Guitar Gear Forum

Hi everyone there,
I'm a new to this topic!Thanks for all the ideas to mod the amps,really the great work!!!After reading through this,I have some little questions about modding my JCA50h because I'm pretty new to mod an amp:
1.The plate resistor change from 220K to 180K,150K or even 100K? Like the R46 and R19 on the JCA50h,seems to reduce the gain and add the clarity,that's definetly what I want for this amp cause I found the gain is too much in od channel(for over 1 o'clock),and the bass heavy od channel make the sound somewhat muddy and sloppy. I'd like a sound that is more clarity under hi gain and with a tighten and clear bass response but still a thick sound.Some one in this topic say this change will thin the sound...
2.The coupling cap change from .022uf to 10nf or 2.2nf ? Like the C24 and C30 on the JCA50h.Seems the same quetion with question no.1...
3.The cathode:I don't really understand the methods doing this,like swap the R36 to 470K and R27 to 820K,someone says you will get a more marshall sound.also the bypass the cathode caps like C19,C26 and C32,is it mean to reduce the gain and add more clarity?
4.The SLO mod:they swap the C10 and C11 caps from .022uf to 47nf in the power amp sections,I don't understand why they do that,also I think changing r19 to 220K in this mod isn't a good idea,cause this amp had lots of gain...
5.The O'netics slo 50W transformer:is it an good upgrade for the JCAs?
Sorry for so many questions.What I want most is too clear the sound but not to thin the sound much.So may be you can suggest the value of resistor and caps...Thanks a lot!

1. Reducing R46 to 180k or 150k reduces gain. I won't go to 100k unless you like a very open sound. It's not suitable for hi-gain then. Try 150k on R46 and increase R19 from 100k to 150k, then you get a modern and dynamic sound. This affects only compression but to get rid of the muddy sound, you better should lower the values of the coupling caps and use good caps overall.
2. Something between 4,7n and 10n for first, 10nF for second coupling cap.
3. R36 to 470k if you reduce the 39k in the cold stage (fe. to "Marshally" 10k). 820k on R27? Are you sure? Stock is 1,8k. I think you mean 820R. But that's way to much. I recommend 1,2k. If you like Marshall specs overall, then change the 1,8k/1u at the 3 tubes to 2,7k/.68u. For more clarity use better caps overall!
4. SLO Mod is not only to increase the coupling caps in the phase inverter. Also 22n is quite enough there, especially, if you don't want a muddy sound.
5. No, they are not better, but different. Not worth the money in my opinion. If you are really planning to exchange the transformers, buy something from Heyboer. But I don't think, that you know which specs and how to install them.

To the guy who asks about Tung Sol in cathode follower - they will easily fail there. I think, this has been discussed enough in several forums, fe. the Gear Page.

Thanks a million,käsebrot!!!I think I fully understand now,great knowledge I've gain from you

Snagged one of the US Custom 2x12 cabs from ZZ Sounds...looking forward to it. Hoping it won't have the fartiness my Avatars did.

käsebrot wrote:
To the guy who asks about Tung Sol in cathode follower - they will easily fail there. I think, this has been discussed enough in several forums, fe. the Gear Page.

Thank you kasebrot.... just to verify.... cathode follower is V3?

Does anyone know what aftermarket footswitch I can use with my 5012c? Boss FS-5U? FS-5L? Any others? Jet City doesn't offer their own branded footswitches anymore, and some of their amps use different footswitch types.

I've been playing with the idea of retubing my JetCity JCA50 ever since i got it, and now i found a site that actually sells a shitload of different tubes for a reasonable price. It's actually a company (if its even a company, or just one guy), that makes high end custom hifi amplifiers and sound systems, and they have all this differnet tubes for sale. To top it of, the thing is like 30minutes away from my house, so i can go pick them up if i want. Here's the link if anyone cares to check it out. Its not in english, but there isnt much written anyway, except tube types:
http://www.trm-audio.si/elektronke.htm

Anyway, now, it turns out, i have a few questions.

First of all, im guessing that if I just replace the power tubes with another pair of 6L6's, i dont have to rebias the amp? Because that is one thing i dont feel comfortable doing.

Second, i belive i can just mix and mach the preamp tubes as i see fit, as long as they are all 12AX7 or ECC 83 type?

Third, there seem to be a lot of different subtypes of each tube, for example, Sovtek 12AX7 tubes have aditional designations like 12AX7LP, 12AX7LPS, 12AX7WA, 12AX7WB, 12AX7WC... Its the same for pretty much all the other makes. Does this make any difference? Or can i just use anyone i want? Also, some are listed as "balanced" and some are not. And all of them have listed things like "double triode" and "output beam tetrode". I hav eno idea what that is, but it seems that the same types (e.g. 6L6's), have the same description, they only differ in the aditional designations (6L6, 6L6GC...). Im kinda lost here.

Fourth, are the 6L6 the same type as 6V6? Or do they have to be strictly 6L6 tubes?

Fifth, the store has tubes from JJ Electronics, Sovtek, Svetlana, Electro Hamonix, Tungsol, Mullard and also some others, including NOS tubes. I play hi gain metal, so what tubes would you recomend, preamp and amp ones. I was thinking either Tungsol or EH 12ax7's and Tungsol or EH 6L6's.

Thanks a bunch for your advice, since this is way more complicated than i thought it would be.

1. The word is JCAs are biased cold from the factory and you can swap power tubes as you please. For optimum preformance and tone I recomend getting it rebiased.

2. You can mix and match 12ax7s as pleased however stay away from tungsol 12ax7s as they dont work in cathode follower spots well.

3. Lots of subtypes for 12ax7. Expeciallyy sovtek brands. LPS of the top of my head means Long Plate, LPS meand Long Plate Sprial filiment. Long plates seem to have a widerbandwith but can be more noisey. Spiral filiments help keep heater hum down. 12ax7--- they just mean they have some speacial feature about that particular tube. Look them up if your interested.

4, 6v6 are not 6l6. Use 6l6 type

Well, im still figuring out what tubes to get, and in the meantime i wrote to JetCity customer support, and asked them which spot in the preamp is the cathode follower stage, because i dont wanna burn out the tungsol tubes, if i get any. They told me there is no reason for a tube to fail in any specific position in the amp, and of course they didnt anwser my question, so i still dont know which one it is. Anyway, is this true, and is the whole internet wrong, or are JetCity being dicks?

Maybe you could just google "Jet City tube layout" and look into the second link you get before calling the guys at Jet City dicks... it depends on the voltage at the CF spot and the specs of the individual tube there, if it fails or not. I wouldn't use Tung Sol there even soundwise.

I would go for Ruby 12AX7 AC5 HG in all positions except V1, there you may use a HG+ or a V1 selected Tung Sol 12AX7 Reissue. If it's too bright, try JJs in V4 and V5.

I suppose I can jump in here...I have a JCA sitting in my music room. Bought it a year or so ago, plugged a guitar in to make sure it worked, and haven't touched it since. lol

If you're happy with the sound there's no reason to do any mods.
But I would check the bias as it isn't set properly and must be adjusted in most cases.

Anyone know where I can find replacement cicuit boards for the 20h? I wanted to rebuild the board with some carbon films and ornge drops and a slight different circuit. Removing the components then sucking out the solder isnt really worth it. Im not even sure if I should cut out all the componets I wont be using off the board first so I can get in their easier. Pcb isnt really my thing it turns out so I might just use some vector board I have left over and hand wird it. If I do that should I keep the same layout as the circuit board?

i read in one of these jca threads that a guy has replacement pcbs from repairing them. you can also contact jet city and tell them there is a crack in your board. i got a jca22 power tranny for 50 bucks from jet city.

if your going to go through the trouble of rewiring it, you basically paid for tubes, transformers, and a chassis. which honestly is a good price if you paid around 200 for the amp. buying a jca and ripping the guts out wouldnt have been my first choice but its by far not a bad one. i probably would have just parted everything out.

I agree shep. I wanted one to tinker with. I havent screwed with it too much. Just two resistors replaced and a cap pulled up. I did remove the pcb so maybe 8 wires I cut at the board.

These amps are supposed to be great mod amps and ill tell you what, moding a pcb is not like working with turrets!
I did buy 50 bucks worth of componets, maybe just doing a few tweeks and a cap job would be easier.

I got the amp for 160$ and it has 60's amperexs el84s in it so I made out very well. The amp for the most part is basicaly untouched.

Edit- Did you upgrade the capacitors? If so was your method for replacing cuting of the cap and then removing the solder?

Also Jca never responded and some repair shop in NYC said they have replacement boards for sale. That was a post from 4 years ago and I never got a responce back.

I don't see the sense in getting a whole new pcb if yours is ok and you only want to change the caps and a few resistors.
If you don't know how to unsolder the stock ones from the pcb, honestly you shouldn't make anything on the interior of a tube amp which works with lethal voltages.

And to the point that you get no answer - I have asked Doug at Jet City (and also Mike) some questions several times per mail, and got an answer very quickly. Really great support and Doug and Mike are really helpful and friendly guys!

If you get no reply, maybe that's because of your way to ask, no clue about that.

Oh, and as I've already said it, if you're really, really going to put Orange Drops 715 or 716 in a JCA, then this will sound very, very bad - harsh, shrill with unpleasent ringing, trebly fizz. But it's your personal amp, so it's your choice.

Drainbamage wrote:I agree shep. I wanted one to tinker with. I havent screwed with it too much. Just two resistors replaced and a cap pulled up. I did remove the pcb so maybe 8 wires I cut at the board.

These amps are supposed to be great mod amps and ill tell you what, moding a pcb is not like working with turrets!
I did buy 50 bucks worth of componets, maybe just doing a few tweeks and a cap job would be easier.

I got the amp for 160$ and it has 60's amperexs el84s in it so I made out very well. The amp for the most part is basicaly untouched.

Edit- Did you upgrade the capacitors? If so was your method for replacing cuting of the cap and then removing the solder?

Also Jca never responded and some repair shop in NYC said they have replacement boards for sale. That was a post from 4 years ago and I never got a responce back.

it may take some time to get a response from them. they also have a couple of official amp techs you can contact from their webpage. you can ask them for a pcb as well. or contact jet city directly. lets see, i got in touch with a doug from jet city who i ordered my tranny from. let me see if i can find his email.
orange drops arent a bad choice as mesa engineering uses almost allllll orange drops besides their cathode bypass caps which are tantalum caps. which käsebrot also says not to use.

also, im able to replace parts on the board without even taking the board out. which is pretty convenient. as the eyelets on the board are double sided. makes things easier. i would suggest using the same board unless you wanna change so many parts that its pretty much a different amp. if you prefer point to point more, than go with it. just make sure that you have ample knowledge on high gain amp circuit layout. and if you need help, i'll be around!

i originally thought of getting more of a mark2c+ sound out of the amp but realized that there would be a lot of component value changes and component additions. so, i built the mark preamp in a stand alone unit and use the effects return on the jet city as the power amp. just made more sense to me at that point.

I've never talked about tantalum caps.
But I've recapped a JCA completely with Orange Drop 715 and it sounded like sh*t. Without any mods, just the same values.
I know about Mesa using these caps. But these are quite other amps, although they copied a lot from Soldano. But a Mesa sounds like a Mesa and a Soldano is quite more of a hot rodded Marshall.
If you want to go in Mesa direction, I would cut off the NFB loop.

Thanks guys, shep, Kas.
So I sucked it up and striped the whole board. It didnt give me a whole lot of trouble just time consuming. A few of the through hole ends (the eyelet on the other side of the trace) came off. I think that happend when some of the componet leads were bent and didnt come out straight. I populatated the board up to the tone stack.

Shep- did you reuse the stock solder when upgrading parts? I pulled the componets then sucked up the solder the best I could and soldered from the trace side. I think that would give the best connection. Pulling the board I also got a visual on the joint. Im afraid of a cold solder joint because finding it will be a nightmare.

i added just a little of my own solder to ensure a good connection. to avoid a cold solder joint you just have to ensure that both the component lead and the board pad are both heated well. i wouldnt over think the potential of a cold solder joint. i also like to wiggle the component a bit to make sure its been soldered well and that the solder doesnt easily come loose from anything.

Finished up the board with some tweeks. All carbon films. The amp is noieser now as expected but not sure what would be normal switching from metal to carbon film. Hopefuly it wasnt do to my pcb soldering lol. Still need to tweek somethings. The amp sounds different now, not really better though. Didnt like reducing feedback. I also removed a voltage divider resistor to ground from the treble pot to the master vol. Not sure if this would help with noise or if it was just there to reduce signal to the PI.

Anyone have a noisy power transformers? It was noisy before any modding just wondering kinda annoying.

Drainbamage wrote:Finished up the board with some tweeks. All carbon films. The amp is noieser now as expected but not sure what would be normal switching from metal to carbon film. Hopefuly it wasnt do to my pcb soldering lol. Still need to tweek somethings. The amp sounds different now, not really better though. Didnt like reducing feedback. I also removed a voltage divider resistor to ground from the treble pot to the master vol. Not sure if this would help with noise or if it was just there to reduce signal to the PI.

Anyone have a noisy power transformers? It was noisy before any modding just wondering kinda annoying.

changing values of the original design may cause noise simply because the pcb layout was designed with original values. also, what wattage did you use for resistors? using lower wattage resistors than original may cause noise.

Drainbamage wrote:Finished up the board with some tweeks. All carbon films. The amp is noieser now as expected but not sure what would be normal switching from metal to carbon film. Hopefuly it wasnt do to my pcb soldering lol. Still need to tweek somethings. The amp sounds different now, not really better though. Didnt like reducing feedback. I also removed a voltage divider resistor to ground from the treble pot to the master vol. Not sure if this would help with noise or if it was just there to reduce signal to the PI.

Anyone have a noisy power transformers? It was noisy before any modding just wondering kinda annoying.

reducing feedback will cause the power amo to be less linear from the way ive understood posts about it. reducing nfb should be similiar to turning the presence knob up. it may also cause noise. such as more feedback

1 watt carbon films. Reducing negative feedback will cause the power section to brake up more. It does that now but to much. The amp gets fluby around 6 on the master even with less gain in the preamp. The PI values arnt really suted for el84s from my understanding. Im thinking lots of negative feedback was Mikes way of rectifing this.

what type of noise was added after the mods? you might could try two watt on any of the plate resistors you changed. calculate the power dissipation using the voltage drop. it probably isnt doing even near a watt but its worth checking or changing just to see. r=v/i, p=(ixi)r. i know its common to use 2watt on power supply and plate resistors