to cut and then the cut segment of trace just lifts up and away?


Dremel. Exacto knife could work though.


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PostPosted: Mon, Apr 27, 2020 4:35pm 
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THANK YOU !!!!


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PostPosted: Mon, Apr 27, 2020 5:05pm 
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Thanks a lot for sharing!
Much appreciated !
The model of the base amp is Jet City 20 or 20h?
Kind regards!


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PostPosted: Mon, Apr 27, 2020 5:18pm 
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Those amps can be made to sound so good, yours sounds incredible. I cut the trace feeding the slope and put the master pre ts, it was easier just to replace the volume pot and run the wires the traditional way


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PostPosted: Mon, Apr 27, 2020 5:55pm 
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Sounds great.


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PostPosted: Mon, Apr 27, 2020 7:15pm 
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Wow.

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PostPosted: Mon, Apr 27, 2020 9:25pm 
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Very cool of you. That amp sounds amazing. :rock:


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PostPosted: Mon, Apr 27, 2020 9:46pm 
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nakoshoko wrote:
Thanks a lot for sharing!
Much appreciated !
The model of the base amp is Jet City 20 or 20h?
Kind regards!


Not sure I know the difference. It’s just a regular old Jet City 20 watt head.


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PostPosted: Wed, Apr 29, 2020 12:10am 
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Thanks for posting this man :rock:
That amp kills!

For the poster above me, the 20 is the single channel, and the 22 is the 2 channel, with 2 gains and 2 masters.
These amps are great value, have good iron, reasonably high quality caps and resistors, and are very easy to work on.
The 20 single channel lends itself more to a CCV kinda thing, and the 22 can be modded to an SLO circuit.
I've got a 22 that's been SLO modded and it sounds F'n killer, as does PsychoDave's.
In order to get them to sound their best, spend some time on the power supply. Don't just swap resistor and cap values in the preamp.
2xEL84's can sound huge, but it takes a bit of work.

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PostPosted: Wed, Apr 29, 2020 12:38am 
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Yup, thanks for sharing. I've always admired that amp of yours. You did a great job modding it.


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PostPosted: Wed, Apr 29, 2020 12:39am 
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Is there anybody here that could install these mods in mine? I dont have the know how and dont have any local tech that could do them unfortunately


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PostPosted: Wed, Apr 29, 2020 7:26am 
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Wow....that sounds really good Dave!

Never played one of these amps, but it seems like there are some really cool mods out there for them.
T

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PostPosted: Wed, Apr 29, 2020 10:07am 
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Sounds awesome!


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PostPosted: Wed, Apr 29, 2020 10:54am 
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apophis wrote:
Is there anybody here that could install these mods in mine? I dont have the know how and dont have any local tech that could do them unfortunately


Now is a good time to learn. Besides, it’s not like the amp is expensive. You fen get them used for $200ish. Then $30 in parts...


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PostPosted: Wed, Apr 29, 2020 11:22am 
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psychodave wrote:
apophis wrote:
Is there anybody here that could install these mods in mine? I dont have the know how and dont have any local tech that could do them unfortunately


Now is a good time to learn. Besides, it’s not like the amp is expensive. You fen get them used for $200ish. Then $30 in parts...


Send it to Dave and he will mod it for FREE!


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PostPosted: Wed, Apr 29, 2020 11:37am 
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Metlupass2 wrote:
psychodave wrote:
apophis wrote:
Is there anybody here that could install these mods in mine? I dont have the know how and dont have any local tech that could do them unfortunately


Now is a good time to learn. Besides, it’s not like the amp is expensive. You fen get them used for $200ish. Then $30 in parts...


Send it to Dave and he will mod it for FREE!


lol. I’ve turned down a bunch of offers for money. I have no desire.


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PostPosted: Wed, Apr 29, 2020 7:28pm 
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fusedbrain wrote:
In order to get them to sound their best, spend some time on the power supply. Don't just swap resistor and cap values in the preamp.
2xEL84's can sound huge, but it takes a bit of work.


Beyond the choke mod, depth mod, and biasing, is there anything specific you would point to?


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PostPosted: Thu, Apr 30, 2020 2:31pm 
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psychodave wrote:
apophis wrote:
Is there anybody here that could install these mods in mine? I dont have the know how and dont have any local tech that could do them unfortunately


Now is a good time to learn. Besides, it’s not like the amp is expensive. You fen get them used for $200ish. Then $30 in parts...


I would love to do it myself , can you recommend a book or tutorial that explains the basics? i can solder but thats it


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PostPosted: Thu, Apr 30, 2020 9:13pm 
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apophis wrote:
psychodave wrote:
apophis wrote:
Is there anybody here that could install these mods in mine? I dont have the know how and dont have any local tech that could do them unfortunately


Now is a good time to learn. Besides, it’s not like the amp is expensive. You fen get them used for $200ish. Then $30 in parts...


I would love to do it myself , can you recommend a book or tutorial that explains the basics? i can solder but thats it


Not really. You just need to do it. Get all of the parts together open the amp up and start modding.


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PostPosted: Fri, May 01, 2020 8:57pm 
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Fission wrote:
fusedbrain wrote:
In order to get them to sound their best, spend some time on the power supply. Don't just swap resistor and cap values in the preamp.
2xEL84's can sound huge, but it takes a bit of work.


Beyond the choke mod, depth mod, and biasing, is there anything specific you would point to?


Yeah, keep an eye on your pre amp voltages and know what target you're trying to hit to get the tone you want.
For example, Dave Friedman likes 130 - 140V on the first triode of his HBE mod, and 330 - 340V @ the PI.
140v is probably going to be too low for a Jose style thing ( mushy ), but those PI voltages are pretty good with a lot of circuits.

The issue that most EL84 amps have is that they start with pretty low B+, so it can be difficult to keep your pre voltages where you need them with such a low starting point.

The B+ string is really just a series of low pass filters. If you need to lower the dropping resistor values to get your voltages, then you need to maintain the cut-off frequency by increasing the value of the filter caps. Try to keep the knee @ 1hz or lower.

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PostPosted: Sat, May 02, 2020 12:43am 
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I was messing around with some more mods. Made it modern and tight with a massive bottom end.



https://youtu.be/zarjEZiNOj4


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PostPosted: Sat, May 02, 2020 6:01am 
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That sounds killer!

I just noticed that some JCA20 heads have a Depth switch to the right of the Presence. So for anyone that gets one of those you might have to get creative to make it optimal (I assume it is the same boards).

I've seen certain amps change the power supply from series to what I call 'spider'. Instead of having each node in series with the previous node, all preamp/PI nodes come from the screen node with their own (larger) dropping resistor and cap. The Dr Z Zwreck is based off of the Trainwreck Rocket, the Zwreck is spider and the Rocket is series. I don't know the difference tonally but it makes the resistor bigger (instead of smaller) so maybe they did that to dial in the knee and voltages?

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PostPosted: Sat, May 02, 2020 8:45am 
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SpiderWars wrote:
That sounds killer!

I just noticed that some JCA20 heads have a Depth switch to the right of the Presence. So for anyone that gets one of those you might have to get creative to make it optimal (I assume it is the same boards).

I've seen certain amps change the power supply from series to what I call 'spider'. Instead of having each node in series with the previous node, all preamp/PI nodes come from the screen node with their own (larger) dropping resistor and cap. The Dr Z Zwreck is based off of the Trainwreck Rocket, the Zwreck is spider and the Rocket is series. I don't know the difference tonally but it makes the resistor bigger (instead of smaller) so maybe they did that to dial in the knee and voltages?



I lowered the voltage a bit at R49. I don’t remember the ow much it dropped, but I wanted it to feel better. I was going to add a switch to be able to have a few different voltage options, if I got lazy.


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PostPosted: Sat, May 02, 2020 10:40am 
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psychodave wrote:
I was messing around with some more mods. Made it modern and tight with a massive bottom end.



https://youtu.be/zarjEZiNOj4

Damn! It sounds like it’s got 6L6’s in it. :rock:


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PostPosted: Sat, May 02, 2020 11:57am 
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Beyond Black wrote:
psychodave wrote:
I was messing around with some more mods. Made it modern and tight with a massive bottom end.



https://youtu.be/zarjEZiNOj4

Damn! It sounds like it’s got 6L6’s in it. :rock:


Yeah, Mark shared some tricks. It was throwing more bottom end than my 5150. My house was shaking from a pair of EL84’s. Makes me laugh when people say amps with them can’t sound massive. :lol: :LOL:


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PostPosted: Sat, May 23, 2020 5:55pm 
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Anyone mod their JCA20?

I’ve been tinkering with mine. Sounds so good.


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PostPosted: Thu, May 28, 2020 11:11am 
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psychodave wrote:
Anyone mod their JCA20?

I’ve been tinkering with mine. Sounds so good.


I might grab one of these to mod. I see you have 47k on the grid stoppers of the el84's, that must have helped make the el84's not sound like a tin can :)

you have a 64k/64k voltage divider with a orange ceramic cap going to ground, what is the value of that cap, 1000p?

With your pictures I'm pretty sure I can sketch out a schematic.

First stage: 33k input grid stopper, 330k load w/47p bypass, and 2.7k cathode with 1.0uf bypass

Interstage: ( is it .022uf?, or .0022uf? ) coupling cap ( the yellow IC cap sticking up) then into 64k/64k voltage divider with orange cap (value? 1000p) to ground.

Second stage: No grid stopper, 100k load w/47p bypass, and 2.7k cathode with 1.0uf bypass.

Interstage: .0022uf coupling cap, gain pot w/bright caps/switch, 470k/470k voltage div w/ 560p peaker

Third stage: 100k load, 820r cathode w/1.0uf bypass.

Standard CF

Jose style MV: 10k into .22uf, MV, clippers, tonestack.

Added (parallel to the .022) .1uf to the PI out


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PostPosted: Thu, May 28, 2020 11:58am 
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CNutz wrote:
psychodave wrote:
Anyone mod their JCA20?

I’ve been tinkering with mine. Sounds so good.


I might grab one of these to mod. I see you have 47k on the grid stoppers of the el84's, that must have helped make the el84's not sound like a tin can :)

you have a 64k/64k voltage divider with a orange ceramic cap going to ground, what is the value of that cap, 1000p?

With your pictures I'm pretty sure I can sketch out a schematic.

First stage: 33k input grid stopper, 330k load w/47p bypass, and 2.7k cathode with 1.0uf bypass

Interstage: ( is it .022uf?, or .0022uf? ) coupling cap ( the yellow IC cap sticking up) then into 64k/64k voltage divider with orange cap (value? 1000p) to ground.

Second stage: No grid stopper, 100k load w/47p bypass, and 2.7k cathode with 1.0uf bypass.

Interstage: .0022uf coupling cap, gain pot w/bright caps/switch, 470k/470k voltage div w/ 560p peaker

Third stage: 100k load, 820r cathode w/1.0uf bypass.

Standard CF

Jose style MV: 10k into .22uf, MV, clippers, tonestack.

Added (parallel to the .022) .1uf to the PI out


The 47k was there in conjunction with the 4k7 screens. Changing values changes tone, so I wanted to bring it back a bit. Tinkering here was to ease some of the strain the EL84 have so they last longer.

Both coupling caps are .002uf. Orange cap is 1000pf


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PostPosted: Fri, May 29, 2020 4:55pm 
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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?

Attachment:
r36.jpg
r36.jpg [ 429.54 KiB | Viewed 464 times ]


Thanks!


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PostPosted: Sat, May 30, 2020 10:50pm 
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CNutz wrote:
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?

Attachment:
r36.jpg


Thanks!


That’s removed. Did you see the trace cuts?


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PostPosted: Sun, May 31, 2020 1:58am 
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psychodave wrote:
That’s removed. Did you see the trace cuts?


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
Attachment:
bot.jpg
bot.jpg [ 204.68 KiB | Viewed 305 times ]


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


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PostPosted: Sun, May 31, 2020 7:47am 
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CNutz wrote:
psychodave wrote:
That’s removed. Did you see the trace cuts?


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
Attachment:
bot.jpg


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


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.


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PostPosted: Sun, May 31, 2020 10:56am 
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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:


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PostPosted: Sun, May 31, 2020 12:12pm 
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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
Attachment:
psycho mod 1.1.jpg
psycho mod 1.1.jpg [ 172.83 KiB | Viewed 46 times ]


Last edited by CNutz on Mon, Jun 01, 2020 8:53am, edited 1 time in total.

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PostPosted: Sun, May 31, 2020 1:06pm 
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CNutz wrote:
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.

Attachment:
The attachment psycho mod.jpg is no longer available


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


Attachments:
8A1E98FA-ED64-47A4-B1A7-A38A4093DA10.jpeg
8A1E98FA-ED64-47A4-B1A7-A38A4093DA10.jpeg [ 355.27 KiB | Viewed 213 times ]
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PostPosted: Sun, May 31, 2020 5:50pm 
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psychodave wrote:
CNutz wrote:
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.

Attachment:
psycho mod.jpg


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


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.


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PostPosted: Sun, May 31, 2020 6:18pm 
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This thread delivers ! Thanks to all who know wtf they’re talking about.


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PostPosted: Sun, May 31, 2020 9:15pm 
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CNutz wrote:
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.

Attachment:
psycho mod.jpg


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

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