Marshall Style 1987 Build, Which Choke?

Cadorman

Cadorman

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I'm collecting parts for a 50 watt 4 hole Marshall build and trying to decide what choke to go with. It appears that there a 3 versions throughout the years and I am trying to figure out which would be best for what I am doing.

3 Hy 250ma
5 Hy 120ma
20 Hy 70ma

I have been working with the Marstran 1987 schematic, which was reverse engineered from a 1968 Marshall. What effect, if any, will using different choke values have on the amp?
 
I've always used the stock value choke in all my builds, which in this case would be the 3Hy C1999 choke. If you look at Marshall schematics they often show a 20Hy choke, though this was only used in very early amps (JTM45's & 45/100's & a few JTM50's/100's)
Some people report feeling a difference in the amp with a larger value choke used, others no difference. I can't comment as I haven't tried it. A larger value choke will drop more voltage across it though as it will have a higher DC resistance (more turns of wire). The RS 20Hy has a DC resistance of 690 ohms, the C1999 3Hy is 110 ohms. Cheers
 
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The general consensus seems to be that with a lower value choke the amp will feel looser, maybe sound a bit more rude. With a higher value choke the amp will feel tighter, maybe sound a bit smoother. If your after the classic four hole Marshall sound & feel Id say stay with the stock 3Hy. Cheers
 
ivan H said:
A larger value choke will drop more voltage across it though as it will have a higher DC resistance (more turns of wire). The RS 20Hy has a DC resistance of 690 ohms, the C1999 3Hy is 110 ohms. Cheers
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A 3 Hy 110 ohm choke versus a 10 watt 110 ohm resistor:

Would both of these components drop the same amount of voltages?

Why use the much more expensive choke versus the much cheaper high wattage resistor?
 
syscokid said:
A 3 Hy 110 ohm choke versus a 10 watt 110 ohm resistor:

Would both of these components drop the same amount of voltages?

Why use the much more expensive choke versus the much cheaper high wattage resistor?
Click to expand...
Yes, they would both drop the same amount of DC voltage (HT rail).


The choke is used to help with the smoothing of AC ripple from the DC HT rail. A choke offers low resistance, so doesn't drop the DC voltage much, while offering high "reactance". Reactance is to AC what resistance is to DC. So the choke opposes the flow of the AC ripple superimposed on the DC HT voltage. This makes it preferable to just a resistor. Cheers
 
Keep in mind that the 3 Hy choke is for KT66 tubes. The 20 Hy choke that is sold by Valvestorm and others is for EL34 type tubes. I've read that running the 20 Hy choke with a KT66 will cause overheating problems with the choke and transformers. I don't know about the other way around. Steve said that a 10 Hy choke in his '63 JTM45 worked very well, with no change in tone. I have not been able to find any source for one of the on the web so far.
 
SG John said:
Keep in mind that the 3 Hy choke is for KT66 tubes. The 20 Hy choke that is sold by Valvestorm and others is for EL34 type tubes. I've read that running the 20 Hy choke with a KT66 will cause overheating problems with the choke and transformers. I don't know about the other way around. Steve said that a 10 Hy choke in his '63 JTM45 worked very well, with no change in tone. I have not been able to find any source for one of the on the web so far.
Click to expand...

The 20Hy choke was used mostly with the JTM45's (KT66's). Some JTM50's used it too, but mostly the 3Hy.
The power tube plates alternately pull large current, which translates to fluctuations (ripple) in the DC rail. This is why the choke is placed in the HT rail between the output transformer CT & the PI & pre-amp, so they (PI & pre-amp) are fed with a nice clean HT supply that is free of ripple. Cheers
 
ivan H said:
The 20Hy choke was used mostly with the JTM45's (KT66's). Some JTM50's used it too, but mostly the 3Hy.
The power tube plates alternately pull large current, which translates to fluctuations (ripple) in the DC rail. This is why the choke is placed in the HT rail between the output transformer CT & the PI & pre-amp, so they (PI & pre-amp) are fed with a nice clean HT supply that is free of ripple. Cheers
Click to expand...
I think I just lost the other half of my brain cells.
 
ivan H said:
The 20Hy choke was used mostly with the JTM45's (KT66's). Some JTM50's used it too, but mostly the 3Hy.
The power tube plates alternately pull large current, which translates to fluctuations (ripple) in the DC rail. This is why the choke is placed in the HT rail between the output transformer CT & the PI & pre-amp, so they (PI & pre-amp) are fed with a nice clean HT supply that is free of ripple. Cheers
Click to expand...


I read at the Amp Garage and a couple of other places that using the 20 HYy choke with KT66 tubes caused severe overheating problems. I saw that Metro amp uses the 7 Hy Mercury Magnetics choke, and that also seems to go with a KT66. I don't know if the 3 Hy or 7 Hy chokes will bring the voltages up to the correct range for an EL34, and you will have to run a 20 Hy.

The early schematic I have shows a 3 Hy choke with a KT66, 6L6, or 5881. And the later schematic using EL34 power tubes show a 20 Hy choke.

Keep in mind, I'm only regurgitating info I've learned from you guys, as I haven't even got all the parts to start mine yet.
 
If we look at the "historical data" section of the Marstran site, which lists specs taken from actual individual amps, we see that it is the JTM45's (though not all of them) that used the RS (Radio Spares, 20Hy, 690 ohm) choke, after which the "U frame" 352-114 (Drake 3Hy, 110 ohm) was used. This was also used on some JTM45's.
If we look at the Solo Dallas Evolution of the 100 Watt circuit: from JTMs to JMP Superleads, it also says the JTM45/100's (KT66's) used the RS 20Hy choke, with later amps using the Drake 352-114, or later still, the Dagnall C1999 choke, both of which are 3Hy, 110 ohm. Cheers
 
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I was going to link that Aiken article.
I used it to help choose the choke for my Micro JCM 800 1w.
Parts are there, chassis stock cut and bend lines are marked up, havent the time.
Sucks too since the weather has been mild here and pleasant for the garage / shop.

I am using Hammond iron and ordered the choke with the PT and OT.
 
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