JCA to SLO mod for 22H, 50H, 100H

[facefirst]

Any chance of posting one of these for the 20H?

[neilrocks25]

Any chance of posting one of these for the 20H?

Please

[facefirst]

Pretty please?

[Jay]

The 20H has 3 gain stages and a cathode follower, the SLO/22H/50H/100H has 4 gain stages and the cathode follower. You would need to jam another tube in there somehow. I'm sure there's other things to do on the 20H.

[liam5150]

I think there is one thing missing from your list.

SLO has a 47pF fizz cap. JCA 50/100H has a 120pF.

Correct me if i'm wrong. Other than that its spot on!

Anyone have the details for the clean/crunch switch?

[Lazz]

Does the SLO mod remove some of the fizz?

If not could somone tell me how to go about maybe reducing the fizz?

[Zozobra]

You can always use a MOSFET for the follower driving the tone stack

[red.guitar]

I found this schematic for the JCA50H, and everything thing looks like it adds up with your parts list, except for C10 and C11 (22n) to 47n: 47n?

Can someone confirm, or tell me what I;m missing?


Thanks!

[technomancer]

So anybody have a SLO Clean / Crunch schematic numbered for the 22H to go along with this? Some decent pics of one wired up would be great as well. Thanks!

[shep]

here is a picture with values if you havent already seen it. i am currently modding the same amp. the slo100 mods for the other jca models are going to be the same for the 22h. check out my thread on ithe jca22h. i would suggest some different mods than the slo100 mods.

[technomancer]

Nevermind figured it out.

Thanks for the schematic with values you posted earlier shep.

[shep]

no problem

[kaoslord]

Hi all

I would like to share my (little) experience with modding my JCA100H

Depth mod - Great addition to this amp and very easy to do !

SLO100 lead preamp mod - This mod is the swap of a resistor, 2 caps and the gain pot (1M to 500K).
I did and... probably better than stock but it is not OMG.
The OD channel is louder than before with still tons of muddy bass.

Swap of coupling cap after V2a (0.02 --> 0.002 uF) + swap of R36 (1M to 470K).
Gain pot is more usable and the lead channel is less bassy/muddy. More marshall raw tone.

Slope resistor into tonestack: i have added a DPDT switch to choose between stock (47K) and 68K.
68K is nice, kind of modern sounding scooped style.
And ​it makes the whole amp quieter, allowing to push master a little more, which is a good thing.

Remove the bypass cap C19 on the cathode follower. This will help to "clear up" both channels: the clean will stay CLEAN up to 4 on gain (depending on pickups) and the lead gain will be more usable, less fizzy/boomy, allowing to dial extreme settings (8-9). Such an easy mod !

Have fun !

[frankencat]

"Remove the bypass cap C19"

Just did this on my JCA50 and it definitely took a lot of the haze off. Nice one thanks!

[Tortuga]

Jay - I don't believe this question ever got answered....

I think there is one thing missing from your list.

SLO has a 47pF fizz cap. JCA 50/100H has a 120pF.

Correct me if i'm wrong. Other than that its spot on!

Is this the correct capacitor, and should that value be changed?




Also, what's the final verdict on changing the gain pots to 500k? Necessary or not worth the effort?

[Hipster Salad]

Changing the gain pots definitely make them more usable and got rid of the muddiness after 7ish on the dial. Have to change the 2 caps and 2 resistors as well but very noticeable when I did it to my 50H and my old 100H.

[Tortuga]

I've been learning a ton about amp theory for a Fender rebuild I'm starting, and decided to see how it applied to the JCA schematics that I was barely familiar with. Making tons more sense now, especially in how tube gain is established with the various resistors, filters, DC blocking caps, etc. Re-read pretty much the entire 28 pages of the JCA thread yesterday and make a ton of reference updates, including to this thread. Now this mod doesn't seem so daunting, and I'm seriously considering doing it. Thanks for the update on the pots (I do remember you stating that it did seem to help) - now I just want to know for sure about the fizz cap.

Not sure what capacitors I'm going with for this. Might get really crazy and upgrade a bunch of 'em, since there's been quite a few people (midnightlaundry, Kasbrot, and others) stating they had a major improvement in performance and tone from just doing that.

[Hipster Salad]

Jay - I don't believe this question ever got answered....

I think there is one thing missing from your list.

SLO has a 47pF fizz cap. JCA 50/100H has a 120pF.

Correct me if i'm wrong. Other than that its spot on!

Is this the correct capacitor, and should that value be changed?




Also, what's the final verdict on changing the gain pots to 500k? Necessary or not worth the effort?

Which one is the fizz cap on the board?

[Zozobra]

The 120p cap in that figure should be shown as being connected between the plates of the V5. It's purpose is to stabilise the the negative feedback loop although there are better ways to do this. It isn't a fizz cap.

There are a few drawing errors on that schem. The big one is that one of the PI plates isn't connected as an output to the power tubes!

[Mudslap]

I found this schematic for the JCA50H, and everything thing looks like it adds up with your parts list, except for C10 and C11 (22n) to 47n: 47n?

Can someone confirm, or tell me what I;m missing?


Thanks!

Old thread I know but any info on this? What do those caps do?

[Ry Manchu]

Don't forget to swap the OT for an Onetics. I believe it's the hi-def. C3amps calls it the SLO OT.

[ibzguy]

Here are the pots you will need as well -

Two 500k PC mount 16mm (mini) audio/log:

http://www.newoldsounds.com/index.php?m ... s_id=11305



Exact drop in replacement, and actually the shafts are better quality IMO because they aren't split or prone to breaking off.

Sorry to quote an old post but that link is dead and I'm currently putting together an order for some mods. It sounds like the perfect replacement though. Currently trying to cover all bases in one mouser order. Can anyone suggest a hassle free drop in mouser carries. Plan on starting with the depth mod but I figured I would just reuse the removed 1M pot during the SLO mod (ideal stolen from much board reading....)

Need to stay off this board while I'm waiting on new tubes to arrive.

[midnightlaundry]

Change C10 and C11 (22n) to 47n

This is the single best mod I did to this thing.. It lets the lowend through and beefs and balances out the tone..