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:
Here is Soldano's own design in the schematic for the HDM - looks pretty similar to me
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.
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.
Guitars: Early 90's Ibanez EX Series, Late 80's B.C. Rich Mockingbird, Fairly new Epiphone LP Custom Amp: JCA22H with 4x12 Ibanez Cab No pedals or any real skills...
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.
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.
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.
Guitars: Early 90's Ibanez EX Series, Late 80's B.C. Rich Mockingbird, Fairly new Epiphone LP Custom Amp: JCA22H with 4x12 Ibanez Cab No pedals or any real skills...
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?
Guitars: Early 90's Ibanez EX Series, Late 80's B.C. Rich Mockingbird, Fairly new Epiphone LP Custom Amp: JCA22H with 4x12 Ibanez Cab No pedals or any real skills...
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 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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Loop wrote:I’m currently shopping for a 1996 Red Dodge Viper with yellow wheels. Who gives a shit about taste?!
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 .
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 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.
Guitars: Early 90's Ibanez EX Series, Late 80's B.C. Rich Mockingbird, Fairly new Epiphone LP Custom Amp: JCA22H with 4x12 Ibanez Cab No pedals or any real skills...
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.
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!
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.
Hello new to the forum and was wondering when doing this mod on the JCA50H, is the cap and resistor in parallel or series? I want to do this mod fixed so I was also wondering If I would just insert the cap and resistor on the yellow wire(I believe that is the feedback wire). Thanx for all the great info and mods!!
I will try this on my amp,but I am wondering this..on which 2 lugs or where does the .0047uf capacitors legs mount ? also I have seen some elsewhere show to use a 500K audio taper pot and I see here to use a 1M A pot so kinda wondering about that as well,,I have seen most showing a 500K pot preferable.. I have seen a few pics of this mod but I never see where the Blue FB wire that gets cut connects on the pot as well..I might have found somethng,maybe this is right...the blue FB wire after getting cut gets sautered onto the outside lug,then on the other side outer lug the opposite side of blue wire gets sautered here..looks like capacitor sautered at the middle lug and outer lug that goes down to the pcb board..I am hoping this is right..I hope somebody can enlighten me,and info greatly appreciated. Also can anyone please tell me how to I guess update my ID so that I can view the pictures ,all I see are boxes where it says to upgrade to view...sorry I am high maintenance,just a little help needed here at the beginning,I am trying ti wear the "New Guy" off here.
Bumping this old thread. I recently got a new JCA100HDM. I like the amp, but it could use a few improvements. First I threw in some old tubes (I took out of my Triple Recto a few years ago when I retubed it with new tubes), and biased it up. That helped a lot with the brittle top end. I thought it sounded pretty good, but the one thing that still annoyed me was that although the HDM has a Depth control already built in stock, I felt like it was in the upper-bass area, and gave a very muddy, thuddy type of palm mute. I think the frequency was too high to get a really nice metal chug. It is like it is tight, yet muddy at the same time. Weird.
After some research, I found that the higher the depth cap is, the lower the frequency. You can change this cap from about .0022 to .01 or so. For reference, the 5150 uses .0068 for the depth cap in the head (.0047 in the old combo). I experimented with a JCM800 2203 based clone I built years ago since I could change it easily. I found .0047 gave the same sort of muddy, thuddy bass I was hearing in the HDM. I found that .0068 was perfect. It cleared up the lower mids / upper bass area, and gives those big 5150 style chugs. I thought that .01 started getting a bit thin as the bass was a very low frequency.
Even though I have only had the HDM a short while I decided to make the same mod to it because I was not happy with the low end. I changed the depth cap from a .0047 to a .0068. I definitely think this helped clear up the low end. It is clearer in general, yet it has more of that metal chug. It has more of that 5150 style low end. It might have a little less low end overall, but it is clearer yet rumbles better overall.
I have considered some other mods, but I think I will leave it at that for awhile since I just got it, and haven't really bonded with it yet. I think that did help the low end though, and it definitely has that modded JCM800 meets 5150 style tone now, especially when using a SD-1 or MXR M77 in front.
Shask wrote:Bumping this old thread. I recently got a new JCA100HDM. I like the amp, but it could use a few improvements. First I threw in some old tubes (I took out of my Triple Recto a few years ago when I retubed it with new tubes), and biased it up. That helped a lot with the brittle top end. I thought it sounded pretty good, but the one thing that still annoyed me was that although the HDM has a Depth control already built in stock, I felt like it was in the upper-bass area, and gave a very muddy, thuddy type of palm mute. I think the frequency was too high to get a really nice metal chug. It is like it is tight, yet muddy at the same time. Weird.
After some research, I found that the higher the depth cap is, the lower the frequency. You can change this cap from about .0022 to .01 or so. For reference, the 5150 uses .0068 for the depth cap in the head (.0047 in the old combo). I experimented with a JCM800 2203 based clone I built years ago since I could change it easily. I found .0047 gave the same sort of muddy, thuddy bass I was hearing in the HDM. I found that .0068 was perfect. It cleared up the lower mids / upper bass area, and gives those big 5150 style chugs. I thought that .01 started getting a bit thin as the bass was a very low frequency.
Even though I have only had the HDM a short while I decided to make the same mod to it because I was not happy with the low end. I changed the depth cap from a .0047 to a .0068. I definitely think this helped clear up the low end. It is clearer in general, yet it has more of that metal chug. It has more of that 5150 style low end. It might have a little less low end overall, but it is clearer yet rumbles better overall.
I have considered some other mods, but I think I will leave it at that for awhile since I just got it, and haven't really bonded with it yet. I think that did help the low end though, and it definitely has that modded JCM800 meets 5150 style tone now, especially when using a SD-1 or MXR M77 in front.
Very Simply put, a .002uf will have more upper mids than a .0047uf. For Marshall style amps, I tend to like .003uf.
Another thing to consider is cap type. Depth circuits seem to sound better using ceramic caps.
Remember, it’s the sum of all parts, so a change here might require a change there.
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.
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