JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Support for JLM Audio Kits

Moderator: Joe Malone

Post Reply
User avatar
Joe Malone
Site Admin
Posts: 2074
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by Joe Malone »

GAP 73, CL 7602 Mk1 and Mk2

These 3 products use the same Chinese input and output transformers.

Step by step mod info ond photo's showing how to change input transformer to OEP/VTX type and output transformer to JLM111DC to give these pres some real fat Neve like Mojo.

If you order the OEP/VTX input transformer and JLM111DC output transformer from us mention in the comments box at checkout “CL7602 mod” and we will add in the 10 x 22uF 16v tant caps for free to the order.
Order transformers here

Overview of changes for CL7602 Mk2.
(Mk1 is the same except mic input transformer mounted on centre metal support panel with direct wiring no pcb and output transformer mounted on left hand side)
Image

Find Mic Input transformer PCB
Image

Remove Mic Input transformer PCB and remove transformer from it.
Replace with OEP/VTX with primary on same side a original transformer.
Image

Cut 3 tracks as shown in red and fit 4 jumper wires shown in yellow.
Refit Mic input transformer PCB and retest CL7602 is working.
Image

Remove card and change 22uF 35v caps to 22uF 25v tant caps.
(The very latest Mk2 versions should have tants already on this PCB)
Refit Preamp & eq amps PCB and retest CL7602 is working.
Image

Remove card and change 10uF 63v and 22uF 35v caps to 22uF 25v tants.
(The very latest Mk2 versions should have tants already on this PCB)
Drill hole in bottom of case under trim pot to allow adjustment when card is refitted.
Refit Output amp & Preamp PCB and retest CL7602 is working.
Image

Find output transformer
Image

Mark wires going to output transformer as shown and remove original transformer.
Fit JLM111DC transformer and reconnect wires to number shown.
(cap and resistor on original transformer not needed for JLM111DC)
Power up and retest CL7602 is working.
Connect meter across JLM111DC +/-1 terminals and adjust trim pot for 1.5v. Enjoy :-)
Image
Joe :-)
JLM Audio
Capturing Audio without Injury

ciscoheat
Posts: 1
Joined: Tue Mar 22, 2011 7:30 pm

Re: JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by ciscoheat »

sorry, I did not understand how to connect the wires to the output transformer, thank you!

User avatar
Joe Malone
Site Admin
Posts: 2074
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Re: JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by Joe Malone »

ciscoheat wrote:sorry, I did not understand how to connect the wires to the output transformer, thank you!
The numbers shown on the above photo are the numbers on the JLM111DC transformer that the wire needs to be moved too so the old transformer can be removed. So best to add paper tape on the wires to write the numbers on all wires and remove the old transformer. Then wire the wires to the new JLM111DC transformer.
Joe :-)
JLM Audio
Capturing Audio without Injury

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Re: JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by chrisp »

Joe Malone wrote:give these pres some real fat Neve like Mojo
... or you might instead just build yourself some BA's for fat AND silky
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

User avatar
Joe Malone
Site Admin
Posts: 2074
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Re: JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by Joe Malone »

chrisp wrote:
Joe Malone wrote:give these pres some real fat Neve like Mojo
... or you might instead just build yourself some BA's for fat AND silky
So true ;-). Just trying to save this gear from being used as door stops or dust collecting rack filler.
Joe :-)
JLM Audio
Capturing Audio without Injury

maosante
Posts: 1
Joined: Sat Mar 23, 2024 2:07 am

Re: JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by maosante »

I have worked on this mod with the GAP Pre 73 MK3 only with the output JLM111DC transformer. At the input I have the carnhill vtb9045m.

For a very long time I used 1.5v at the primaries, but after a while I found the sound was a bit muffled, comparing it to IC preamps. So I looked for alternative voltage in forums , how to calculate the voltage/amps or any other way to get a better coloration but there’s not really much info about this. So I found myself trying step by step from 0.3v to 1.5v at the primaries finding the sweet spot at 1.415v

With this voltage it just sounds right, similar to the neve color. I don’t know if it is the correct voltage, yet I found it’s the most pleasant to my ears.

User avatar
Joe Malone
Site Admin
Posts: 2074
Joined: Wed Jan 24, 2007 11:35 pm
Location: Brisbane, Australia
Contact:

Re: JLM Transformer mods for GAP 73, CL 7602 Mk1 and Mk2

Post by Joe Malone »

maosante wrote:
Sun Mar 24, 2024 8:56 am
I have worked on this mod with the GAP Pre 73 MK3 only with the output JLM111DC transformer. At the input I have the carnhill vtb9045m.

For a very long time I used 1.5v at the primaries, but after a while I found the sound was a bit muffled, comparing it to IC preamps. So I looked for alternative voltage in forums , how to calculate the voltage/amps or any other way to get a better coloration but there’s not really much info about this. So I found myself trying step by step from 0.3v to 1.5v at the primaries finding the sweet spot at 1.415v

With this voltage it just sounds right, similar to the neve color. I don’t know if it is the correct voltage, yet I found it’s the most pleasant to my ears.
The correct 120mA to 130mA bias can be set by measuring 1.3vdc to 1.4vdc across the 12R 1Watt resistor feeding a neve output circuit. You have done very similar by measuring across the transformer primary which happens to be close to the same resistance.
Joe :-)
JLM Audio
Capturing Audio without Injury

Post Reply