Dig it! Jet City JCA20H Choke Mod

[Modern Saint]

What does a choke do one might ask, chokes help to reduce ripple and improve the stability of the DC voltage. Be careful if you perform this type of task because you are working with high voltages.

So after reading several forum post such as HC and others, I acquired the parts necessary to perform this improvement. This mod will tighten up the DC voltage and will remove any type of inherent 60 cycle noise that maybe prevelant. With the improvement. The supply voltage will be more stable. Both of my Jet City Amps will receive this mod along with a future depth mod as well (parts need to be purchased).

Before I disassembled the amp, I did take voltage reading so I have a comparion of DC and AC ripple voltages using my DVM. This will be my starting point. I will remeasure the voltage again once the choke is installed. That way I have a record for future repairs if necessary.

The Parts
I bought the hardware from my local OSH

• Machine Screws - M4-.7x12MM
• Flat Washers - M4
• Nylon Lock Screws - M4-.7

Philmore Split Washers from my local Electronics Supply
The choke - Hammond 156M - from Mouser Electronics.

Total Cost - about $20-$22

Board Layout
The choke will replace R3 on the board

Parts Removed to Perform Mod

Choke Installed

Close of Installed Choke

Shot of Screws Installed on Board Side

Notice the new M7 nylon lock screws to the left of the board. One is located above the blue and black wires and the other to the left of D2. Notice resistor R3 which is Pink, this is what the choke will replace.

I will be wiring this up tomorrow and then work on installing 1 ohm resistors for easy bias adjustment. Pots are already installed.

 

[Mark Wein]

 

[Chad]

very cool. Those look very easy to mod, the layout is pretty clean.

 

[Modern Saint]

A lot of it is just patience and a plan. I am fickle and do not like to rush things so I will measure twice and cut once. I was looking at the board last night thinking of where to add the 1 ohm resistors to make bias adjustment easier and just figured it out. The same goes for the depth mod that I will add after I aquire a couple of parts. With the depth mod, I want to be able to remove the mod if I ever sell the unit. As for the choke, it improves the amps performance and should also improve the bottom end because of the improved voltage filtering.

For those who care to know more.

http://www.aikenamps.com/Chokes.html

 

[Help!I'maRock!]

i have no idea what you replaced. but good job.

 

[Modern Saint]

i have no idea what you replaced. but good job.

lmbo.....

The Hammond Choke was added.

 

[Help!I'maRock!]

lmbo.....

The Hammond Choke was added.

well, you say it will replace R3 on the board. but i have no idea where R3 is. i can see the new choke installed but i have no reference for what the old one looked like.

 

[Modern Saint]

R3 is the lonely "PINK" resistor.

 

[Prages]

Why do amp companies put bias pots on their amps, and no bias test points?

 

[Modern Saint]

Why do amp companies put bias pots on their amps, and no bias test points?

Great question....

 

[Modern Saint]

Got a chance to work on the JC amp again. Here is what I have done so far.

Board Ready For Mods

Resoldering the solder blobs on the wires.
Typical of cheap labor I guess. Sorry for the not so clear pic but if you can see the wire attached to the board, there are solder blobs - ugh.

R3 Removed and wires are soldered properly now.

Board before PC traces to be cut

PC Traces Cut

One of the 1 Ohm resistors added

All Components Wired Up

Closeup of Mods made

All Assembled and Ready to be Biased

Biasing will be done later today along with taking all of the new voltage measurements for the maintenance record. I will get a chance

I keep hearing about a depth mod that requires two parts - a pot and a cap. I will order the parts this weekend to try it out. I have heard that it helps when playing on low volumes and adds to the sound, the question is what is the effect on stage volumes and when I play at the amps sweet spot.....

 

[ugameus]

I just picked up a JCA20H. It is a pretty sweet little amp. I am definitely going to put a choke in it.

Does anyone have any experience with other noise reduction mods? How about this depth mod I keep reading about?

 

[Modern Saint]

I just picked up a JCA20H. It is a pretty sweet little amp. I am definitely going to put a choke in it.

Does anyone have any experience with other noise reduction mods? How about this depth mod I keep reading about?

I don't know about the noise reduction mod. The amp runs quite quiet with the JJ's I have in them. As for the depth mod, almost did it but from what I read, it is more effective at low volume levels. I like to use the amp master between 6-7 (max 8) and the preamp about 4-5 (max 7) on live gigs. That is the sweet spot of the amp and the need for the depth mod is really null for me.