Here are the mods I did to my JCA20

[fusedbrain]

Thanks for posting this man
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.

 

[CornNutz]

Yup, thanks for sharing. I've always admired that amp of yours. You did a great job modding it.

 

[apophis]

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

 

[mctallica1]

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

 

[dawnofdreamx97]

Sounds awesome!

 

[psychodave]

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...

 

[Metlupass2]

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!

 

[psychodave]

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.

 

[Fission]

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?

 

[apophis]

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

 

[psychodave]

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.

 

[fusedbrain]

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.

 

[psychodave]

I was messing around with some more mods. Made it modern and tight with a massive bottom end.

https://youtu.be/zarjEZiNOj4

 

[SpiderWars]

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?

 

[psychodave]

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.

 

[Beyond Black]

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.

 

[psychodave]

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.

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.

 

[psychodave]

Anyone mod their JCA20?

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

 

[CornNutz]

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

 

[psychodave]

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