JTM45 Choke Impedance

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Re: JTM45 Choke Impedance

Postby David B » Fri Dec 05, 2008 11:07 pm

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
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Re: JTM45 Choke Impedance

Postby RS DeLuxe » Sat Dec 06, 2008 10:01 am

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.
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Re: JTM45 Choke Impedance

Postby adrian » Mon Dec 08, 2008 5:07 pm

RS DeLuxe wrote: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...
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Re: JTM45 Choke Impedance

Postby RS DeLuxe » Mon Dec 08, 2008 6:09 pm

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.
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Re: JTM45 Choke Impedance

Postby David B » Wed Dec 31, 2008 5:44 pm

RS DeLuxe wrote: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



David B wrote:
So if I got this right the choke filters just the screen grids of the output tubes and the preamp section ..
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Sat Jan 30, 2010 9:41 pm

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

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Re: JTM45 Choke Impedance

Postby David B » Thu Feb 04, 2010 6:32 pm

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 ?
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Sun Feb 07, 2010 1:37 pm

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.
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Re: JTM45 Choke Impedance

Postby Roe » Wed Feb 10, 2010 6:00 am

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)
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Wed Feb 10, 2010 7:14 pm

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.
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Mon Jun 25, 2012 1:05 pm

Just measured the choke on a 1989 Marshall 2210 100W. The choke number is DE C-3064J and it measured 3H @ 104.7 ohms.
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Re: JTM45 Choke Impedance

Postby Roe » Tue Aug 28, 2012 6:40 am

I removed the 7h from my metro 45, installing a 3h instead. I also dropped the flying resistor to a 470R one
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Re: JTM45 Choke Impedance

Postby neikeel » Wed Aug 29, 2012 7:44 am

Hi Roe

I tweaked my Metro JTM45 as it never really broke in. It sounded crisp and clean with no real harmonics or interest, ok with pedals and a pair of golds in a BB combo but I was always waiting for it to bed in (usual Mullards and perfectly matched NOS GEC KT66s). I removed the MM OT and replace the KT66s with another slightly mismatched pair (7 or 8mA difference at idle, IIRC). I also replaced the OT with a Metro one George sent me. He sent me a -114 choke too but I kept the 10H MM one in situ expecting to do a swap later. I could not believe the transformation with the OT change and left the choke in (in fact have it in my part assembled JTM50 at the moment). Only other mod I did was to replace the V1 cathode cap with a 10uF. I have not put a flying resistor in just 470R on the screens it is lead spec but with 100pF bright cap. Now it is a 'do not meddle' amp - just play.

Other experience of choke change was my JTM100 that sounded pants with a choke from a Carlsboro but drop in a -114 and normal service resumed 8)
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Re: JTM45 Choke Impedance

Postby Roe » Fri Aug 31, 2012 6:05 am

I serviced a metro 45 with MM that sounded ok but not like great 45 should sound. the MMs had a more modern sound with more bass.

Mine has a george's proto RS dlx OT running at 6k6 (sounds good to me). The PT has been changed to a classictone one that delivers around 440-450v on the plates. You don't really need the flying resistor at these voltages. its a matter of preference. the amp will certainly feel softer with the flying resistor.
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