Jet city JCA20H mod - can't find schematics or instructions

[spunko]

Hello,

I have searched the forum and found some threads but the attachemts won't open.

Also I have been looking for any info of this mod all over the web, but all the threads are really old, the fles won't open, or the links are down.

Maybe someone has kept the schematics or info about it, and could share it.

Thank you in advance.


(sorry for the bad english)

[Jazz P Bass]

What specific 'mod' are you looking for?

Try the HC Jet City JCA20H page: https://www.harmonycentral.com/forum...la-28/1376468-

[spunko]

What specific 'mod' are you looking for?

Try the HC Jet City JCA20H page: https://www.harmonycentral.com/forum...la-28/1376468-

Thank you, already been there. All the links and pics are down. No pdf files.

I want to make the amp cleaner

[J M Fahey]

OK.
Mods in general do not make amps "cleaner", just lower gain.
Maybe they clip some cathode capacitor off, replace 12AX7 with lower gain tubes, etc.
So after one maybe you can set amp to 7 before clipping, while before it started at 4 ... not a real change; amp puts out, measured by ear, exactly as before, you could simply have turned volume down a little.

If when , say, playing along a drummer (which are NOISY) you had good distortion but not clean sound, no matter what you do that wonīt change either, youīll still have around 15/16W RMS.

If you want bto improve real headroom, as in acoustic headroom, not just "a number on a knob"; I suggest you use more sensitive/efficient speakers.
That is a real change.

[Pedro Vecino]

Look here: http://www.pedrovecino.com/JCA20H.pdf

[alexradium]

with gain on 2 and master on 5/6 the amp is clean,if its too dark just add bright cap on the 1M before the gain pot,for less gain put a 100k in parallel of gain pot.

[Jazz P Bass]

Mike Soldano did a pretty decent design on this amplifier.

Unfortunately, the Chinese manufacturers got a hold of it.

I had one of these in for a signal dropping out issue.
It was the head version.
Turned out to be a crappy 1 uf bypass film capacitor on the first stage cathode that was shorting.

I was not impressed with the soldering quality at all.

The clean channel starts to get rounded at a gain setting of 3 (100 mv input).
The dirty channel was over the top gnarly in a good way.

All in all, with a little TLC, I found it to be a nice amp.
With a good speaker.

[spunko]

Mine has too much HISS, even with the gain pot at 4 and volumen at 7.

Changed the chinese tubes with tunsols in pamp and EHX in pre. It is the same.

I haven't tryed a 12aY7, the only one I have is 12aT7 but I read that is a kinda sterile, used on reverb circuits. is that correct?

[nickb]

***Yikes! That schematic is full of errors Suggest you look at what you actually have and feed back a corrected schematic.
**Scrub that. It's just sheet one that is misleading.

Turn vol to zero - how is the noise?
Vol at max, pre gain at zero. How is the noise?
Raise the pre gain. How is it now?

[J M Fahey]

I read that is a kinda sterile, used on reverb circuits. is that correct?

Its basically like 12AX7, somewhat less gain, somewhat higher current (so itīs preferred in Reverb drivers and Phase Inverters), "sterile" is Marketing Dept. babble (just not to call it BS)

Please do the tests suggested by nickb and post results.

But in general Mods are basically not much more than gain adjustments; if yopu post a trusted schematic we might suggest a couple .... generally not needing much more than a resistor or two.

Way more flexible and powerful than tube rolling and definitely cheaper, in a good way.

[spunko]

***Yikes! That schematic is full of errors Suggest you look at what you actually have and feed back a corrected schematic.
**Scrub that. It's just sheet one that is misleading.

Turn vol to zero - how is the noise?
Vol at max, pre gain at zero. How is the noise?
Raise the pre gain. How is it now?

vol 0 gain 0 - no noise
vol 10 gain 0 - no hiss, just normal noise
vol 10 gain 4 - a little hiss, but lots of gain
vol 10 gain 5 - lots of his, lots of gain

Look here: http://www.pedrovecino.com/JCA20H.pdf

Pedro uploaded the schematics, thank you.

How can I mod it to clean the sound, or lower the gain?

[J M Fahey]

vol 0 gain 0 - no noise
vol 10 gain 0 - no hiss, just normal noise
vol 10 gain 4 - a little hiss, but lots of gain
vol 10 gain 5 - lots of his, lots of gain

Thanks, thats about what I expected, meaning amp is not broken, just high gain.

How can I mod it to clean the sound, or lower the gain?

Start by adding 150k in parallel with R33 (330k).
Itīs about 10dB attenuation, I guess that should be enough.

If you want more, add 39k instead, that should be about 20dB attenuation.

If still too much, then simply a Soldano is not for you, trade it for a small Fender tube one or a Roland SS (maybe a Katana)

Please do the tests and comment results.

Hint: you may add a small toggle switch, (like those phase/split ones on Guitars) to the front panel, to switch the attenuator in-out at will.

[spunko]

Thanks, thats about what I expected, meaning amp is not broken, just high gain.



Start by adding 150k in parallel with R33 (330k).
Itīs about 10dB attenuation, I guess that should be enough.

If you want more, add 39k instead, that should be about 20dB attenuation.

If still too much, then simply a Soldano is not for you, trade it for a small Fender tube one or a Roland SS (maybe a Katana)

Please do the tests and comment results.

Hint: you may add a small toggle switch, (like those phase/split ones on Guitars) to the front panel, to switch the attenuator in-out at will.

Thank you!! I made the changes you suggested and also something from another forum.
Tha amp will stay like this.

- R28 lower 220k to 100k
Lifted C27
Lifted C15
R33 330k in parallel with 150k

I want it to be switchable, and I noticed that the three changes in bold go to ground, so that can be switchable.
The problem is that R28 doesn't go to ground, so it can't be in the same switch, at least as I imagine. Position one everything lifted, position two everything grounded, with a dpdt on/on switch.

Is there any way to make this with one swicth?

[g1]

Adding another 220K in parallel with R28 will make it 110K.
Use a DPDT switch, one side as you described, the other side add 220K parallel with R28.

[J M Fahey]

You can have 3 attenuation settings with a single switch and no high voltage involved

You need an SPDT Center OFF switch.



it means (as seen in this picture):
* toggle has 3 positions: left, center and right.
* when toogle is at left, center and right legs are shorted
* when toggle is at right, center and left legs are shorted (so far, like on any other switch)
* when toggle is at center, center leg is connected to neither left leg nor right leg (i.e. "center OFF")

Connection:
* 150k resistor to left leg
* 39k resistor to right leg
* free resistor ends twisted and soldered together, go to R33 top end.
* centerb leg goes to ground.

Effect:
* toggle center: unmodded JCA20
* toggle right : 10dB attenuation
* toggle left: 20dB attenuation (same as 10dB attenuation + 3 Mods you already made, which are now unnecessary)

You can try it before fully committing to it.

The beauty of the resistive attenuator is that:
* it can get any attenuation you can get by using other, more complex methods, simply use different resistor values.
In general: smaller resistor value: higher attenuation.
* simple and cheap.

I said "right and left" to match the picture I found online, I guess youīll mount switch so toggle goes Up and Down.