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

[Jazz P Bass]

Quote:"About the harshness he says: "The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic"

But it doesn't tell what type of capacitor, and if it goes in parallel or series."
ACROSS implies a parallel connection.
Any capacitor type will do. (ceramic disc is fine)

[spunko]

"Cut out the preamp gain pot and get a new 1M log one.

From middle of pot (reconnect left/right pins to the board, flipped because the pins are now upside down) run into a switch (on/on).
One side back to the middle of the pins on the PCB (the original channel)
The other side to R36. This way you create a cleaner channel with just two gain stages"

I'm lost with these instructions. Do I have to install the pot backwards? The left lug to the rigth pcb hole, and rigth lug to left pcb hole? I don't understand what he means with "flipped because the pins are now upside down"

Can I use the original pot? it says A1M, I guess is audio taper or logarithmic as well.

[vintagekiki]

I'm lost with these instructions. Do I have to install the pot backwards? The left lug to the rigth pcb hole, and rigth lug to left pcb hole? I don't understand what he means with "flipped because the pins are now upside down"

Your English is good enough to understand you, and here is friendly translate.google.com.

Middle pin on the potentiometer is always a slider.
If the potentiometer is not foreseen for the PCB, when wiring potentiometer, end pins are cross when with the wire connect to the PCB.

http://sound.whsites.net/pots.htm
Beginners' Guide to Potentiometers

[spunko]

[COLOR="#000000"]If the potentiometer is not foreseen for the PCB, when wiring potentiometer, end pins are cross when with the wire connect to the PCB.

Oohh ok, I get it. Thank you very much.

what I don't get now is why do I have to get a new pot?
He said to use a Log pot, and the original is marked as A1M, which stands for Audio taper or log taper, that's what I've read; and B1M stands for linear taper. Is this correct?

[vintagekiki]

... which stands for Audio taper or log taper ...

There are 'old' and 'new' standard for marking potentiometers.

http://sound.whsites.net/pots.htm
Read Pot Markings

https://www.vintage-radio.net/forum/showthread.php?t=132235
Markings on Potentiometers ('A,B,C,L,F')

http://www.potentiometers.com/potcomFAQ.cfm?FAQID=29
Potentiometer Frequently Asked Questions

[spunko]

Thank you.
I will get the gain pot out to measure it with my multimeter.
Probably it is a commercial log pot.
I really appreciate the help everyone is giving with this thread, and all the link provided.

=)

[spunko]

This is the way I understood the instructions.

[vintagekiki]

...

Approximately
View Marshall Clean / Crunch circuit, and analyze the signal path of the part schematics V2a / V2b (switch SW5)

https://drtube.com/schematics/marshall/6100-61-04%20iss9.pdf
Clean/Crunch circuit

https://drtube.com/library/schematics/69-marshall-schemas#AN30
Marshall 6100

[spunko]

Approximately
View Marshall Clean / Crunch circuit, and analyze the signal path of the part schematics V2a / V2b (switch SW5)

https://drtube.com/schematics/marshall/6100-61-04%20iss9.pdf
Clean/Crunch circuit

https://drtube.com/library/schematics/69-marshall-schemas#AN30
Marshall 6100

I haven't tried this solution in the amp yet.
I have looked at the marshall circuit, but I don't understand what I need to get this mod right.
I have followed the signal path (I think) and it is basically the same as I have understood, my AC signal comes amplified from the plate of A (1st half) and gets inyected into greed of the second half to get amplified and goes out from the plate of the second half. That would be the normal path.
With the mod, the singal bypasses the second half of the tube and goes to the second tube.
The marshall switch circuit is more complex for my understanding, I don't get why it feeds the signal back to the grid of the second half, if the intention was to bypass that half.

[J M Fahey]

Have you at least tried the simple solution I suggested?

[Dave H]

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

I'd try connecting a resistor in parallel with the gain pot VR6
Gain reduction would be 15dB using 100k, 20dB using 56k.

Both sides of the pot are at ground potential so there shouldn't be any pops when switching the resistor. Tone should be unchanged unlike when switching out the cathode bypass caps.

[Dave H]

Have you at least tried the simple solution I suggested?

Too easy

[vintagekiki]

...

The Clean / Crunch schematics is as an idea of how the signal path is going.
Tube V2b gives gain for crunch.
Schematics for Crunch is your personal choice, it is important is select choice Clean / Crunch. In Clean mode g1 V2b (crunch tube) via SW5a and R24 binds to ground so that V2b does not oscillate.



Path Signal for Clean (Red)
Path Signal for Crunch (Green)
Choice Clean/ Crunch (Blue)

[spunko]

I'm sorry for the late reply, I was out of town.
Thank you for the help. As I understood, it is necessary that 2b goes to ground via a resistor to avoid oscillation.

is this ok?


Also, I'm going to try Fahey solution.

[spunko]

**Edit: I was asking for help in this post, because the amp started to oscillate, even at stock (no mods). Sorry for the double post

It turns out I forgot to clean the flux from the PCB and it got conductive. Just after I asked for help, I cleaned it and the oscilation stoped.

What do you think about the diagram above?