Guitar Gear Forum

mesa3077boogie wrote:

8Len8 wrote:I made some mods (slope, depth, scoop) to my JCA50H and have some questions, just to make sure I did things right.

- For the depth mod, you remove the red wire going to the FB point on the PCB, wire the 0.1uF cap end of the circuit to the PCB, and the pot end of the circuit to the red wire?

- A little goes a long way with the depth mod. It seems like turning the pot just 1/4 turn has a big impact.

- With the depth at 1/4, and both the scoop and slope mods enabled it seems like there's too much low end on the crunch channel. I included a 22uF across R45. I wonder if that's boosting the lows too much?

- The Crunch channel seems to sound a little "quacky" with the scoop and slope mods on. Anyone try playing with the values in that notch filter?

Yes 22uf anything on this amp is too much. Lower the cathode bypass caps as low as .047 or .068 ( think Im using my gain channel at .0047 and its plenty bassy and thinned out compared to stock

Thanks. Yeah, a smaller bypass cap definitely help. Now I'm noticing something strange. When I play the crunch channel I think I hear some distortion from the 2nd channel bleeding over. I thought I read another post about this but I can't find it again. Any ideas?

In response to your question, user "Pandemic 2" commented in post #456.3 (on the harmony central forums) 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.

Thanks for the info. I don't recall if there was bleed through before the mods, but in the past couple of days I've read many posts of people talking about the stock amp having bleed through. Since your doesn't I can just attribute it to unit to unit variation due to the design and inconsistent build in China.

470pf is smaller than 0.001uf so that shouldn't be a problem.

I'll look into the other things. Thanks.

Where are you seeing people talk about bleed through on the JCA amp heads? Can you show me the thread?

I read the posts. Im not sure what the deal is with the OD bleeding over to the clean. ITs hard for me to say since I have done so much to the amp. After I did the SLO mod to the OD channel I noticed less noise but I attribute that mostly to reducing the 1M volume pot to a 500K volume pot. You can just swap the volume pot out on the OD channel without doing the rest of SLO mod just to experiment. If you do that you will have to double the bright cap value since you are cutting the the volume pot in half. If you don't double the bright cap value in conjunction with reducing the value of the volume pot, you will be passing only highs starting at about 5 or 6K. Moving the bright CAP up to about 1000pf in conjunction to reducing the volume pot to a 500K will get you the upper mids passing through starting at about 1K instead of the 6K which is what the 1M pot and stock bright cap are currently passing. Reducing the volume pot in general should reduce noise but its not guaranteed to stop the bleed. if you buy a cheap step bit from harbor freight for $6.00 you can actually bore the VR2 hole to fit a guitar volume pot instead of the mini pot unless your local parts shop happens to sell them.. you will have to run some wires but a full sized pot will clear the board

I'm looking for experiences of different values of the choke mod(I have found different values for a choke in a JCA 22h, between 3-20 Henries)! So anyone that has the coke mod installed? What Henry value did you choose?

New Custom 22 designed by Martin Kidd of Victory/Cornford fame.

http://www.jetcityamplification.com/#!custom-22/c1dk6

Dang, I wanted to be the first to post about it..

Ya it seems like it will be pretty promising. I cant wait to try one. I wonder if it will be tighter than the 22h and or more recto ish?

mesa3077boogie wrote:Dang, I wanted to be the first to post about it..

Ya it seems like it will be pretty promising. I cant wait to try one. I wonder if it will be tighter than the 22h and or more recto ish?

Hopefully it's not more Recto-ish. I already have a Recto.

Yeah me too. But what else could you mean by "modern" gain ? My mind immediately goes to rectifier and the 5150 and such.. BUT things are very different with EL84s. I love the demos of the mini rectifier.

Sinical wrote:New Custom 22 designed by Martin Kidd of Victory/Cornford fame.

http://www.jetcityamplification.com/#!custom-22/c1dk6

If this new Custom 22 sounds anything remotely close to my Earhart, then holy shit watch out guys.

I've been reading the crap out of this forum. Thank you guys for your info and experiments. Love modding! I'm going to try a few on my 22h.

Looking to gain some low end on both channels for those gigs where the owner expects a rock band to play at bedroom levels. Also need to reduce some excessive high end on the high gain channel. While I enjoy the amazing "punch through the mix" the high end provides, it is a bit much. I wanted to pursue a separate treble knob for the high gain channel but I'm with try the mods first.

Could someone please take a look at the pic to see if I have the correct electonical bits labeled before get started.

1) Low end boost at low volume.
2) Slope.
3) Depth. Where do I connect??
4) Gain Redux at plate.

Thanks.

If you are looking for a low end boost, the depth mod will cover that.. Some say to use a 1M pot for the depth mod. Turning the knob of the depth mod pot to 2 (out of 10) will significantly boost your usable bass. It will boost the good bass below where the mud usually is (mud is usually around 200-300hz on guitar). I would highly recommend using a 250K pot though for the depth mod though. Its more usable. I don't know how to explain it but it is.. Try a few different pots to experiment though. The depth mod can be found here http://toboy-diy.blogspot.com/2013/10/j ... h-mod.html .

Im not sure how to reduce the gain in the OD channel but I do know that reducing the pot value will; reduce a bit gain, reduce some shrill, and will make the channel more quiet.

If you are looking to add a separate treble control for the high gain channel its because you are dissatisfied with where the treble Q lies on the clean channel when switching back over to it from the high gain channel. I believe a happy medium can be found in the shared treble control by swapping out the OD channels bright cap. Try reducing the bright cap on the gain channel to make it brighter and increasing the value to add in both high mids and high end. if you are not familiar with that, you need a google crash course in bright caps.

R26 is the slope resistor. This acts like a tight switch and changing the value (from 47K) down to 33K will remove some mud but also thin it out (in a good way) remember mud is in the lower mids. Reducing R26 to 30 or 33K will take some low mids out and will make it seem like you are "boosting" the good treble range, making it a bit more marshall like.

Say, has anyone ever tried to add a single pot/knob tone control to just the JCA50 Normal channel?

I am coming up on a year with my 2011 JCA100HDM...
No mods, just a new set of Mesa Boogie 12AX7's, and she's one sweet sounding beastie.
I may change up the tubes a little next year as follows:
V1 - Tung-Sol 12AX7
V2 - Mullard 12AX7
V3 - JJ ECC83S
V4 - JJ ECC83S
V5 - Sovtek 12AX7LPS

May consider the SLO mod for her as well... However, as-is, she totally gave my friend's '04 SLO a run for her money!

8Len8 wrote:Say, has anyone ever tried to add a single pot/knob tone control to just the JCA50 Normal channel?

I have note heard of it, let us know what you come up with..

Mesa 3077. Thanks. You are so right the depth mod saved this amp for me. I now have adequate low end. I play pretty clean and needed the boost. Used a 250k pot and went with a 5800pf instead of 4700pf.

The gain redux reduced the gain a lot!! Like down to barely audible. Weird? I did not keep it.

Keep slope, low end boost and depth.

With my flux capacitor faceplate I'm pretty good for now.

Soooooo, have you ever seen the new Jet City Custom 22 by Martin Kidd??

This is what it looks like from the inside



http://extrazoom.com/image-48876.html?s=huln50x50

The crunch channel is much cleaner and the whole amp is more to the British side of things. I Just got it las Friday and haven't had the time to test it properly.

I'll do a video review as soon as I can, and as soon as I cab take it to a place where I can play it LOUD as this amp screams for volume and its freakin loud to play at my apartment.

Nice! I'm definitely interested in getting one. I've read that there are plans for a 50 watt version as well, so I may hold out for that.

Anxious for clips!

BTW when are we getting those stateside? Should be sometime this month...

StanD wrote:Mesa 3077. Thanks. You are so right the depth mod saved this amp for me. I now have adequate low end. I play pretty clean and needed the boost. Used a 250k pot and went with a 5800pf instead of 4700pf.

The gain redux reduced the gain a lot!! Like down to barely audible. Weird? I did not keep it.

Keep slope, low end boost and depth.

With my flux capacitor faceplate I'm pretty good for now.

So did you experiment with different pots for the depth or just go straight to the 250K ?

capota wrote:Soooooo, have you ever seen the new Jet City Custom 22 by Martin Kidd??

This is what it looks like from the inside



http://extrazoom.com/image-48876.html?s=huln50x50

The crunch channel is much cleaner and the whole amp is more to the British side of things. I Just got it las Friday and haven't had the time to test it properly.

I'll do a video review as soon as I can, and as soon as I cab take it to a place where I can play it LOUD as this amp screams for volume and its freakin loud to play at my apartment.

Thanks capota. Cleaner than what? the JCA22H crunch channel?

Is the OD channel really "modern" sounding? I cant imagine it is. I think recto, 5150 etc when I think modern. Is it bassy and cluttered sounding or a more well rounded than the stock JCA22H ?

Not the greatest pics, but here's what I did to the 5012 combo

Pay no attention to the extra hole in the front from my semi-failed master volume attempt lol

Just stick a pot in there to fill the hole. Nobody will ever know that it doesn't do shit.

Producer pot!!