Guitar Gear Forum

Howdy gang,
Just stumbled across this thread while I was Googling something else. (I was wondering why the old "Jca50H / 100h/ 22h mod thread and owners club" thread on HC was so dead. I followed it pretty regularly the past few years).

Anyway, thought I'd share this nugget. I posted it on the HC thread earlier this month:

One thing I didn't like about the factory Jet City switch is that it takes up so much space on a pedalboard.I bought a few parts from Pedal Parts Plus (an enclosure, LED/bezel, DPDT switch, & a jack) and basically just copied the existing wiring from the factory switch (even reused the resistor). Works like a charm, and the LED turns blue when the overdrive channel is selected:

Yarbicus wrote:I'm in! I started with a JCA20h and JCA12 cab. Last year I pulled some strings (I don't work for JCA but dud write the user manuals) and scored a JCA22H that Mike Soldano modded so that the Crunch channel is a true clean channel. Also switched to chickenhead knobs. I loves it so!

Hey Yarbicus,
Do you know exactly what Mr. Soldano did to the crunch channel on that 22H to make it clean?

Ok, one more post for the night...then I'll shutup!

Just thought I'd share a pic of my JCA22H. I ordered one of the Jet City appearance upgrade kits from Doug White/AmpFactory a few weeks back. I'm super pleased with how everything turned out:

So what do you do about the buzz in the JCA50?

Nobody has pointed out that these amps don't have a B+ fuse..

Hey guys, another update on my humming issue here: I talked to Doug and he said one option to look into would be the heater traces and the need to be modded to be further off the board.
Could someone explain this to me? I think I know someone qualified to make this mod here where I live, but I'd like to inform myself first. Thanks!

midnightlaundry wrote:Nobody has pointed out that these amps don't have a B+ fuse..

And what does that do for toan?

JackOfBlades wrote:Hey guys, another update on my humming issue here: I talked to Doug and he said one option to look into would be the heater traces and the need to be modded to be further off the board.
Could someone explain this to me? I think I know someone qualified to make this mod here where I live, but I'd like to inform myself first. Thanks!

YOu can cut the traces and elevate them like a Dumble.

GRIMESPACE wrote:

midnightlaundry wrote:Nobody has pointed out that these amps don't have a B+ fuse..

And what does that do for toan?

If a power tube arcs out, it will save your amp you jackass..

Midnight, I don't understand what you are suggesting, that's why I asked what it means on the first place. So, what does it mean? I can't find concrete information on the internet.

midnightlaundry wrote:

GRIMESPACE wrote:

midnightlaundry wrote:Nobody has pointed out that these amps don't have a B+ fuse..

And what does that do for toan?

If a power tube arcs out, it will save your amp you jackass..

Was just joking, dude - maybe you stop bitching about it and propose a solution? Impress us with your wisdom, rather than name calling, eh?

GoKart Mozart wrote:Ok, one more post for the night...then I'll shutup!

Just thought I'd share a pic of my JCA22H. I ordered one of the Jet City appearance upgrade kits from Doug White/AmpFactory a few weeks back. I'm super pleased with how everything turned out:

Looks great, and welcome to the club! Like the switch, too!

JackOfBlades wrote:Midnight, I don't understand what you are suggesting, that's why I asked what it means on the first place. So, what does it mean? I can't find concrete information on the internet.

On the JCA circuit board the heater traces run from each 12AX7 to the next one. Here is a close up of the board. The heater traces connect to pin 9 on one side and pins 4+5 on the other side. As you can see the heater traces pass under and close to other components on the board. This can lead to induced hum from the heater traces into other parts of the preamp, especially sensitive high gain circuits.



Now compare the JCA to the pic in the post above which shows the more traditional way of wiring heaters. The yellow and orange heater wires are twisted together and elevated away from the tube sockets and the chassis. Twisting the wires reduces radiated hum and elevating the wires keeps them away from sensitive parts of the circuit like the tube grids.
To mod the circuit board to raise the heater leads off the board is a lot of work. You have to cut the traces at each tube socket and rewire with twisted wires from one socket to the next. You can also use a shielded twisted pair to reduce the hum even further. This would not have to be done to the power tubes or the PI.
Even after modifying the PCB you may still have some residual hum from inside the tubes themselves. The best way IMO to reduce hum from the heaters is to convert the heater supply for the preamp tubes to DC. This much easier and can be done with a hand full of parts.
Many high end/ high gain amps have DC filament (heater) supplies at least for the first tube in the high gain channel. Another, even easier, low budget way to reduce heater hum is to elevate the heater winding center tap to about 70-80 VDC right from the B+. This takes two resistors and one capacitor. Any decent tech can do either of these mods for you if you are not inclined.
Of course if the hum is not from the heaters any mods to the heater circuit are not going to solve your problem. I think Aiken recommends to use a 6 volt lantern battery to power the filaments to see if there is an improvement before you do any modifications.

The JCA50 (and I think the 100 too) has a jumper that connects the heater supply circuit from the power tubes to the 9 pin tubes. This is a convenient point to disconnect the preamp heaters for testing and modification.
My personal JCA50 has been modified with a second filament transformer to power just the 9 pin preamp tubes. This new transformer has an elevated center tap to reduce hum. It also lets me run any power tube I want without possibly drawing too much current from the stock PT.

Good info. Do you have a part # for that filament transformer?

How would you convert the existing filaments to DC? I read that you can't just put diodes on it but it wasn't explained why. Any schematics or more info on that?

I was noticing just now on the Amp Factory page that you can order a custom JCA22H or JCA50 with a "minimal gain" option on the Crunch channel.

Anyone have any firsthand knowledge as to how they're doing that particular mod? I run the crunch channel with the pregain at about 8-9 o'clock (2-3 on the dial) with the post up high. Having a more clean Crunch channel kind of peaks my interest.

midnightlaundry wrote:Good info. Do you have a part # for that filament transformer?

How would you convert the existing filaments to DC? I read that you can't just put diodes on it but it wasn't explained why. Any schematics or more info on that?

The transformer is a Hammond http://www.tubesandmore.com/products/P-T166L6
Google is your friend......
http://www.valvewizard.co.uk/heater.html
http://s1245.photobucket.com/user/Lebow ... c.png.html
http://blueguitar.org/schems.htm

Here is one from a Carvin amp I think...DC for just V1 and V2
http://img224.imageshack.us/img224/2060 ... ply4oi.png

Cool man! I Remember reading about the lantern battery tip from Aiken. I'll do that test asap.

This is a really good sounding amp. I would be a great amp if I can kill the buzz.

GoKart Mozart wrote:I was noticing just now on the Amp Factory page that you can order a custom JCA22H or JCA50 with a "minimal gain" option on the Crunch channel.

Anyone have any firsthand knowledge as to how they're doing that particular mod? I run the crunch channel with the pregain at about 8-9 o'clock (2-3 on the dial) with the post up high. Having a more clean Crunch channel kind of peaks my interest.

Firsthand? they are probably doing the same mods as in this thread. One is the SLO style clean/crunch switch and the other is like the Decatone clean channel. they lower the gain coming out of V1 by adding some resistors to the crunch circuit. The decatone has a bridged t-filter in the clean channel to scoop the mids.

After investigating further, the buzz in the JCA50H might be an endemic property of the amp- natural of him. I'd like to try the mod you're sugesting, ironman. Are there any other easy mods to attack this buzzing/humming issue?

New here with new JCA 2212. Question to all who have played one:

RE: the drive tones of the amp, (either channel) is there an inherent harsh/fizzy sizzle to the high end similar to a solid state distortion/or a software based amp model with cab IR bypassed?

It's unpleasing to my ear, I was anticipating a much smoother high end. We played several different Les Pauls, each with different pickups and the fizz was present with each.

Played through and external 212 cab (Classic Lead 80 and GT-12 75) and the fizz was somewhat less.

Bypassed preamp using Mesa V-Twin into effect loop return, and the fizz was even less yet, but still apparent using either internal speaker, or the external cabinet.

I really didn't hear that in any clip/video I've watched.

The crunch channel with gritty clean settings is worth the price of admission alone. Still, I am expecting a two channel gigging amp covering all bases with limited drive/boost pedals, preferably none aside from the amps channel select footswitch.

(BTW - love the mid-range voicing in the amp, much nicer in person than I thought it was going to be)

glepko wrote:New here with new JCA 2212. Question to all who have played one:

RE: the drive tones of the amp, is there an inherent harsh/fizzy sizzle to the high end similar to a solid state distortion/or a software based amp model with cab IR bypassed?

It's unpleasing to my ear, I was anticipating a much smoother high end. We played several different Les Pauls, each with different pickups and the fizz was present with each.

Played through and external 212 cab (Classic Lead 80 and GT-12 75) and the fizz was somewhat less.

Bypassed preamp using Mesa V-Twin into effect loop return, and the fizz was even less yet, but still apparent using either internal speaker, or the external cabinet.

I really didn't hear that in any clip/video I've watched.

The crunch channel with gritty clean settings is worth the price of admission alone. Still, I am expecting a two channel gigging amp covering all bases with limited drive/boost pedals, preferably none aside from the amps channel select footswitch.

(BTW - love the mid-range voicing in the amp, much nicer in person than I thought it was going to be)

Heya, and welcome!

Most new Jet City owners have greatly benefitted from upgrading the tubes from the lower-quality stock units, as well as setting the bias properly. This, alone greatly improves the amp, and many stop there. I would suggest starting there and see if it makes a big diffence in the fizz you're experiencing.

I've got a JCA50H - similar in many regards, but still has some notable differences - power tubes (yours is running EL84s where mine has 6L6s) and the effects loop is also supposedly changed to make yours more compatible with pedals.

The amps are great modding platforms as well, and if you take the time to read through the pages, you'll see numerous other updates you can make. The mods I've done (which apply to the 22 design as well) are depth, clean & scoop switches (crunch channel only), and a slope resistor switch that made a huge different in upper-midrange voicing. Of those, I highly recommend the depth and slope resistor - especially if you already like the crunch channel.

Best of luck - there's quite a few regulars here that will probably be watching and pipe in as well. Keep us posted on what you find.

Thanks for quick reply. So much information in this group, and the old H/C mod thread. I wanted to make sure this is normal before diggin in an modding a new amp, which, openly, I'm a bit timid to tear into. You know, the old (based in truth) tales of injury and death lurking inside amplifiers.

I'll start with email to Eurotubes and taking suggestions on V1, V2, V3, and possibly V5 in attempts to smooth it out (approach more of an MKIV lead tone without losing the mid-range bark) Then finding someone locally to set bias.



Actually, I'm going to start by putting a parametric EQ in the effects loop to locate the frequency center my ear isn't enjoying. I'll come back here once I figure that out (next week)

I just checked my stock power tubes, and they were 26 ma's out of match! WTF?

glepko wrote:Thanks for quick reply. So much information in this group, and the old H/C mod thread. I wanted to make sure this is normal before diggin in an modding a new amp, which, openly, I'm a bit timid to tear into. You know, the old (based in truth) tales of injury and death lurking inside amplifiers.

I'll start with email to Eurotubes and taking suggestions on V1, V2, V3, and possibly V5 in attempts to smooth it out (approach more of an MKIV lead tone without losing the mid-range bark) Then finding someone locally to set bias.



Actually, I'm going to start by putting a parametric EQ in the effects loop to locate the frequency center my ear isn't enjoying. I'll come back here once I figure that out (next week)

I had good luck working with Eurotubes, but I'd really suggest reaching out to a few of the better dealers to see what comes up. I did buy a bias tester from them, but worked very carefully whilst learning the techniques of draining the filter caps (the ones everyone rightfully warns you about). If you're not experienced in doing so, I agree that you'd be better off taking it to a qualified tech.

Don't skimp out on the power tubes - they do make a difference. I've read that you can skip V3 and V4 if you're trying to save cost, as they have less impact on tone.

Do tell us what you discover with the EQ in the loop. I'm getting a new studio ready, and plan to do some experimenting of my own very soon.

Good luck!

midnightlaundry wrote:I just checked my stock power tubes, and they were 26 ma's out of match! WTF?

You mean one side is drawing 26ma more than the other? Maybe one tube is bad. Try swapping the tubes to either side to see if the current draw follows a particular tube.
All tubes drift with age. All new tubes drift for the first 8-10 hours sometimes dramatically. When you buy matched tubes they must be burned in for the initial 8 hours then matched.
The only way to properly bias unmatched pairs is to have a bias adjustment per tube