Guitar Gear Forum

Spec shows Audio pot

Linear pots have a different resistance curve from Audio - Audio pots have an exponentially increasing amount of resistance the closer you get to the end of the turn, whereas linear resists a steadily increasing amount along the turn of the dial.

Not sure that makes sense. Just use an audio pot like the fakking spec sez

And the pics that Sahl posted really help clarify the circuit path. As far as I can tell, you have your wiring exactly right.

We need to get this info into the Depth Mod thread - Dogger - would you please document out what you do with Sahl's pics and yours, so people can see how it's supposed to look on the more modern JCAs?

GrimeSpace - sent link for image to use in the DIY section. Let me know if that is what you wanted.

Is this right for the Channel Switch Mod?

GRIMESPACE wrote:Sahl - any chance you could send me some higher-res / close-up gut shots? I'd love to see more about how those switches are functioning.

It really sounds like the voicing switch is similar to the slope resistor mod on my 50 that gave it a much more 'british' tone that I love.

Can't wait to compare my 50 to Jer's modded 22

I'll get a pic uploaded here in a minute.

I've got a question on mine. There was a 1nf cap laying on top/parallel of the R19 resistor (I have removed and measured it) - any idea what that would be for?

High res gut shot of my "Valve 20":

https://goo.gl/photos/PrN13NcgyvKpZPR8A

May be getting a JCA 20 Head for this CV Tele I have to throw on the 70's Kustom cab I am suppose to get Friday. If the guy backs out it's cool though....I just scored an 82' JCM 800 combo

Ran the JCA 50H through a Mesa V30 4x12 this weekend - at volume.

Holey Ballz... I'm in

I definitely want to do the SLO mod, and get a decent V30 2x12 for it

I'm back in, bitches! I picked up another Jet City, the JCA50H, a few weeks ago and finally got the Avatar Signature 2x12 loaded with Vintage 30's to run it through. Massive sounding! I sold my old JCA100H last year and have regretted it since. I had that amp for 5 years. This JCA50H still has that tone. I'm considering modding this time around, but nothing too crazy. Probably SLO voicing mods, but I already like it as-is, so I'm no hurry to do it. Cheers to the Jet City Lounge

If anyone is interested

viewtopic.php?f=25&t=32820

Dogger - what are you buying next?

not sure.....probably going back to a Mesa Boogie Single Rec w/ 6v6 pwr tubes. Really like that amp and sold it a couple years ago or a EVH5150 III 50 watt...(has a head phone jack)

I am nothing more than a bedroom hack that like everyone else is looking for that tone in their head! Problem is do I really know what that tone is?

Some what a gear junkie and also have gotten into building my own replica cab. Currently this is what I play through. Made them both my self.









4x12 has 2-V30 and 2-G12T-75
2x12 has 1-Greenback and 1-Greenback 55hz

VERY impressive work!

Hey just picked one of these up and I notice the power transformer hums pretty impressively when I take it off standby - is that normal?

To me, it sounds like its saturating from overload but I dunno - never had a 50w before.

Amp plays fine, no red plates, pt not hot or anything - all seems well except for the PT humming like its shorted or something!

theDogger wrote:not sure.....probably going back to a Mesa Boogie Single Rec w/ 6v6 pwr tubes. Really like that amp and sold it a couple years ago or a EVH5150 III 50 watt...(has a head phone jack)

I am nothing more than a bedroom hack that like everyone else is looking for that tone in their head! Problem is do I really know what that tone is?

Some what a gear junkie and also have gotten into building my own replica cab. Currently this is what I play through. Made them both my self.









4x12 has 2-V30 and 2-G12T-75
2x12 has 1-Greenback and 1-Greenback 55hz

Kick ass!

Hi, first post so be kind

I managed to pick up a JCA20H last night for a decent price, sounds great but I have a few questions.

I want to change the tubes (probably just preamp for now), without going through endless pages can anyone suggest a good tube config for this amp?
Had a quick rummage at home last night and I found a Sovtek 12AX7 which is a start but that was it so I'll need to buy a new set.

For modernish higher gain metal sound would I be better off buying a distortion or overdrive pedal for this amp? I used to have a Bad Monkey which worked great with my old Krank Rev Jr however I fancy something different this time. I was contemplating an old MXR Distortion III but I haven't pulled the trigger yet. Any recommendations?

Lastly, I'm a bit of a tinkerer and I love modding stuff (nothing too serious mind). Are there any good resources for modding ie drawings, instructions etc? Had a quick read about changing some caps blah blah but that's about it so far.

Sorry about the long first post, I've got a lot to ask

Jcmatt wrote:Hey just picked one of these up and I notice the power transformer hums pretty impressively when I take it off standby - is that normal?

To me, it sounds like its saturating from overload but I dunno - never had a 50w before.

Amp plays fine, no red plates, pt not hot or anything - all seems well except for the PT humming like its shorted or something!

Sorry - not sure. If it's abnormal, I'd highly recommend returning it.

Beezerk wrote:Hi, first post so be kind

I managed to pick up a JCA20H last night for a decent price, sounds great but I have a few questions.

I want to change the tubes (probably just preamp for now), without going through endless pages can anyone suggest a good tube config for this amp?
Had a quick rummage at home last night and I found a Sovtek 12AX7 which is a start but that was it so I'll need to buy a new set.

For modernish higher gain metal sound would I be better off buying a distortion or overdrive pedal for this amp? I used to have a Bad Monkey which worked great with my old Krank Rev Jr however I fancy something different this time. I was contemplating an old MXR Distortion III but I haven't pulled the trigger yet. Any recommendations?

Lastly, I'm a bit of a tinkerer and I love modding stuff (nothing too serious mind). Are there any good resources for modding ie drawings, instructions etc? Had a quick read about changing some caps blah blah but that's about it so far.

Sorry about the long first post, I've got a lot to ask

JJ's from Eurotubes have worked well for me. Loads of other options, though - much of which is covered throughout the pages in this thread. For preamp tubes, you'll get the most benefit from changing V1, V2, and the Phase inverter (V5). Power tubes in these things pretty well suck - changing them out, along with setting bias is highly recommended.

Boosts are covered throughout the thread, but not heavily. I suggest checking with Sahlmonic, as he's probably our biggest 'metal' player that uses JCAs quite a bit.

For mods, see references on page 1, post 3.

GRIMESPACE wrote:Sorry - not sure. If it's abnormal, I'd highly recommend returning it.

Cool, thanks for your input. Does your jca power transformer hum?

I plan on checking the tube bias tonight. Maybe the previous user cranked it up too far .

I've been bothered by hum/hiss through the speakers, but not in the amp itself. Does it do that when it's on standby?

And yeah - I guess bias being up too high might cause a significant power draw from the power transformer.

Beezerk wrote:Hi, first post so be kind

I managed to pick up a JCA20H last night for a decent price, sounds great but I have a few questions.

I want to change the tubes (probably just preamp for now), without going through endless pages can anyone suggest a good tube config for this amp?
Had a quick rummage at home last night and I found a Sovtek 12AX7 which is a start but that was it so I'll need to buy a new set.

For modernish higher gain metal sound would I be better off buying a distortion or overdrive pedal for this amp? I used to have a Bad Monkey which worked great with my old Krank Rev Jr however I fancy something different this time. I was contemplating an old MXR Distortion III but I haven't pulled the trigger yet. Any recommendations?

Lastly, I'm a bit of a tinkerer and I love modding stuff (nothing too serious mind). Are there any good resources for modding ie drawings, instructions etc? Had a quick read about changing some caps blah blah but that's about it so far.

Sorry about the long first post, I've got a lot to ask

I had a Rev Jr. also. I definitely like the JCA better. I have JJ power tubes and Ruby pre-amp tubes & put an ET-65 in it as suggested in one of the threads here and it sounds great. I am actually picking up a head version Friday. At the moment I prefer my JCA dirt more than my JCM800 but I just got the 800 so haven't had a lot of time with it.

Nope, quiet on standby. There is a whump leaving standby (as I'd expect, filling the B+ caps) but then sort of a constant buzz.

I'll check the bias and take a video if its still doing it.

Aha! Twas the bias cranked way the hell up - bringing it back down a bit quieted the power transformer. This amp is used, and perhaps the previous owner just cranked it up to make it sound metal AF or something.

It's a little odd that there are 0 Ohm jumpers in the cathodes of the output tubes and not actual resistors. I mean, the cost is the same to stuff a 0 ohm resistor as a 1 ohm - what the heck, jet city?

Well anyway, I'm back up and running - and with a heater mod too! That certainly does quiet down the hum, thanks all for the info!

Hey - that's great, man!

I'm wondering how much damage to the output tubes, though - surprised it didn't redplate.

Thanks for the replies, I've ordered 3 JJ preamp tubes as per the Eurotubes Jet City high gain kit but from a UK supplier.
I'll wait to change the power amp tubes but they will be done. I'll have a bash at adjusting the bias once the new tubes are in.
I've read quite a few of the threads, lots of pico farad this and nano watt that that which makes hard reading, I do fancy some simple component swaps but I need to read more before I attempt those.

GRIMESPACE wrote:Hey - that's great, man!

I'm wondering how much damage to the output tubes, though - surprised it didn't redplate.

Yes, thanks! I'm also surprised it didn't redplate. I'm going to get some new 6L6 anyway - like you said, these have probably taken a beating.

Hi all!
So I joined the club of happy JCA22H users club few months ago
Needles to say that I love that overdriven sound, reminds me of a crossing between the 80s-90s hard rock sounds (thing Skid row and the likes) and some more modern sounding drive.

So in a nutshell here's what i've been using on this amp rather successfully so far, and sorry but some contradicts what has been said so far I think

Guitar > Tuner pedal > Clone central TS9/TS808> Donner/Mooer Noise gate >amp
Effects send > Ibanez EQ > Joyo D-SEED (yes it works brilliantly) > TC Dito (as well) > Effects return.

So the DSEED and Dito do actually work flawlessly on this amp.

Now the issues i've encountered were with the noise gate.
If i put it in the effects loop, which i initially wanted to do, i'd hear a loud POP when switching it on, and the frequencies would really be cut on the signal.
I'm looking for a gate that could be used on this effects loop, so if anyone has a reference... I heard of the GString but it's way ove army budget.
Maybe the TC electronics one ? (I had a quick discussion with whoever takes care of the JCA Facebook page, and he was sort of pointing at TC Electronics gear generally) or maybe the EHX Silencer ?
Has anyone tried any of these ?

Next, a reverb. I'm about to order T-Rex's Ghost Tone. It's cheap and sounds promising.
Has anyone tried that one in the loop ?

Thank you all for your feedback (if i can say so )