JTM45 Choke Impedance

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JTM45 Choke Impedance

Postby RS DeLuxe » Wed Nov 26, 2008 4:26 pm

I've been experimenting with and wondering about the different value Chokes that have been used in
Marshall JTM45 & JTM50 amps over the years.
Has anyone accurately measured chokes found in these amp? I know the real early Marshall schematics show a "20 H"
choke; and then there's the C1999 8 Henrie choke... but looking at numerous chassis pics there seems to be quite a range
of different size chokes used.
What effects have other people found with varying Choke Impedance?
From my own limited testing, it seems like 5 Henries is the absolute smallest size you'd want to use with a JTM45 or 50.
8-10 Henries gives a nice responsive feel to the amp (in a JTM45) and the larger values like 20 Henries really gives the amp balls in the low frequency response. Also, hum suppression seems directly related to choke size with larger values keeping it to a minimum.
Anybody else have observations regarding Chokes?
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Wed Nov 26, 2008 5:12 pm

I have measured quite a few different chokes found in the Marshall 45, 50 and 100W amps and have found them to be in the 3H range including the C1999. I will make these measurements again and snap a picture of the readings on my meter to verify the numbers this week.

Here are a couple of reading I did that was posted on the Metro forum.

"OK, the 1471-263 choke measured 3.43H and has a DCR of 106.5 ohms.

I also measured an open frame choke I have and it measures 3.63H and has a DCR of 103.7 ohms. So 3H it is for these two chokes found in Marshall amps
."
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Re: JTM45 Choke Impedance

Postby RS DeLuxe » Wed Nov 26, 2008 5:20 pm

Where those chokes in any particular amp, or just by themselves?
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Wed Nov 26, 2008 6:10 pm

They are not in any amp right now but the open frame choke is from Mr. Twistyneck's 70 something Marshall and the 1471-263 is from a RI JTM45.
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Wed Nov 26, 2008 6:18 pm

Here are the first pictures.
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Re: JTM45 Choke Impedance

Postby Roe » Thu Dec 04, 2008 6:24 pm

andy/flames said that the 20h choke in the early 45/100s was supposed to handle only 70mA, if i remember correctly. this suggests that it will drop below 20h as when you dig in
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Thu Dec 04, 2008 6:31 pm

Yes, the RS choke is only rated for 70mA. It also has a DCR of 690 ohms. I'm not sure on how much drop the choke will have under load but I do remember reading that the Henry will drop in this condition.
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Re: JTM45 Choke Impedance

Postby David B » Thu Dec 04, 2008 9:46 pm

Hey Brian,

How's that meter working out for ya?
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Thu Dec 04, 2008 10:22 pm

The new one is fine but that's the old one in the pictures. :)
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Re: JTM45 Choke Impedance

Postby David B » Fri Dec 05, 2008 2:19 pm

Brian Wallace wrote:The new one is fine but that's the old one in the pictures. :)



Same model meter?
btw would the DCR of 690 ohms would equate to more voltage drop and less regulation?
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Re: JTM45 Choke Impedance

Postby RS DeLuxe » Fri Dec 05, 2008 4:07 pm

A JTM45 (and most other old Marshall amps) have what's known as a Capacitor Input Filter for cleaning up the rectified output of the power supply. That is, the raw DC electricity from the GZ34 tube, or Diodes in a solid state version, first flows through a large Electrolytic Capacitor, then the Choke; and finally a second Capacitor. A Capacitor Input Filter is used when the current needed is relatively low; in say a JTM45 thats 450 volts at maybe 150ma. maximum. In addition, a Capacitor Input Filter allows for a smaller Power Transformer than if another type of input filter, called a Choke Input Filter, was used. (In a Choke Input Filter, the raw DC flows first through a Choke and then the Capacitor)
The Capacitor functions in the filter to smooth the Voltage variations, and the Choke smooths the Current.

To answer your question, in a JTM45 the 32uf electrolytic capacitor is doing most of the work cleaning up the DC.
The Impedance of the Choke (Henries) is more of a critical factor since its resistance is just part of the total Load resistance that the power supply "sees", and for KT66 tubes or similar, it's in the many thousands of Ohms. The goal in power supply filter design is to get the smallest amount of Voltage Ripple (variation) as is practical with smallest practical components (i.e. bigger or more = more $$$).
There are standard formulas & charts for calculating these things. Below is a chart I lifted from the Radiotron Designers Handbook which goes into the subject much deeper. The chart illustrates the relationships between Capacitance & Inductance & Load Resistance vs. percentage of output Voltage Ripple.Image
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Re: JTM45 Choke Impedance

Postby David B » Fri Dec 05, 2008 8:01 pm

Thanks RS great explanation :D

So if I got this right the choke filters just the screen grids of the output tubes and the preamp section ..besides the capacitance ,inductance & load resistance how does the current capacity of the choke such as the 70mA of the old RS have on its ability to filter under heavy load?
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Fri Dec 05, 2008 10:43 pm

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Re: JTM45 Choke Impedance

Postby David B » Fri Dec 05, 2008 10:56 pm

Oh OK so when the current rating of the choke is exceeded the inductance drops, suppose this could measured, guess one could just measure the screen current ?
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Re: JTM45 Choke Impedance

Postby Brian Wallace » Fri Dec 05, 2008 11:01 pm

As I said before, there is a thread on this drop somewhere on the Metro forum, I just couldn't find it tonight.
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