Members kaoslord Posted November 3, 2014 Members Share Posted November 3, 2014 Hello all. I need some guidance, help and advice on a mod I want to do to my JCA50H custom head. I’ve got this head through ampfactory and one of the mods I requested was the “minimal gain” mod on the clean channel which makes that channel as clean as it can be. I like the mod for the most part as the channel can be very clean BUT it is a dark sounding channel. When compared to the overdrive channel, the clean/crunch channel has very low highs. This is a big problem for me as if I EQ the amp for the clean channel to sound good, then I engage the overdrive channel the tone is ice-picky with very excessive highs and brittle tone. If I EQ the amp for the overdrive channel to sound crisp and full, then when the clean channel is engaged the tone is dull and with no sparkle at all. I do have notice that the highs do get closer between the channels if I set the gain of the clean channel all the way up, which defeats the purpose of the mode in the first place. What options do I have here? What could I do to bring up the brightness on the clean channel? Thank you all for any help you can provide. Strange because in the jca100 (my amp) is the exact opposite of what you are describing... The clean is too bright compared to the lead... can you post/describe the mod you have in your amp ? Link to comment Share on other sites More sharing options...
Members Pandemic 2 Posted November 5, 2014 Members Share Posted November 5, 2014 Hi guys! I am new to the thread, but have been following it for a while, and it has inspired me to do some mods on my own JCA 100... i have done the marshall upper mids mod and another cap mod... but.... i would like to do something more radical. Now i am a big fan of the sound thar Slash has on the AFD record and always wanted something simmaler, but i am broke and cant afford an old marshall... so...After some searching on the net i found a thread on the Metro amp forum that supposedly had schematics for the infamous S.I,R amp that slash stole after the recording of the album, but the thread had been sealed... but i managed to to dig my way through the web ,and low and behold i found a guy in france going by the name A-Wai who had modded a jca 20 to the specs of the S.I.R amp... I got in touch with him and he was kind enough to send me the actual schematics for the jca 20 mod... but... i can replace a cap and solder, but i cant translate a schematic and check if it is possible to do the mod on my jc100h... so i am looking for some help on doing just that. I know edge11 had contributed in the original thread on the metroamp forum... but i dont know if he actually got arround to fiddeling with the mod for the jet city. Cheers Link to comment Share on other sites More sharing options...
Members franknstuff Posted November 18, 2014 Members Share Posted November 18, 2014 I have both a JCA20H and a JCA50H - I ordered identical parts for the depth mod for both. After installing the depth on the JCA20H there was definitely a very noticeable difference - MUCH better tone! On my JCA50H I have the same "later" PCB as does brian1234, above; I installed the depth mod at the same point, using the red wire going from the FB point on the PCB to the 4 ohm speaker tap. The problem I have with it is that the effect is very minimal (if anything at all) on the JCA50H - nothing like with the JCA20H. I was expecting much more of a bass response; I can hear it working through the speakers as I turn the knob clockwise/counterclockwise - just little or no difference in resonance. Any ideas? Link to comment Share on other sites More sharing options...
Members Pandemic 2 Posted January 26, 2015 Members Share Posted January 26, 2015 Does anybody know what resistor i have to pull and replace with a choke on the JCA100h ? Link to comment Share on other sites More sharing options...
Members Pandemic 2 Posted January 26, 2015 Members Share Posted January 26, 2015 I just did this mod with the C19, i liked the effect on the lead channel, but... this led to the lead channel bleeding in to the clean. It was very faint, but it was there... Link to comment Share on other sites More sharing options...
Members destined2nate Posted April 5, 2015 Members Share Posted April 5, 2015 hey guys, was looking at the schematic for the SLO and comparing them to those of the JCA100HDM. In the JCA's schematic there are 2 caps that i cant's seem to find the equivalent to in the SLO's schematics. the respective Caps are located at C31 and C34. Any insight fellas? Link to comment Share on other sites More sharing options...
Members BowerR65 Posted May 16, 2015 Members Share Posted May 16, 2015 Easiest mod...Change out the tubes and set the bias properly.Outside of that' date=' my JCA22H sounds fine stock. Even takes pedals well (Hardwire delay/chorus/reverb in the loop, overdrive / poly tuner in front)[/quote']When i got mine it had 2 JJ power tubes and V3-5 preamp tubes V1 and V2 still have the metal can over the tube and those were still stock. The amp had a weird noise that seem to follow or shadow the distortion it was like extra noise in the distortion. All i did was swap those last 2 stock tubes and it instantly sounded better.I agree with a few othe rposts that a slightly less gain tube in V1 and even V2 helps the clean up the clean crunch channel. Its one of those things that it depends on yoru style of playing. If you want clean or if you want crunch. You can always back off on the volume on the crunch channel to clean it up. Link to comment Share on other sites More sharing options...
Members BowerR65 Posted May 16, 2015 Members Share Posted May 16, 2015 Strange because in the jca100 (my amp) is the exact opposite of what you are describing... The clean is too bright compared to the lead... can you post/describe the mod you have in your amp ? Same with mine, my clean channel got very clean when i swapped the V1 and V2 tubes with 12AU7 or ECC81 the 12AU7 in V1 only is enough but i only have those 2 to try. Im thinking 2 ECC81s would be the best in V1 and V2 i think i read they are 70% gain compaired to 100 from the 12AX7 12AU7 is the lowest at 20% i believe. Link to comment Share on other sites More sharing options...
Members BowerR65 Posted May 19, 2015 Members Share Posted May 19, 2015 My friend and i went to a new guitar center that opened near us a few weeks ago and they had a few used items. I noticed the JCA50H and i had heard alot about them but never tried one. I told my friend to plug into it and he was hooked first strum he didnt want to try anything else. He would up putting a down payment on it and a few days later payed it off. After some time with it he decided his Dark Terror was good enough and let me have it for what he had paid ($325.) looked mint. After i got it i opened it up to find out 5 of the 7 tubes had been replaced with JJs. The V1 and V2 were still the stock china tubes i swapped them with better ones and my noise and fizz issues went away. I put in slightly lower gain ones and my crunch channel is WAY cleaner. My OD channel is way cleaner with less gain as well. So here is my issue. Since he got it the footswitch LED never has worked. I started to wonder if it was even an LED till i noticed an image on google with the JCA50H and in the image was the footswitch with the LED lit up blue. So i took mine apart and i didnt notice anything loose or disconnected. I put a meter to it and found 16v going to the OD side of the switch. Measuring the output side of the resistor i wasnt getting anything. The resistor measures 68K looking up on the LED builder site it said that i would need a 680ohm resistor for a 3V led so i replaced the stock resistor and the LED then started working but the FW stopped working correctly. I wound up trying several resistors till i found one that lit the LED and switched the channels. 35K is what i wound up using, half the value of the stock resistor. Now the LED lites up and it switches. I had to go with about half the value as the stock one for some reason. Im really thinking about drilling a few holes in the face of my amp and adding 2 LEDs so i know what channel im on. Green for clean, maybe stay wit blue for the OD channel. Link to comment Share on other sites More sharing options...
Members destined2nate Posted May 24, 2015 Members Share Posted May 24, 2015 My friend and i went to a new guitar center that opened near us a few weeks ago and they had a few used items. I noticed the JCA50H and i had heard alot about them but never tried one. I told my friend to plug into it and he was hooked first strum he didnt want to try anything else. He would up putting a down payment on it and a few days later payed it off. After some time with it he decided his Dark Terror was good enough and let me have it for what he had paid ($325.) looked mint. After i got it i opened it up to find out 5 of the 7 tubes had been replaced with JJs. The V1 and V2 were still the stock china tubes i swapped them with better ones and my noise and fizz issues went away. I put in slightly lower gain ones and my crunch channel is WAY cleaner. My OD channel is way cleaner with less gain as well. So here is my issue. Since he got it the footswitch LED never has worked. I started to wonder if it was even an LED till i noticed an image on google with the JCA50H and in the image was the footswitch with the LED lit up blue. So i took mine apart and i didnt notice anything loose or disconnected. I put a meter to it and found 16v going to the OD side of the switch. Measuring the output side of the resistor i wasnt getting anything. The resistor measures 68K looking up on the LED builder site it said that i would need a 680ohm resistor for a 3V led so i replaced the stock resistor and the LED then started working but the FW stopped working correctly. I wound up trying several resistors till i found one that lit the LED and switched the channels. 35K is what i wound up using, half the value of the stock resistor. Now the LED lites up and it switches. I had to go with about half the value as the stock one for some reason. Im really thinking about drilling a few holes in the face of my amp and adding 2 LEDs so i know what channel im on. Green for clean, maybe stay wit blue for the OD channel. i have the jca100hdm, and my other guitarist has a jca50h. we got our footswitches mixed up once at a gig and he was having channel switching issues. perhaps you got the wrong model footswitch with your amp. as far as i know the switching circuits are slightly different between models Link to comment Share on other sites More sharing options...
Members BowerR65 Posted August 26, 2015 Members Share Posted August 26, 2015 Check out what i did to my amp today. Been working on it a while finally got it all done. I noticed when i did the heater mod there was a 12v point so i got some some blue LEDs from walmart and just wired em into the case. Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted September 13, 2015 Members Share Posted September 13, 2015 start a site PYA.com pimp your amp.com.. just saying.. Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted September 22, 2015 Members Share Posted September 22, 2015 The following quote is regarding removing C19 on the 50/100 (C13 of the 22H) Hi All, another great mod that I have found: remove the bypass cap C19 on the cathode follower. this will help to "clear up" both channels: the clean will stay clean up to 4-5 on gain (depending on pickups) and the lead gain will be more usable, less fizzy/boomy with extreme settings (> 8). i was planning to make it switchable but it sound so good that will leave it unbypassed. Hello I have I am wondering if anyone else experienced what Pandemic 2 stated above "#456.3 Pandemic 2 commented 01-26-2015, 05:56 PM I just did this mod with the C19, i liked the effect on the lead channel, but... this led to the lead channel bleeding in to the clean. It was very faint, but it was there..." This seems like a good MOD to try to help me get rid of boomyness hand hazyness. I play the 22H My goal is to get tight and throaty marshallesque style of tome but sill retain some of the gain sweetness of the SLO sound. I have done: *The SLO spec OD channel *Reduced the coupling cap after V2 to .0022uf *exchanged the bright cap on the OD channel for a 620pf cap *switchable 33K/68K slope resistor *.0047 w/1M resonance I still have wayyyy too much gain and somewhat loose base. The resonance is useful but is still way too basey after 9 oclock. Is there a way to reduce its effectiveness maybe by bypassing it with resistor or some value? has anyone else experienced the bleed by removing C19 (C13 on the 22h) ? I have not tried this one yet. Any other suggestions to reducing gain, tightening base and making the amp more growly? Thanks, mesa3077boogie Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted September 25, 2015 Members Share Posted September 25, 2015 Ok I removed C13 on my 22h (C19 on the 100/50) I didnt hear any bleed between the 2 channels as member 'pandemic 2' mentioned Removing the cap did reduce the gain in both channels but not in a good way.. You need the mids and low mids that the cap helps gain up. I found it more effective way of helping to clear up the mud and it was by reducing the coupling cap on the OD channel to .0047. This and a few other changes I made reduced the woofyness and added some clarity to the OD channel big time! I also achieved a "sparkling clean" tone on the crunch channel on my 22H and I can honestly say that it is more satisfying than my Vox AC30HW was.. Im pretty satisfied with a bunch of the changes I have made this far. Im in the middle of a move and unfortunately packed the amp in pieces as I ran out of time but in the next few weeks I will provide a list of my changes once I get it all put back together and include some pics as well. Link to comment Share on other sites More sharing options...
Members len88 Posted October 8, 2015 Members Share Posted October 8, 2015 Can anyone tell me if the scoop mod effects both the JCA50's crunch and overdrive channel, or just the crunch channel? Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted October 9, 2015 Members Share Posted October 9, 2015 Can anyone tell me if the scoop mod effects both the JCA50's crunch and overdrive channel' date=' or just the crunch channel?[/quote'] the crunch channel only, what are you looking to achieve? Link to comment Share on other sites More sharing options...
Members len88 Posted October 9, 2015 Members Share Posted October 9, 2015 the crunch channel only, what are you looking to achieve? I like mid heavy distortion, and scooped sounding cleans. I want to set the mids on the distortion channel high (~3 o'clock), and keep the gain of the crunch channel low for the cleans. It sounds like the scoop mod will then get me what I want. Thanks. Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted October 11, 2015 Members Share Posted October 11, 2015 I like mid heavy distortion, and scooped sounding cleans. I want to set the mids on the distortion channel high (~3 o'clock), and keep the gain of the crunch channel low for the cleans. It sounds like the scoop mod will then get me what I want. Thanks. Ya the scoop mod definitely does just that, however the scoop mentioned in this forum for the JCA's is most commonly used in conjunction with the clean mod. I don't foresee the scoop mod with merely reducing the gain on the clean channel enough to give you what you are looking for. You have to do both the clean and scoop.. There is also a cathode bypass that is part of the "edge 11 scoop mod". Its my opinion that the 22uf capacitor for cathode bypass is too large of a value and boosts too many low mids and lows. However that all depends on what values you choose in conjunction with each other and what sound you are looking for. Link to comment Share on other sites More sharing options...
Members fajen Posted October 19, 2015 Members Share Posted October 19, 2015 Hey guys, I have just got a JCA 5212 Combo and I would like to Mod it a bit but I can't find any reviews about mods on this amp type. I read the Amp is quite similar to the Lucky 13. Soldano offers a Gain mod for this Amp to open up the Gain to be equal to the SLO.Do you know if this mod can also be done on the 5212 Combo? Could you please help me with these mod? At the moment I have only done the depth mod. I realy like the Clean channel of this Amp but the OD channel lacks a bit Gain I think. Thank you for your help. Link to comment Share on other sites More sharing options...
Members len88 Posted October 22, 2015 Members Share Posted October 22, 2015 So I just added the scoop mod to my JCA50H. I anded up using a 470 pF cap instead of a 0.001 uF since that sounded to "quacky" for me. I noticed something odd now though. When I play the crunch channel with he gain lower than 12 o'clock I can hear heavy distortion quiet in the background. I thought I had run across a post a while ago about this same thing but I can't find it again. Could the 2nd channel now be coupling on the crunch channel? Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted October 22, 2015 Members Share Posted October 22, 2015 Hey guys, I have just got a JCA 5212 Combo and I would like to Mod it a bit but I can't find any reviews about mods on this amp type. I read the Amp is quite similar to the Lucky 13. Soldano offers a Gain mod for this Amp to open up the Gain to be equal to the SLO. Do you know if this mod can also be done on the 5212 Combo? Could you please help me with these mod? At the moment I have only done the depth mod. I realy like the Clean channel of this Amp but the OD channel lacks a bit Gain I think. Thank you for your help. If you can post the schematics with the values I can see what I can come up with. You may be able to apply the mods in this thread such as the SLO or predator etc. for more gain. So I just added the scoop mod to my JCA50H. I anded up using a 470 pF cap instead of a 0.001 uF since that sounded to "quacky" for me. I noticed something odd now though. When I play the crunch channel with he gain lower than 12 o'clock I can hear heavy distortion quiet in the background. I thought I had run across a post a while ago about this same thing but I can't find it again. Could the 2nd channel now be coupling on the crunch channel? In response to your question, user "Pandemic 2" commented in post #456.3 that he was experiencing bleed through the channels after reducing the value of a cathode follower (C19 for the 50 /100, C13 on the 22h). As stated in my response I did the same mod (in addition to the scoop and many other mods as well) and didn't experience the channel bleed issue. I have read every post on this thread and the similar JCA mod thread in Guitar Amp Board and have not heard of this issue when the scoop mod was performed. Without being an experienced tech, which neither of us are (or we wouldnt be here on the forums for answers in the first place) here are a few logical questions to ponder regarding isolating the issue in order to identify the problem: 1) Did the issue make itself even faintly known before the mod? 2) Does the bleed come only when the scoop mod is engaged? 3) Have you tried swapping some preamp tubes around? 4) Did you leave any length of new wires (related to added/non stock/aftermarket MODs) near a transformer or B+ 5) Did you leave any scraps of wire or solder on the circuit board? 6) Did you change guitars, cables and speaker cabinets? If the answer to all of these questions is favorable and you have isolated the issue then my estimated guess based off of the simple process of elimination would be that the issue itself lies within the 470pf cap you chose. I dont know the science behind it but I do know that 470pf=.0047uf which is nearly 5 times larger than the recommended value of .001uf. The following definition of the scoop mod is found @ http://toboy-diy.blogspot.com/2013/1...a-srv-mod.html "The mid scoop is activated by grounding a capacitor between two resistors which are bypassed by a second smaller value cap. This is called a Bridged T Filter. The top cap is usually small allowing high frequencies to the skip the filter. The lower cap is large enough to bleed off relevant frequencies but not large enough to bleed low end. Low end is attenuated as normal through the resistors. By disconnecting the cap from ground, we end up with a normal RC (Resistor Capacitor) Network. Highs skip the resistors and everything else is attenuated. The values of the components determine the effect." Maybe there are parameters of resistance that were breeched by using a different value cap that are either allowing too much or too little signal/frequency in to the next stage due to the 470pf cap.. Just a guess. Im not a licensed tech. This is all of the info that I can give you to help you make a better informed decision on whats causing the issue. If anyone else has something to say, please correct me and/or chime in. Link to comment Share on other sites More sharing options...
Members fajen Posted October 23, 2015 Members Share Posted October 23, 2015 @mesa3077boogie I have found some schematics on the internet but without values. If you need the exact values I need to first trace them out. Link to comment Share on other sites More sharing options...
Members fajen Posted October 24, 2015 Members Share Posted October 24, 2015 Hey, I have just spoken to Douglas and he send me the Schematics of the JCA 5212 with values. [ATTACH]n31622308[/ATTACH][ATTACH]n31622309[/ATTACH] Link to comment Share on other sites More sharing options...
Members mesa3077boogie Posted October 30, 2015 Members Share Posted October 30, 2015 fajen, I apologize for the late response, I looked over the schematics and cannot correlate the two. I am pretty basic in my understanding of schematics and know as much as needed to get me through the projects that I have done. I apologize, keep searching and you will find a way to get the gain you need. Link to comment Share on other sites More sharing options...
Members kaoslord Posted November 2, 2015 Members Share Posted November 2, 2015 As for the C19 modification that I have proposed and the problem mentioned by "Pandemic 2", there is no leakage from the lead to the clean. But: as the effect is lowering the gain of both channels you might notice differences in the relative levels of the channels. The mods bring back some "old-style" feeling of (good) marshalls amps, the amps responds better to pick attack (more dynamic).Another good mod is the slope resistor (I like 68k here), to lose some of the excessive low-mids these amps have.This will reduce the gain also of both channels, allowing to push the master a bit more (good). You can compensate it by increasing the plate resistor of 4stage to 220K like the slo100 (stock JCA is 100K). Now, The leakage problem.In my amp (stock version, no mods) the lead channel was also bleeding into the clean.I found out that there was a bad soldering in the footswitch connector. A bad solder there means that less current is going to the LDRs and you might have malfunctions. If everything looks OK, the problem might come from a bad LDR, or from the switching circuit. Link to comment Share on other sites More sharing options...
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