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Choke questions

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3/23/2004 12:52 PM
Matt	Choke questions What is the inductance rating of a choke for a 100W Marshall? What difference (if any) would using a 3 Henry choke make? Would it sound that much different. This is actually for a two-channel (BF clean - Bogner Red), 4xEL34 amp using a "plexi" style OT. TIA!

3/23/2004 1:45 PM

I would use something like 8-10 Henries good for 80-100 ma. A Hammond 157M would be ok.

I'm not sure if a 3 Henry choke would do much. A choke in a power application is there for regulation. It needs a minimum value before it will work. Anything higher is overkill - anything lower and there's no choke action, it's just a resistor with a value based on the DC resistance of the wire.

In the Marshall circuit the choke just feeds the screens rather than the screens and preamp stages as with Fenders. The idea is to provide some screen voltage regulation as the tube starts to work. The screen current rises and the choke starts to work. Too small a choke value and it won't do anything. This definitely affects tone - maybe for the better! Doesn't do much to protect your tubes though...

3/23/2004 1:55 PM
Matt	Thanks Bill. Are you sure about the choke not also supplying the preamp in a Marshall? It doesn't look that much different than a Fender in that regard but maybe I'm missing something.

3/23/2004 3:16 PM
Bradster2k	5 Henries for 1471-263 used in the 2203, no idea about the new stuff, and the older stuff like the 1959SLP was different I'm told.

3/23/2004 3:22 PM
Bradster2k	BTW the choke in the classic Marshall circuits like JTM45, 1959SLP, 1987, 2203, 2204, actually does feed everything except the power tube's plates which are feed B+ via the center tap of the OT.

3/23/2004 7:16 PM
Bruce /Mission Amps	You mean you haven't noticed any dropping resistors after the choke and filter cap node? Every voltage drop acts like a new power supply node designed to act as a new filter section and distribution point that is "decoupled" from the others. Bruce

3/23/2004 7:55 PM

I don't follow you Bruce (not unusual for me to be the last in the room to 'get' the joke).

After a little research, it appears the inductance rating on the Marshall chokes varied from as low as 3H (early JCM era?) up to maybe as high as 20H.

I piddled around with Duncan's Power Supply Calculator and it appears to be yet another balancing act between L and DCR of the choke and the C of the node. But I still don't know how any of this 'sounds'. With high L and low DCR, it appears there are some voltage 'spikes' or overshoot so then you require a lot of C, but then it looks like there is a longer recovery time. With high L and high DCR, everything looks pretty good but Randall Aiken has an article that says too high of a DCR is not desirable because the regulation isn't as good (and you lose some voltage downstream).

Mmmmkay. But what's it sound like? I'm wondering if all this choke business really starts to matter (tonally) when you push the amp hard.

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