Guitar Gear Forum

I almost picked up a JCA50 head.... I know the power tranny is beefy enough to run a pair of 6550s or KT88s xD, Hell at least ditch the Chinese/JJ 6l6s they come with and put in my Sylvania STR 387s or old GEs, GE preamps also then this amp would actually scream xD

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GRIMESPACE wrote:@flowmastergfunk - welcome! Standard response for getting the most from your JCA: tubes and bias first, then assess what you're looking for before digging into mods. I really liked the results from the depth control and making a switchable slope resistor in the tone stack. I did the clean mod on my JCA50H, but was really surprised at how drastic a volume drop I get when I turn it on. I'm having to compensate by cranking the crunch channel master - gawd help my ears if I kick the clean switch back off without bringing the channel master back down first. I'm not certain what that means for you, since you've got a single channel amp, but I'm guessing it's not going to be something that you can expect to flip on and off without a corresponding change in master volume, and I'd think your max volume with the clean switch engaged would be quite low. Not a huge mod to try out - just be somewhat cautious with your expectations.

I definitely did the bias as soon as I put the new tubes in! Next time I order electronic parts online, I am certainly going to get parts to do the depth and slope mods on both of my heads.

It's been a while, but since I was here last, I found a matching Peavey 1x12 cab. It does still have the stock Sheffield speaker in it, but I am excited to have two identical amps to run in stereo.

Ben Verellen has finished modding my first JCA, and I should probably have it back in a week or two. I can hardly wait to compare and contrast. Not sure when I will be able to run both at the same time though. On my pedal board, I do have several pedals that have stereo inputs/outputs, but not sure how well they will distribute a signal without sharing a buzz. I am making the educated guess that I will end up needing to make/buy an isolated ABY pedal if I want to run them together :/

I have been spending a lot of time working on my pedals, patch cables and a sweet backup guitar lately...trying to entertain myself while waiting for my modded amp to come back to me, but I ended up adding ANOTHER amp to the fam yesterday

I couldn't pass it up...I saw a mid 70's 2x12 Kustom 11-L, or Lead II, on craigslist this week for pert near a handfull of dirt. It said that it was broken, but I had noticed in one of the pictures that it was loaded with a pair of Peavey Scorpion 12's. They have always been some of my favorite speakers, and I have actually been hoping to score an old Peavey combo just to rob a pair out of it. I had to grab it.

At first, I was thinking about putting one scorpion in each of my 1x12 cabs, putting the JBL into the Kustom, and just running it as a 1x12 combo (if I could fix it). I got it home only to find that the amp actually WORKS, but I believe it needs a few components updated. VERY cool vintage piece though. Lots of legit components. Fine craftsmanship for a solid state unit. It will make a nice backup project/practice amp

But I digress...

Turns out, the scorpions are 4ohm 12's that were wired in series to be 8ohm :/

The 4ohm versions are pretty hard to find, so I guess it's not a bad thing...but I don't know about using them in a 1x12 setup. Some amps would take it, but does anyone have any advice on running a single 4ohm speaker on a JCA? Do you think it would be too much for the output transformer or other components? I assume the amp would actually be louder with a 4 ohm speaker, so I could be a little more gentle on the volume knob. Then again, I could just leave them in the Kustom. Sigh Advice?

Well back to Jet City again, just snagged a JCA50 combo...totally forgot about the fx loop fiasco

Times are tough... I now have to sell my JCA 22H

Do we have anymore updates on pedals that work with the 50?
What am I looking for in specs on pedals that will work? -10db/+4db?

Elessar [Sly] wrote:Times are tough... I now have to sell my JCA 22H

Hell man, why don't you sell one of your 27 Laneys?

justinjag77 wrote:Do we have anymore updates on pedals that work with the 50?
What am I looking for in specs on pedals that will work? -10db/+4db?

I've been posting any effects loop / pedal updates on the reference list of this thread (first page). Nothing new in quite some time. From what I've read, the loop runs at line-level, meaning many pedals (especially cheaper digital ones) are out.

I can tell you from what I have that the TC Flashback seems to work fine in the loop. The EHX Small Stone does not. Alesis Quadraverb is hit/miss (have to boost the output gain on the unit a LOT) and the patches don't sound all that great. Have tried 4-cable method with my POD HD500X, and it seems to work fine.

Hey Grime, I haven't been on in a while, did the other jet city forum we where on close? anyways I've got one of those new Zoom G1on's it doesn't work, neither does my BBE Sonic Stomp the Zoom has a +2db not sure about the BBE. I hope the +4db and up pedals is what I'm looking for

Just wanted to add my 2¢ to this cool lounge, which I found trying to search for mods. I mainly hang out at the MLP forum.

I'd picked up a pair of JCA5012C's recently and noticed the heater hum thing right off the bat like a few folks here had mentioned. What fixed it for me was cutting the backside traces and wiring in new heater lines.

Big John wrote:Just wanted to add my 2¢ to this cool lounge, which I found trying to search for mods. I mainly hang out at the MLP forum.

I'd picked up a pair of JCA5012C's recently and noticed the heater hum thing right off the bat like a few folks here had mentioned. What fixed it for me was cutting the backside traces and wiring in new heater lines.

whoa nice job with the wiring! Looks great! Welcome to the forum.

justinjag77 wrote:Hey Grime, I haven't been on in a while, did the other jet city forum we where on close? anyways I've got one of those new Zoom G1on's it doesn't work, neither does my BBE Sonic Stomp the Zoom has a +2db not sure about the BBE. I hope the +4db and up pedals is what I'm looking for

Glad to see you back. Yeah, if you're talking about the Jet★City Amplification Community forum that Andy Mast set up back in '12, yeah - it's gone. He moved on to another amp and lost interest in keeping the board going, especially since the hosting funds were coming from his own wallet. I'll tell you, though - this place has been a huge boost for me.

I'll try to add those pedals to the list in the next day or so.

Big John wrote:Just wanted to add my 2¢ to this cool lounge, which I found trying to search for mods. I mainly hang out at the MLP forum.

I'd picked up a pair of JCA5012C's recently and noticed the heater hum thing right off the bat like a few folks here had mentioned. What fixed it for me was cutting the backside traces and wiring in new heater lines.

Heya, Big John welcome to the club! Between you and the other guy posting his DC heater mod, I've been busy researching just what I want to try. I really like the DC mod, since it doesn't require permanently modifying the PCB. But then again, this looks really clean, and it's nice that it's just wiring, rather than sourcing and finding space for a DC power unit.

Any chance you have some pics of the other side of the board?

Also, I found your recent posts over at MLP. What's the deal with the cap removal mod you talked about there?

Sorry, I forgot to snap a pic of the back side. Removing that cap (C21) has the same effect as the Warren Haynes mod on the SLO. When the gain knob on the OD channel is below half, the tone is warmer because that little wiper bypass cap isn't there to let the highs thru. The "treble bleed" mod that players put on their guitar volume pots works the same way as that C21 does.

So I got this wild hair...I want to turn one of my 5012C's into a 2x12 combo. I just got done messing around with one of them and I think I can nail it one of two ways:

1. install a co-planar baffle (speakers are slightly angled to face each other side by side)
2. diagonal mount the speakers flat to a new baffle

The first way would be the quickest, but if I use a front frame for the grill cloth to maintain a stock look, then I won't be able to close-mic either speaker. That rules out using my E609 since it's a supercardioid and needs to close to the speaker.

The second way will require a couple things: (A) going full height with the grill and losing the blue crossbar trim; (B) relocating the choke inside the chassis so that the speaker edge will fit (it hits the choke as is)

Opinions?

Ok guys had this amp jca50c 112 for a week now and have a couple questions

1. Does the jet city flood delay work in this loop?
2. My amp comes with a depth pot from factory, haven't seen any of these amps, is this a newer combo?

I had to join the heater club. Thanks for the inspiration Big John.

midnightlaundry wrote:I had to join the heater club. Thanks for the inspiration Big John.

Damn, dude - you beat me to it. How'd it work out? Any pics of the traces you cut on the back?

Also gotta do the ext bias when I bust it open...

This thread keeps getting better and better!

It worked real good. I used the 18 gauge wire which is a little harder to work with, but it's worth it. It took away that ratty buzz my amp had.

I used my Dremel with a cutoff wheel for the traces. It's fast that way. Just cut them off around each tube pin so they're out of the picture. No big deal.

midnightlaundry wrote:It worked real good. I used the 18 gauge wire which is a little harder to work with, but it's worth it. It took away that ratty buzz my amp had.

I used my Dremel with a cutoff wheel for the traces. It's fast that way. Just cut them off around each tube pin so they're out of the picture. No big deal.

Thanks for the tip! Glad to hear it worked well for you. Do you think that changing both the heaters and rectifier diodes were equally beneficial, or one more than another?

And two other questions, if you don't mind: are all the signal caps you changed out in view in that pic, also is that middle leg from the bias pot just going to ground, hence not needing to carry it to the external bias pot?

Excellent job on the wiring, by the way. Really appreciate all you guys sharing all the kno-how with this newb

Great wiring job, midnight. 18g!...you're brave. I've nixed the 2x12 idea. Going to throw it in a headshell instead. One more thing I want to do is warm up the crunch channel like pulling C21 did for the lead channel.

just got JCA22H for couple days.can't wait till my parts coming and want to stick my solder for the MOD

*sorry for my poor english,not my primary language

adding some photo so it becomes official

Oh man...I stumbled into a black hole on the internet.

Since I didn't get an answer about the jca20h supporting a 4 ohm speaker, it has sent me into a firefox session with a dozen opened tabs, regarding output transformers, and negative feedback theory. I am starting to understand the science of the depth knob (in depth :P ) , but now my head is starting to ache a little bit.

Since I didn't get any feedback (negative or positive ) I would like to rehash my question without my jibberjabber about a different amp. Does running a JCA20h through a single 4ohm 12" Scorpion speaker smell like catastrophic failure? Is that output transformer rated at an 8 ohm minimum?

I hope by the end of my weeding through these articles, I will have a true understanding of negative feedback instead of "snip this wire and hook it to a potentiometer. You'll like it." hahaha! I know how to play, I know how to solder, I have Ohm's law forever imbedded into my memory...but I still don't have a proper understanding of the depth knob. If I can narrow it down to one or two useful links, I will post them for anyone else that may be as curious as myself! To my understanding thus far, it does have a direct correspondence the the impedance and load of the speaker, so it makes sense to discuss NFB along with speaker/cabinet choice.

Looks like the twisted heater wires are the official fix from Jet City. Dude over on the MLP forum got his 50w chassis back and this is what got done. Owner says the hum is now gone. Looks like they left the PCB traces in-place between V3-V4-V5 and just flew the wires between V1-V2-V3...



Interesting that they curved the heater wires between V1-V2. May have to try that.

Quick and dirty "How I did it" for putting in a new heater supply for my JCA5012C (combo version of the JCA50H)...

I removed all knobs, nuts, & washers on all the pots and jacks front and back, and the PCB screws that hold it to the stand-offs. I have to get the speaker/footswitch/fx-loop jacks loose and out of the way to make the flip possible, as well as snipping the zip ties...





The long traces that go from preamp tube to preamp tube are the target. They attach at the top and bottom of each socket...





I used a Dremel with a cutting wheel. Keep a steady hand and go slow. Don't need a lot of speed or pressure on the wheel: "hot knife thru butter". Found that out on the first cut which you can see on the lower trace...







While the PCB is flipped, took the opportunity to de-solder the blue/black jumper wire on the front of the PCB that goes from the power tubes to the preamp tubes (JP5-JP5).

Now it's time for the new heater supply. I used a $7 spool of doorbell wire from Home Depot; 22g solid core. The wire itself is fine, but the insulation shrinks damn fast if you park the soldering iron on it. Gotta be quick with the heat...



Getting the twist is easy. Cut the wire to about 6-8". Chuck one end into a drill and hang onto the other end. Don't have to twist them too tight; this shit insulation will tear if it's too tight...







First leg to put in is the jumper that goes from the power tubes to the preamp tubes. This is just old school bending/trimming/dry-fitting/repeat. Don't pay attention to my redneck band-aid...



Happy with the fit, so tin the ends. I didn't strip the insulation back at all with this wire. It shrank back on its own from the heat.



One jumper down, four more to go.



For the last jumper, I curved the supply to mimic what Jet City does. Done deal...





While the PCB was flipped, I took the opportunity to de-solder and take out C28. After studying Soldano and Jet City schemos for a week, I took a guess that this was the cap that mimic'd the crunch channel bright switch on the SLO (which I never used on my SLO's). Once I got the amp buttoned back up, I think I was right. The crunch channel is now warmer.



I'd previously removed C21 for a permanent Haynes mod on the lead channel...



With all new tubes and a re-bais, the amp is now much quieter, and both channels are warmer. Right on.

Big John, are you running any pedals in the fx loop?