JTM45 Choke Impedance

[David B]

that's OK I get the general idea but at some point I would like to measure the drop in inductance and see if it corresponds to some sonic difference

[RS DeLuxe]

The Choke isn't just regulating power for the Screen Grids, but most importantly the B+ high voltage that is supplied to the Plates of the vacuum tubes (KT66's, ECC83, etc). Obviously the pair of O/P tubes plates use the most current as they're what produce the final signal to the O/P transformer and then speakers.

[adrian]

The Choke isn't just regulating power for the Screen Grids, but most importantly the B+ high voltage that is supplied to the Plates of the vacuum tubes (KT66's, ECC83, etc). Obviously the pair of O/P tubes plates use the most current as they're what produce the final signal to the O/P transformer and then speakers.

That's correct for some other amps, though not for Marshall's. JTM45's and later Marshall's have the output tube plates connected directly to the first filter cap (obviosly through the output tranny). The choke supplies only output tube screen grid and PI plus preamp currents.
Some of the very earliest prototype Marshall's might or might not run plate current through the choke, but it's hard to tell as most of those pictures on the web are from bogus amps...

[RS DeLuxe]

Yes, I shouldn't have included the KT66 Plates ; the point I was making, is that the Choke doesn't just supply filtering to the Screen Grids as David indicated, but rather is part of the first filter (Pi filter)that cleans up the power headed downstream to the preamp plates, etc.

[David B]

Yes, I shouldn't have included the KT66 Plates ; the point I was making, is that the Choke doesn't just supply filtering to the Screen Grids as David indicated, but rather is part of the first filter (Pi filter)that cleans up the power headed downstream to the preamp plates, etc.

actually I did indicate the preamp section but the way I worded it you could have missed it, probably better to say the choke filters everything being fed past the choke.. sorry man

So if I got this right the choke filters just the screen grids of the output tubes and the preamp section ..

[Brian Wallace]

Had a chance to read a Dagnall choke from a BB reissue.

[David B]

Brian,

Seems most of the C1999 and 352-114 chokes measure in the ballpark of 3H+...
Besides the RS 20H are there any other known/measured variations ?

[Brian Wallace]

For all the ones I have measured, except the RS choke, I have only seen things in the 3H range. I'm sure things could hit 4H with tolerance though. I will set up a thread in the Marshall data base forum showing the pictures of different chokes and their measured values. If anyone would care to send me Marshall chokes to measure that are not pictured that would be very helpful to this study.

[Roe]

the low henry rating seems to be part of the marshall sound, perhaps even an important part. however, many of the chokes used today in plexi clones are 5h, 7h, 8h or even 10h. metro's 352-114 is 5h and their c1999 is 8h. I guess the drakes could have varied from 2 to 4 or even 2 to 5 H. but 8 H seems high for a dagnall that's supposed to be spec'ed at 3. Of course, all of these chokes can sound nice - its a matter of taste. I have the marstran 3H in two amps and a MM 3H in another. however, in my 67 with low filtering I prefered the 5H. (also my 45 has 7h and my 1987 a 7 or 8h)

[Brian Wallace]

Well, all I can say about the Drake 352-114 is that the spec they gave me for this choke was 3H @ 100 ohms. The C1999 I rewound was in this range as well and this is what I offer to keep it period accurate.