switching plexi between shared and split cathode on V1A/V1B

[jzucker]

What's the best switch configuration to be able to go from a JTM-45 style 250uF/820 shared cathode to .68uf/2.7k & .68 on V1?

[Mickey_C]

A SPDT switch will work, if that's all you're doing. Disconnect the jumper on V1 pin 8 to pin 3. Connect the SPDT common to pin 8 of V1. Run a wire from V3 to one side of the switch (leaving V3 connected to the existing RkCk), and run a wire from the other side of the switch to your new 2k7/680nF bypass res and cap. Flipping the switch now splits the cathodes. I highly recommend tightly twisting all those wires from the tube socket and new Rk/Ck to the switch.

If you wanna be uber-plexi, use a DPDT, and connect the other side common pole to the 820R V2a cathode bypass resistor, and on the same end of the plexi side of the switch, connect the switched pole to a 250uF - 330uF cap. Twist those wires as well, but keep the twisty pair separate from the other three.

Now when you flip it, you are splitting V1 and also adding the bypass cap to V2a!

[jzucker]

Thanks Mickey,

You have a diagram of that for amp-dummies like myself? The way the switch is currently wired (DPDT) channel 2 doesn't work in plexi mode!

A SPDT switch will work, if that's all you're doing. Disconnect the jumper on V1 pin 8 to pin 3. Connect the SPDT common to pin 8 of V1. Run a wire from V3 to one side of the switch (leaving V3 connected to the existing RkCk), and run a wire from the other side of the switch to your new 2k7/680nF bypass res and cap. Flipping the switch now splits the cathodes. I highly recommend tightly twisting all those wires from the tube socket and new Rk/Ck to the switch.

If you wanna be uber-plexi, use a DPDT, and connect the other side common pole to the 820R V2a cathode bypass resistor, and on the same end of the plexi side of the switch, connect the switched pole to a 250uF - 330uF cap. Twist those wires as well, but keep the twisty pair separate from the other three.

Now when you flip it, you are splitting V1 and also adding the bypass cap to V2a!

[Mickey_C]

There might be on the jtm45 yahoo group.

If there's not, I can pull one together for you. You really want to switch the NFB too... as it will be too high for a plexi (meaning less gain) if you want to do this right. If you do all three, you will have a decent plexi sound. A switch for a solid state rectifier would help too.

Good luck!

[jzucker]

There might be on the jtm45 yahoo group.

If there's not, I can pull one together for you. You really want to switch the NFB too... as it will be too high for a plexi (meaning less gain) if you want to do this right. If you do all three, you will have a decent plexi sound. A switch for a solid state rectifier would help too.

Good luck!

Thanks. I think what I really want to do is forget about the switch and just wire the split cathode, switch to the 47k/8ohm tap for NFB and bag the tube/SS rectifier switch since the way it's implemented, I'd have to re-bias when switching that. It's just that I have 3 switches in the panel now that would be doing nothing and if I ever sold it it'd be an eye-sore. I believe the amp uses a weber chassis so I suppose I could buy new panels but I couldn't find the panels on weber's site. I know they're there somewhere.

[thousandshirts]

Thanks Mickey,

You have a diagram of that for amp-dummies like myself? The way the switch is currently wired (DPDT) channel 2 doesn't work in plexi mode!

Hi. First of all, love the sheets of sound videos. Always enjoyable. I started catching them last year when you posted links at TGP, and it's high time I checked back in again.

As for your question:

Getting a good split/shared switch in a Marshall type circuit is covered quite nicely, with pictures and diagrams, at the MetroAmp forum.

http://forum.metroamp.com/viewtopic.php?t=14442
http://forum.metroamp.com/viewtopic.php?t=832
http://forum.metroamp.com/viewtopic.php?t=11966
http://forum.metroamp.com/viewtopic.php?t=1405
http://forum.metroamp.com/viewtopic.php?t=12039
http://forum.metroamp.com/viewtopic.php?t=13997

The popular thing is to use a push/pull pot setup. Have a closer look! IIRC someone there also made a footswitch.

[jzucker]

Thanks! You guys are great and very helpful!

[islandamp]

I used a DPDT switch and put a 150 ohm 10watt resistor on the unused half of the switch to emulate the sag of a tube rectifier. Works great, but I eventually went to full time Jtm mode due to the Plexi being to bright for my Tele. Good luck

[GainMaster]

What's the best switch configuration to be able to go from a JTM-45 style 250uF/820 shared cathode to .68uf/2.7k & .68 on V1?

http://www.ax84.com/october/AX84_Octobe ... 080601.pdf

Check out this schematic for a switchable shared cathode. There is even a second schematic with a series/paralell switch. I have both on my October.

Brian