Marshall JTM 60 choke mod

[torcamaniac]

Hi,
ordered a choke (Hammond 159P at 10Henry 125mA 155Ohm 500V) and will install it in place of 270Ohm dropping resistor in my JTM60.
Also beacuse of possible voltage spikes because of lower chokes resistance, I'll replace each filter cap (47uF 550V) with two 100uF 350V caps in series. That way I'll get 50uF at 700V. Hopefully enough. I heard the spikes could go up to 680 Volt.
Than I'll parallel each cap with a 470kOhm equalizing resistor.
I hope this will work well.
I know many amps don't need a choke and it doesn't make much difference soundwise but the JTM60 supposedly do and benefit from it. Especially at quieting the slight buzz down these amps have...

Should be something like this:
Sorry about hand drawing...

[Chuck H]

Well that looks fine. And you may like the tone I wouldn't count on reduced buzz though. I don't see where changing to a choke would alter that. As far as voltage spikes, I don't see that changing to a choke should increase that either. In fact I would expect it to reduce voltage spikes because the choke acts sort of like an equalizer in that regard compared to a resistor. That, and the resistor is only 270 ohms. The choke itself has a DCR of 155 ohms which isn't much smaller at these low values in this circuit. And the choke has an impedance that should be much greater than the resistor. That's should actually squelch spikes. But I've never bench tested these particulars.

[torcamaniac]

Well that looks fine. And you may like the tone I wouldn't count on reduced buzz though. I don't see where changing to a choke would alter that. As far as voltage spikes, I don't see that changing to a choke should increase that either. In fact I would expect it to reduce voltage spikes because the choke acts sort of like an equalizer in that regard compared to a resistor. That, and the resistor is only 270 ohms. The choke itself has a DCR of 155 ohms which isn't much smaller at these low values in this circuit. And the choke has an impedance that should be much greater than the resistor. That's should actually squelch spikes. But I've never bench tested these particulars.

Dunno but take a look here

The Valve Wizard

Maybe I'll just try it with only a choke first...

[Chuck H]

Ah, Yes. I hadn't considered the resonant aspect. I was considering voltage "spikes" and inrush voltages when the caps initially charge. With the cap values you have you'll be fine. Changing to the totem arrangement won't buy you anything here because the decoupling frequency won't change significantly between 47u and 50u and you already have 550V rated caps. That should be sufficient to handle any inrush and the peaks you wish to mitigate occur when the amp is operated at loaded voltages when the HV supply is pulled down lower by current. So yeah, just try the choke first. But don't expect an apples/oranges difference. More like a Gala/Fuji difference (two different kinds of apples)

[torcamaniac]

Thanks Chuck...
The biggest problem will be finding a spot inside the chassis to place the choke... The amp is a mess inside...

[Chuck H]

Well you do want the choke on the OUTSIDE because it's an enormous inductor operating under significant current and as such has a generous EMF. If you put it inside there's a good chance it could impress hum onto the signal path due to proximity. But you can certainly test it without cutting any holes and decide if mounting it is worth the effort.

[torcamaniac]

Well you do want the choke on the OUTSIDE because it's an enormous inductor operating under significant current and as such has a generous EMF. If you put it inside there's a good chance it could impress hum onto the signal path due to proximity. But you can certainly test it without cutting any holes and decide if mounting it is worth the effort.

I guess I have to try. There is very few spots if any to mount it outside. Take a look at the back and how stupid of a design is it to put the tubes like this.
Maybe there would be enough space outside between the preamp tubes and PI and if there is would that mean I'd have to shield the tubes?
Other thing to consider would be the tube cage maybe...
These are not pics of my JTM just some random ones I found on the net...

[Enzo]

Why not tack solder it into the circuit, on longer wires and just screw it to the wall in the back. Then see if it makes any real difference to the sound. if it does, THEn worry where to put it. if it makes no discernable difference, then just don't install it, and return the resistor.

[torcamaniac]

Thanks Enzo,
that's what I'll do first anyway.
I'll keep you guys posted as soon as I have time to try it...
And if its good and done I'll post some pics.

[torcamaniac]

Hey guys,

just tried the choke and didn't like it or better said, I didn't hear any difference. The amp wasn't quieter and sonicaly I didn't notice that the sound is softer or whatever as some say...
I had to mount the choke on the cage underneath the tubes. Anyway I've put the resistor back now...
And as promised some pics:

[Chuck H]

You can't know if you don't try! Save the part for a different amp or build perhaps.

[torcamaniac]

You can't know if you don't try! Save the part for a different amp or build perhaps.

Yeah, I'll save it...
I do plan to make an amp from scratch in future.
Probably based on Bassman but with 2 or 3 channels... Probably lower wattage also... 18 - 30 Watts
We'll see...
I was even thinking to do it the "primitive" way. No PCB, no turrets.
Idea was a wooden plate with nails as connection points...
The way old radio techs did a lot of stuff especially when prototyping...
Or maybe the basic breadboard with holes and solder terminals...