JCA OWNERS!!! (RE)JOIN THE JET CITY LOUNGE!!!

[Hipster Salad]

Got in some new Jet City speakers today. The 'Vintage' series. Much bigger magnet than the original Jet City 100 watt speakers, and the Vintage ones are 55 watts. Sort of like a smoothed out V30 but not as crisp. I'm very anxious to hear how much they open up after they fully break in.

I will say that they are fantastic. Gigged them last night with my Earhart and I was all smiles.

[video]https://youtu.be/-ocbcpsgkCA[/video]

[theDogger]

A while back in a conversation about adding a resistor to assist with a the depth mod at lower volumes

RE the fizzle: The primary frequencies of nuisance for me are between 3-4k. A judicial cut with EQ in the loop reduces the fizz. However - opening up the volume does the same thing, much to my surprise. The low end grows into too much when letting the master section work - that's a good thing. Very pleasant surprise with that. Gorgeous with mid gain on crunch right through ripping high gain on overdrive channel.

There is an known problem with master volume controls that connect the wiper of the master volume pot to the input of the PA section. This is common on all the JCA amps. The master volume becomes a high pass filter at low settings on the master. This gives a thin, fizzy sound with the master set below 3-4. You open up the master and the low end jumps up. The fix is easy, one 100k resistor in series with the MV wiper restores the low end below 3-4 on the master. I find this far more effective than the standard "depth" mod to restore lows and it only takes one resistor.

Whoa - any potential drawbacks to this mod? Will it reduce the max volume potential of the amp if it's cranked? Should this be done to both masters (on the dual channel amps)?

With a 1M master and a 100k series resistance you lose 10% of the signal at full volume but by that time your power amp is well into clipping. Its all relative anyway like knobs that go to 11. if you need that extra 10% to push you over the cliff then you probably don't set your master to 2. JCA amps only go to 9 anyways If you have a 500k master try a 47k. I have used as high as 220k on some amps. If you routinely run the master above 5 this mod will not be noticeable. You only need to install one resistor at the PA input in series with the input cap for dual masters.

This is really cool. Definitely on my list of things to try next time I open the chassis.

I could use a serious hand holding on this fizz fix. Confident in soldering, confident in following directions and photo instruction recipes and such, heck I can even reason how and why this is going to work - but have no idea where the PA input (power amp input?) and input cap are looking at the schematics.

So my question is this, What resistor is needed?
What Watt? (3W, 1/2W, 1/4W)
1% or is 5% OK?

Can you help out?

thanks for your time!

B

[JerEvil]

Custom 22 H clip from last nights rehearsal. Crunch channel set clean'ish and Mojo Hand Sacred Cow (Klon type) for drive. PRS SC245.

[video]https://www.youtube.com/watch?v=mWWl675b1Gk&feature=youtu.be[/video]

[theDogger]

Need some help troubleshooting the noise issue on the JCA 22H

So I just bought this off Reverb and everything looks great. I checked the Bias and it was pretty cold (12ma), I set it to 22ma.(343 plate volts) Has the what looks like the OEM Tubes.

I fired it up and it plays great and everything works but seemed to have more noise that it should. So while trying to locate what might be the cause I unplugged the guitar and I get the sound you hear in the short video clip.

When the master Volume is turned up the noise gets louder.
http://www.bigfootdesigns.com/jca22H.mp4
[video]http://www.bigfootdesigns.com/jca22H.mp4[/video]

Tubes maybe? I am going to open it back up tonight and move tubes around and see if that make any type of difference.

Any thoughts?

B

[Hipster Salad]

It could be the heater wires not being elevated. I know some guys mod the 50H/100H to elevate them and eliminates a lot of the noise. Not sure if this is relevant to the 20 watters though.

[theDogger]

Ya something to look at. I saw the post about the heater re-wiring in the JCA50H, I am opening it up tonight to do the Depth Mod and add a Choke.

Any thoughts on the POT value. Everything that I have see talks about a 1 meg pot I would think that a 500k would give you more precise control.

thanks

[theDogger]

Noise issue resolved

More details of the issue.

So with the guitar or no guitar plugged into the input off the amp there was a horrible buzzing noise that got louder when either channel pre-amp was increased and a little increase when the master volumes were turned up.

Before:
http://www.bigfootdesigns.com/jca22H.mp4
[video]http://www.bigfootdesigns.com/jca22H.mp4[/video]

I started troubleshoot by plugging the guitar into the return and the noise went completely away. SILENT!

Not sure why, but I am assuming that this eliminates the pre-amp section.

So I started at V5, pulled the tube inspected it and re-seated the tube and the noise decreased to about 10% of what it was...

So new tubes are on the way from EuroTubes.

Any thoughts as to why this was???? I am really trying to learn more about how Tube amps work, I enjoy modding the amps but would really like more of an understanding

[theDogger]

Now that I have the noise issue resolve I am wanting to do the depth mod. Which one of these is right? I googled the Depth mods and find several different references.

If both are right what is the difference in the out put?

NFB wire cut and attached to the first post.
Cap attached to the 1st and 2nd post the other end of the NFB wire to post 2?
No jumper wire to 3rd post.


Jumper wire from 2nd post 3rd post then to NFB wire that is cut?


Also what difference would the 500k pot have over the 1 meg?

[mbranford]

As I understand it, you should use the 1 Meg pot, as it is standard in amp design to use it for bass frequencies. Soldano uses the 1 meg pot on his depth control in other amps. However, old posts so that people were happy with the results using a 500k pot.

As to jumping the 2nd and 3rd lugs, the German amp tech, (Edge11 called him Dues/VH4) did it that way, so that is the way I did it. It is one of the first posts on Edge 11's JCA20 page.

http://www.harmonycentral.com/forum/for ... 8/1376468-

[Tortuga]

Check page 1 of this thread for depth mod link(s) in the references.

[theDogger]

Thanks for the quick replies...I have checked both of the those post and that is what brought me to post the questions..

So I am really looking for more of why behind the how.

So what does jumping the 2nd to the 3rd do ans is it needed?

What difference in sound would you expect between the 500k and 1 meg pot?

I have both pots and two. 715P Orange Drop Capacitor .0047 µF.

Just concerned, I don't want to add to much bass and muddle up the low-end

thanks

[mbranford]

Just did a quick check: 1st lug is input, 2nd is output and, 3rd is ground. In a guitar, the 3rd is bent to the back of the pot and star soldered to the ground. So probably could be left alone or grounded; probably not the best to jump it. As for the pot, the 1 meg variable resistance is better for bass. The presence control, also based on a NFB loop is for treble, so it uses a 25k pot.

[theDogger]

Ok so makes sense....so this image is wrong then?
it jumped from the output to the ground.



So it should be wire this why then?????



thanks again guys

[theDogger]

OK did the two mods tonight and not that happy with them. I did the depth mod and a choke.

The bass now is way too pronounced (boomy) and heavy or if I go the other way it gets very thin sounding. I am playing through a Ibanez MTM1 w/ Ducan Mick Thomson Blackouts standard tuning no effects straight in.

I wired it like this. but used a 500k pot. Will the 1 meg make a difference?



One would assume that adding a choke would not contribute to the bass getting boomy.

The JCA22H sounded better before the mods
thoughts?

[mbranford]

Did you remember to move the lead of the NFB wire from the depth pot from the 16 ohm jack to the 8 ohm jack? Originally, the NFB wire is on the 16ohm jack in the stock amp, and you need to resolder it to the 8 ohm to finish the mod.

[theDogger]

Did you remember to move the lead of the NFB wire from the depth pot from the 16 ohm jack to the 8 ohm jack? Originally, the NFB wire is on the 16ohm jack in the stock amp, and you need to resolder it to the 8 ohm to finish the mod.

OK then here is how I hooked it up not finding a clear image to under stand.

I cut the NFB Blue wire and attached it to the input of the CTS pot.
Jumped Input to the Output with the .0047U
Jumped the Output to the Ground with wire.
From the Ground connected the other end of the NFB

You are suggesting that the output is suppose to connect the NFB wire which is on the 16ohm jack in the stock amp, and need to resolder it to the 8 ohm to finish the mod?



Can anyone take a pic of their depth mod and post it. (picture is worth a thousand words!) Most all pics in forums are no longer available.

thanks

[Tortuga]

Like I stated above, there's a link to at least one Depth Mod article in the references I posted on page 1 of this thread.

I'll go one further... here's the link (http://www.guitarampboard.com/forum/viewtopic.php?f=53&t=19992)to the specific post I made about the JCA depth mod, including photos of everything I did for the mod, so when someone came along asking me about this a year or two later, and I'd completely forgotten about it (which is exactly what has happened), they could easily go there and find the link.

You can see all the wiring and the Pot / caps I used there. Best advise is to use what everyone recommends for values. If it calls for 1M and you're using 500K, you're on your own.

Sorry if this is coming across harsh - you really should read through the entire thread. I put a lot of time into making that references section with all the links.

[theDogger]

Sorry if this is coming across harsh - you really should read through the entire thread. I put a lot of time into making that references section with all the links.

Don't apologize! I am not a Hillary supported so I won't cry!

*(The 500k pot was suggested by Brian at BFG when I spoke with him on the phone. Said he prefers it over the 1meg pod and that the .1uf is not really needed as the noise is not that noticeable)


I have read all of that, and thanks for taking the time for the referencing all of the links, that is a lot of work, I have read through and still some what confused, and the pics are from a JCA 20H not a JCA22H, then the post goes off into some other mod. So please don't take offense in what I am going to post.

What I am asking for a GUT shot of a JCA22H (like I posted) that has the mod done so I can look at it and compare. Schematics are OK but I don't read them that well.

(only help here is the values of the parts)


This is a 20H...a lot different as you know from the 22H


I have looked though other forums but all pics are no longer available.

So going back to my original question? Is this setup correctly??
Where is the impedance switch or speaker +? Where is this located on the image that I have posted below? Is this where the NFB wire is originally connected on the 2nd pcb board marked NFB? I circled it in the pic in Yellow?


thanks

[Hipster Salad]

I'm pretty sure it's the same as the Custom 22, so here's a shot of mine (I had to re-solder them for the new OT I put in last week so I took this for reference)

[Hipster Salad]

Also, I see indicators of 8, C, and 16. Should be self explanatory

(8 ohm tap - Yellow, Common ground - Black, 16 ohm tap - Violet/Purple)

[theDogger]

Ok thanks for trying to help but I see no Depth mod in the pic. Other than the wires from the OT that you soldered on top

I get the FB - GND
8 (ohm) -(C)ommon- 16 (ohm)



What does this mean in this pic??
Connect to impedance switch or speaker?
Where is this and what is it? Is the impedance switch the second part of the NFB wire that remained connected to the secondary PCB board marked FB?
or
What is being referenced by as speaker and where to connect it?



So based on my pic that I photochopped to represent what I did is it right? Other than using a 500k pot?
The way I did the mod:
1 - drilled the pot hole and mounted the pot
2 - cut the NFB(blue wire) coming off the main pcb board
3 - attached the NFB (blue wire) coming off the main pcb board to the input terminal of the pot
4 - jumped the .047uf across the input to the output terminal on the pot
5 - jumped a wire across the output terminal to the ground terminal on the pot
6 - connected the NFB (blue) wire from the smaller pcs board labeled FB to the ground terminal on the pot.

So I photochopped a 22h gut shot to represent what I did. The question is simple....Is this right or is it wrong?

What I am questioning is from the pot ground terminal back to the second PCB board does it connect to the (Blue) FB wire that I cut.

Where does it connect? On the second PCB board at FB or some where else?



Sorry for the trouble but still confused
I will take actual pics when I get home and see if that helps, I did not think that this was going to be this hard!

thanks

[Hipster Salad]

Looks like your drawing is correct on wiring the Depth mod in. I've done the depth on the 20H and not my Custom 22 (yet), so the board I worked on was a bit different. Looks right, though.

Great thing, if it isn't, it won't blow up your amp and you'll know right away if it doesn't work, if that's at all comforting haha

[theDogger]

Looks like your drawing is correct on wiring the Depth mod in. I've done the depth on the 20H and not my Custom 22 (yet), so the board I worked on was a bit different. Looks right, though.

Great thing, if it isn't, it won't blow up your amp and you'll know right away if it doesn't work, if that's at all comforting haha

Ok thanks that what I was looking for that was not so hard! So now I can go home tonight and swap the 500k pot for a 1 meg and see if the bass is still really boomy when turned up and thin when turned way down.

I will also swap to two V30 speakers and remove the WGS Retro and ET65 before to see if that helps.


thanks again...now on to adding a channel switch

[clipless bumper]

Are you using a linear taper, or audio taper pot?

Which is recommended?

[theDogger]

As far as I know I ordered audio taper cts pots.

As I said earlier in a post Brian from BFG mods suggested using the 500 K instead of the 1 Meg CTS audio taper pod as it would give a more granular or more control over the adjustment.

I am going to swap out the 500 k for the 1 meg tonight or tomorrow and see what the difference is and I'll post