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JCA Depth Mod 
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GABmiral
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Lots of threads out there regarding this mod, although most are missing a key ingredient - a .1uF cap inline with the pot. This helps with 'scratchiness' that develops when adjusting the control.

Parts needed:
* 1M A pot
* .0047uF 400v (or higher) - have used mylar/film
* .1uF 400v (or higher) - same type

Here's a modified drawing showing how it's wired in:

Image

Here is Soldano's own design in the schematic for the HDM - looks pretty similar to me :D

Image

And a couple of pictures of mine. Not pretty, but it works. Note - I didn't want to cut the existing red NFB wire, so I disconnected it from the speaker jack and installed a new wire (grey in the pic) to it from the pot.

Image

Image

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Tue Feb 03, 2015 4:35 pm
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I did the depth mod on my 22H and it is indeed scratchy for the first third of the knob or so. I thought it was a shit pot...A couple questions...

Is my scratchy pot / sound due to not having this cap in the circuit?

Will this change the tone at all?

I don't change this setting on the fly, for the most part, just set it and forget it...If the only benefit is fixing the scratchy sound of the pot then I may pass. If it contributes in some way to changing the toan then I may give it a go and see if I prefer it.

Also, thanks...I never even knew.

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Tue Feb 03, 2015 5:42 pm
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GABmiral
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I would say yes, this is likely the cause of the scratching you're hearing. My understanding is that it filters out DC, which causes noise on the line when the pot is adjusted. Not sure if this filters any part of the tone, but would sure be a great thing to know. If you do make the change, please let me know what you experience.

While you're in there, try changing the slope resistor to 33k - makes a huge improvement in the tone stack,imo. :thu:

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  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

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Tue Feb 03, 2015 6:04 pm
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GRIMESPACE wrote:
I would say yes, this is likely the cause of the scratching you're hearing. My understanding is that it filters out DC, which causes noise on the line when the pot is adjusted. Not sure if this filters any part of the tone, but would sure be a great thing to know. If you do make the change, please let me know what you experience.

While you're in there, try changing the slope resistor to 33k - makes a huge improvement in the tone stack,imo. :thu:


Yea, you're talking to a shit head...You'd have to give me a resistor number for me to find that. If I had one of those caps laying around I'd do it right now but I don't have anything close, well, sort of close. I don't know that I will get out to the electric supply joint until next week. Besides, I have a guitar torn apart and that is sort of my priority right now. Will keep you posted though, for sure.

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Tue Feb 03, 2015 7:05 pm
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GABmiral
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Hey - no worries, man. If you do get a chance to try the blocking cap and let me know what you find sometime, I'd be seriously grateful.

As for the slope resistor (voicing) mod - if you have a 50H, slope resistor should be R24. I put links to two discussions about this mod in the References section of the main JCA thread (at 3rd posting), but I can save you a trip: http://www.guitarampboard.com/forum/viewtopic.php?f=5&t=1615&start=50#p50598 and http://www.guitarampboard.com/forum/viewtopic.php?f=5&t=1615&start=100#p53042

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  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

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Tue Feb 03, 2015 10:08 pm
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I've got the 22H. I believe the slope resistor is r26...Also, looking at removing c6 to help tame some of the fizz. Maybe r21 from 220k to 440k as well?

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Wed Feb 04, 2015 3:28 am
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Attachment:
JCA22H Schematic Page 1.png


Here is the JCA22H schematic.

R26 is indeed the slope resistor. Tune to taste.

To tame fizz you've got a couple of options. I'd leave C6 alone as it will be a fundamental ingredient to the tone of the amp and it might sound a little flat/lacking without it.
Upping R21 to 470K will roll off a little of the highs but the damage it already done in terms of the high frequency harmonics by that point so I'd look at curtailing some of the high frequencies earlier in the chain. Also cutting here may make the crunch a little too dark. I'd consider adding a snubber to the first stage of V2. You can add a cap across R13, something in the regions of 100p-1000p adjusted to taste. This is basically what JFA mods charge $100 for doing to a 5150 :shh: Another option would be to add a 1n cap across R18 to shunt some treble to ground. You see this trick a lot in Splawn designs.

You may also want to consider some of the mods the cathode follower described here depending on your playing style:
http://valvewizard.co.uk/dccf.html
I've not tried it myself but I'm itching to mod my Shiva preamp with a smaller cathode resistor to coax more compression from it


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Wed Feb 04, 2015 6:58 am
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GABmiral
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Just looked up the 22H diagram - yes, slope resistor is R26. Unfortunately, I'm not up to speed on fizz or what will tame it. It'd probably be good to either start a new thread asking for help to fix what you're after or put it in the main JCA thread, so maybe we can flag down some of the 'smarter' guys :D.

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  • CEOwLP (Sig T), Carvin DC135, Squier strat
  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

Help out MISTER NOBODY: https://www.gofundme.com/34f76sf4


Wed Feb 04, 2015 7:14 am
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GABmiral
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...too late - looks like one of the smarter guys just showed up. More for me to learn :D

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Co-founder of the Jet Setters 2.0 (Jet ★ City Lounge Redux)

Gear
  • CEOwLP (Sig T), Carvin DC135, Squier strat
  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

Help out MISTER NOBODY: https://www.gofundme.com/34f76sf4


Wed Feb 04, 2015 7:22 am
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Zozobra wrote:

Here is the JCA22H schematic.

R26 is indeed the slope resistor. Tune to taste.

To tame fizz you've got a couple of options. I'd leave C6 alone as it will be a fundamental ingredient to the tone of the amp and it might sound a little flat/lacking without it.
Upping R21 to 470K will roll off a little of the highs but the damage it already done in terms of the high frequency harmonics by that point so I'd look at curtailing some of the high frequencies earlier in the chain. Also cutting here may make the crunch a little too dark. I'd consider adding a snubber to the first stage of V2. You can add a cap across R13, something in the regions of 100p-1000p adjusted to taste. This is basically what JFA mods charge $100 for doing to a 5150 :shh: Another option would be to add a 1n cap across R18 to shunt some treble to ground. You see this trick a lot in Splawn designs.

You may also want to consider some of the mods the cathode follower described here depending on your playing style:
http://valvewizard.co.uk/dccf.html
I've not tried it myself but I'm itching to mod my Shiva preamp with a smaller cathode resistor to coax more compression from it


Nice. Thanks for the tips. My main goal is to get rid of some of the fizz and tighten the bottom end up a bit - it's a little muddy IMO. I don't want to shit in this thread any more than I have. Thanks again.

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Wed Feb 04, 2015 10:31 am
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Change C2 to 3n3 to tighten up low end


Wed Feb 04, 2015 10:50 am
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With 3n3 at C2 the crunch channel will sound quite thin.
I would go for 10n at C2 and 10n at C9 (C9 only affects OD channel).
You could also swap out C13 and go for 1k2 at R23, this will give you a more rough and less compressed sound - but it's less compressed then and maybe more difficult to play accurately.

And on this topic - I would not use the 100n or 1uf cap in front of the depth, this will lack sound, although the effect is hardly noticeable. For the "why?" you could read what Larry has written in Metro forum.


Thu Feb 05, 2015 9:44 pm
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GABmiral
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Hey - great to see you back, käsebrot!

Interesting about the depth knob - I may remove the cap and see what I think about it.

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Co-founder of the Jet Setters 2.0 (Jet ★ City Lounge Redux)

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  • CEOwLP (Sig T), Carvin DC135, Squier strat
  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

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Thu Feb 05, 2015 10:44 pm
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Hello All. Brand new to this forum and to the JCA100H. So far i'm loving this amp! I'm interested in trying the depth mod but am feeling over my head, i know nothing about electronics. the walkthru above looks pretty basic but what was the verdict on the cap in front of the pot, use or don't? Thanks!


Mon Jan 25, 2016 12:30 pm
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GABmiral
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Never wound up taking the cap out to see if there was any real difference. Kind of lost interest, as I've since moved on with a Marshall DSL. My JCA sits unused for almost a year now.

Would still like to do the SLO mod at some point, though.

The Depth mod was the first thing I did with my JCA. Well worth the effort (along with tubes & setting bias). However, the lack of electronics knowledge makes me a bit concerned. There are plenty of dangers that lurk in the chassis of any tube amp, and you need to understand certain procedures for ensuring your safety, ie: discharging caps, measuring voltages, and the like. Be safe.

And, :welcome: to the forum

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Co-founder of the Jet Setters 2.0 (Jet ★ City Lounge Redux)

Gear
  • CEOwLP (Sig T), Carvin DC135, Squier strat
  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

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Tue Jan 26, 2016 7:04 am
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Hi,

Is it possible to show the pictures again for the depth mod ? I have an old Jet City 100 Watts amp that i would like to modify.

Thanks !!

Paul


Sat Sep 30, 2017 11:29 am
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GABmiral
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Hiya - I'll try to get them posted tomorrow for you :thu:

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Co-founder of the Jet Setters 2.0 (Jet ★ City Lounge Redux)

Gear
  • CEOwLP (Sig T), Carvin DC135, Squier strat
  • DSL100H, 1960AX, Mark III, homebrew 2x12 (C90 / EVM12L Thiele), '73 Fender Pro Reverb, POD HD500X (Church rig), JCA50H
  • PLX GABion, Crybaby, Small Clone, DL8, MXR108, Algal clone, Pharaoh (thanks Greg :bow:)

DIY

Help out MISTER NOBODY: https://www.gofundme.com/34f76sf4


Sat Sep 30, 2017 2:57 pm
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Thanks, that will be excellent, I wanna try and do this mod.


Wed Oct 11, 2017 11:48 am
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