New JTM45-based Project

[BenTobith]

Let me restate things a bit, so it’s clear what I’m looking for help on:

I’m working on a JTM45-based build, with some modifications to hopefully improve grounding, make it a bit more humbucker-friendly, split cathodes, and so forth. So the end result is obviously not a JTM45 replica.

All that aside, one area that I’m not quite sure about or happy with as-is is the switching, power, and mains/fuse wiring. My starting point is a Metro/Valvestorm chassis, and I’ve been using their instructions for the most part. I put together my own DIYLC layout to help think through the build ahead of time (see below).

I believe it is true that it is best practice (safer?) to wire the Hot/Neutral and fuse differently than how a JTM45 would’ve originally been done (see my layout). But, I’m not sure what that translates to, and don’t want to muck around with this and get it wrong. Based on the layout and components I’m using, could someone offer advice on how to improve this aspect of the build? I’m pretty far along with the build itself, but this is one thing I want to possibly redo or improve before I call it done.

For reference, the PT is from Modulus/Dagnall, the indicator lamp is an amber/neon 120v lamp (e.g. from Valvestorm), the switches are Carling SPST. Since I’m using 120VAC (US), I have to wire the two individual 0v (blue/violet) and 120v (grey/brown) leads from the PT in parallel. Makes for a bit of a rough connection on the Mains switch, due to 3 18 AWG leads on one switch lug, but I digress…

I think I’m going to just disconnect the Standby switch altogether rather than leaving it as-is, adding a resistor to it, or changing the wiring altogether. I’ll leave the switch for “looks,” but that’s it.

But again, would appreciate some guidance on the mains/fuse/IEC wiring, please.


DIYLC File

 

[dan40]

Regarding point 1/2, can you clarify how I would do this, based on my existing layout? EDIT: Could the following changes work?:
1. Move hot/black wire from IEC to "tip" of fuse holder
2. Move neutral/white wire from IEC to "side" of fuse holder, where it connects with the PT 0v common/lamp (?)
3. Add wire from "side" of fuse holder to mains switch (?

Many old layouts from Marshall and Fender show the neutral (white) wire being fused but modern code dictates that the hot wire (black) be fused. With a simple switch like a standard Carling with only two solder lugs, The hot (black) wire enters the chassis and goes straight to the fuse. From the fuse it heads to the power switch where it connects to one of the solder lugs. The other solder lug on the switch would connect to one of your PT primary wires. In your case it would be two because you have to pair them up with your PT. The neutral (white) wire would then connect directly to the other PT primary wire. This completes the power circuit through the transformer when the power switch is closed. The only problem with this is you have no where to firmly attach the neutral wire and PT primary wire so you are usually joined together with solder, heatshrinked and tucked down against the chassis. I like to drill a hole and mount a tag strip near the power switch so that I have a solid point to mount the neutral/primary wire pair. Just be sure to solder them to the ungrounded side terminal and not the center grounded terminal.

Many builders will use a dpdt switch for the power switch so that they have four solder points. You can land the Hot wire/primary wire on one side of the switch and the neutral/primary wire can be soldered to the other two lugs on the switch. This gives you a solid place to mount the neutral wire and you now have the added safety of switching both the hot wire and neutral wire. This arrangement is mandatory in Europe I believe and many new American made amps also use this setup. If you decide to go this route, do not add the jumper between the two sides of the switch because you need the hot and neutral to be isolated from each other.

I hope this made sense and hopefully one of our members could draw up a quick sketch of this arrangement.

 

[BenTobith]

Many old layouts from Marshall and Fender show the neutral (white) wire being fused but modern code dictates that the hot wire (black) be fused. With a simple switch like a standard Carling with only two solder lugs, The hot (black) wire enters the chassis and goes straight to the fuse. From the fuse it heads to the power switch where it connects to one of the solder lugs. The other solder lug on the switch would connect to one of your PT primary wires. In your case it would be two because you have to pair them up with your PT. The neutral (white) wire would then connect directly to the other PT primary wire. This completes the power circuit through the transformer when the power switch is closed. The only problem with this is you have no where to firmly attach the neutral wire and PT primary wire so you are usually joined together with solder, heatshrinked and tucked down against the chassis. I like to drill a hole and mount a tag strip near the power switch so that I have a solid point to mount the neutral/primary wire pair. Just be sure to solder them to the ungrounded side terminal and not the center grounded terminal.

Many builders will use a dpdt switch for the power switch so that they have four solder points. You can land the Hot wire/primary wire on one side of the switch and the neutral/primary wire can be soldered to the other two lugs on the switch. This gives you a solid place to mount the neutral wire and you now have the added safety of switching both the hot wire and neutral wire. This arrangement is mandatory in Europe I believe and many new American made amps also use this setup. If you decide to go this route, do not add the jumper between the two sides of the switch because you need the hot and neutral to be isolated from each other.

I hope this made sense and hopefully one of our members could draw up a quick sketch of this arrangement.

Thanks for the reply.

I was discussing this on the Gear Page, as well. I’m currently planning to do this (which involves replacing the SPST for Mains with a DPST), while omitting Standby altogether:

 

[Lowerleftcoast]

I’m currently planning to do this (which involves replacing the SPST for Mains with a DPST), while omitting Standby altogether

That will work if you add the wire from rectifier pin8 to the reservoir cap.

Here is how I changed the layout for the single pole switch. You can get rid of the standby.

 

[BenTobith]

That will work if you add the wire from rectifier pin8 to the reservoir cap.

Here is how I changed the layout for the single pole switch. You can get rid of the standby.

View attachment 975447

Thanks!

Oh yeah. Forgot about the rectifier to cap can..

 

[Wyatt]

How I would do the mains wiring, but I think it's already what is being discussed.

Use a DPST power switch.

1. HOT to tip of fuse holder, side of fuse holder to one pole of DPST
2. NEUTRAL to other pole of DPST
3. PT and 120v lamp to the switched throws of the DPST

This keeps HOT > fuse > switch, plus switches the NEUTRAL as well, and goes you a spot to tie in the PT and lamp wires to the 120v.

If you want to keep a SPST, then you go HOT to fuse to switch to PT (you would still connect the lamp at the switch), and have to install a terminal strip to connect the NEUTRAL to the PT and lamp.

 

[BenTobith]

How I would do the mains wiring, but I think it's already what is being discussed.

Use a DPST power switch.

1. HOT to tip of fuse holder, side of fuse holder to one Pole of DPST
2. NEUTRAL to other pole of DPST
3. PT and 120v lamp to the switched throws of the DPST

This keeps HOT > fuse > switch, plus switches the NEUTRAL as well, and goes you a spot to tie in the PT and lamp wires to the 120v.

If you want to keep a SPST, then you go HOT to fuse to switch to PT (you would still connect the lamp at the switch), and have to install a terminal strip to connect the NEUTRAL to the PT and lamp.

Thank you!

Based on my updated layout above (using a DPST), does it appear I’m heading in the right direction?

 

[Wyatt]

Thank you!

Based on my updated layout above (using a DPST), does it appear I’m heading in the right direction?

Yes. Looks right.

 

[BenTobith]

Yes. Looks right.

Thanks!

I’m also considering 48uF filtering for mains (16uF PI) vs. 32uF mains + 32uF PI.

Would the correct way to do that be to run a wire from pin 8 of rectifier -> 16uF lug of PI cap can -> first 32uF lug of mains cap can?

 

[Wyatt]

You have to run the power string.

Rectifier > plate/screen filter cap > choke (some voltage drop) > PI filter cap > dropping resistor (lots of voltage drop) > preamp filtering

 

[Lowerleftcoast]

I’m also considering 48uF filtering for mains (16uF PI) vs. 32uF mains + 32uF PI.

Would the correct way to do that be to run a wire from pin 8 of rectifier -> 16uF lug of PI cap can -> first 32uF lug of mains cap can?

That could work.

Why not just use the 32uF/32uF in parallel (64uF) for the reservoir cap, then 16/16/16 for the rest of the filters? Or... add a 32uF cap in parallel with the the reservoir cap and maintain the 32/32/16 for the rest of the filters? There are a lot of options for adding/changing filtering.

 

[BenTobith]

Decided to leave it as 32, 32, 32 (mains, screens, PI). Some good progress. Just waiting on DPST switch to finish redoing the power/fuse wiring.