# JTM45 choke question



## Jean GODBOUT

I'm presently sourcing parts to build a JTM45 clone, most builders are using a choke rated at about 5 to 7 Henries while the original Marshall schematic call for a 20 Henries choke. 

Can someone explain me the difference it can make to use a 5 H or a bigger like 20H choke ?

I understand that the 20H will filter more but what are the effects on tone and on power ?

Thank for your help


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## hollowbody

I can't help with an answer, but I'm interested as well. I have a Marshall JTM45 reissue that I'm slowly modding to vintage-correct specs. Choke isn't something I had thought about before, actually.


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## Wild Bill

Jean GODBOUT said:


> I'm presently sourcing parts to build a JTM45 clone, most builders are using a choke rated at about 5 to 7 Henries while the original Marshall schematic call for a 20 Henries choke.
> 
> Can someone explain me the difference it can make to use a 5 H or a bigger like 20H choke ?
> 
> I understand that the 20H will filter more but what are the effects on tone and on power ?
> 
> Thank for your help


Go with the smaller choke. There will be no difference in tone!

When building a power supply, the most inefficient and frankly goofy way to try to get better filtering is to make filter caps or chokes bigger and bigger! Once the cap or choke is big enough to do it's job you rapidly get diminishing returns. Making the value bigger gives mice nuts for improvement in taking out the ripple.

The best and most cost effective way is to do what virtually all tube amps do and that is to have several cascaded stages, using either a resistor or a choke with a capacitor. Each stage will take out 90% of the hum/ripple. Doubling the cap value might give you an extra 1%. Adding an extra stage gives you another 90%!

When Jim Marshall first built his JTM45 he was copying the Fender 6G6 Blonde Bassman. The choke had a Fender part number. He likely had no idea of its value! What's more, few techs had an inductance bridge on their bench that could measure the value of unknown power chokes. Only the techs who learned some deeper electronics theory understood chokes well enough to calculate the minimum necessary value. Usually they just used one so big they knew it would have to work! So he used 20 Henries, a value that was huge overkill!

Later, he was kinda forced to go back and spend some time thinking about that choke. Chokes are like transformers. They are among the most expensive parts in an amp. The bigger they are, the more they cost, by a LOT! That's why in the newer Marshalls the value was scaled back to 7-8 Hy or so. That's still overkill but not by nearly so much. It probably cost less than 25% of the 20HY unit.

The cost of the choke is why many amps replace them with a large power resistor. This works reasonably well but is a lot cheaper. There is a bit of tone difference when going from a choke to a resistor. A choke is a better current regulator and tends to keep the screens of the output tubes better regulated. It's all taste, of course.

Hope this helps!

WB


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## Jean GODBOUT

Thank you Wild Bill, your explanation is 100 % clear, I will save some money here  + I start to understand what I'm doing !

Thank.


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## hollowbody

Ditto. Thanks Bill!


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## Jean GODBOUT

*The JTM45 is now completed*

Here a few pictures of my completed JTM45 clone

Audio circuit = Allen Bradley NOS carbon comp resistors
Power circuit = Takman - REX 1 Watt Carbon film (2 %) tolerance from Parts Connexion 
Capacitors = Mullard/Philips NOS
Capacitors Bias circuit = Sprague Premium
Choke = Mercury Magnetic USA
Transformers = Magnetic components USA
Sozo 0.047uF 400V capacitor and 100K 2 Watts kiwame resistor on Standby Switch (very silent switching)
UK Cliff input and output jack
CTS potentiometers
Larry goounding schem
Hand made Paxolin dark brown circuit board, turreted
Metro-Amp aluminium chassis (polish)
Weber 25K bias Potentiometer
Ceriatone plexi front panel - Hand made rear plexi panel
v1 & v2 = Mullard 12AX7 RI
v3 = Mullard Blackburn factory UK 12AX7 NOS
v4 & v5 = Valve Art KT66
v6 = GZ34 Blackburn factory UK Mullard NOS (label G.E.B.)


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## greco

CONGRATULATIONS !! ...beautifully made. 

You have my total admiration.

Cheers

Dave


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## keeperofthegood

WELL DONE!


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## mrmatt1972

If that sounds as good as it looks you've made something special. Congratulations.


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## Jean GODBOUT

*I need better speakers*

Thank guy, indeed I'm proud to be very close to the look of a ''real'' 1965 Marshall JTM45, it wass one of my goal.
I'm not a electronic tecnician, just a hobbyist and to my big supprise the amp run very well from the first startup, this mean that I made zero error. I've read a lot in the Metro-Amp forum and I build the amp witch include the tweeking that help the amp to run perfectly. Example, most of the builders using the stock layout complaint that the Standby switch squeel, so I corrected the issues right away in the building process, the modified grounding layout prevent the ground loop noise etc...

Yes it sound very good but ... I'm running the amp into a 2 X 12'' Jensen P12Q cabinet, witch is not the best match for a Marshall, so I'm in the market for a pair of British voice speakers. ANY SUGGESTION ?

Thank again, Jean


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## mrmatt1972

If you're going to run a pair of speakers with a British voiced amp I don't think there is a better choice than a Weber Blue Dog and Silver Bell (both ceramic) together. I once had that rig (sold it before a move) and it was simply awesome.

I bought the Neo mag version of the Silver Bell before they discontinued it, it's really good too, but I definitely preferred the ceramic magnet for rock tones.


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## hollowbody

Very nice work Jean. I've been thinking about dropping a turret board into my JTM to replace the stock PCB for a while.

As for speakers, I'm using a Celestion V30 and a G12H-30 in a 2x12 right now and they sound FANTASTIC with the JTM45. I can't recommend this pair of speakers enough. I was running a pair of G12H-30s for a while, and that sounded pretty good too, but it sounded a bit dark/muddy with two of them. It's much more open sounding with a V30 in there.


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## WCGill

I'm a big fan of the Celestion G12H Hertitages. They overdrive with a lot of character, not heavy in the mids, fizzy on top, big bottom. These speakers are not for everyone and you will probably have to pull some bottom out of the amp, but they're quite excellent with the right tweaks. One of these paired with a V30 is also an option. BTW, beautiful work, mine don't look nearly as nice inside and I know how much work it is to make it so.


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## Jean GODBOUT

I'm very tempted by the Weber Blue Dog and Silver Bell ceramic because of their relatively low cost. What is the differece (tonewise) with the ceramic and the more expensive Alnico magnets ?

Thank, Jean


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## Wild Bill

Jean GODBOUT said:


> I'm very tempted by the Weber Blue Dog and Silver Bell ceramic because of their relatively low cost. What is the differece (tonewise) with the ceramic and the more expensive Alnico magnets ?
> 
> Thank, Jean


A bluesman once told me that alnico is great for Delta blues and ceramic is more "Chicago". By that he meant that alnico is more clean and traditional, while "Chicago" was more "in your face", like when Muddy Waters took blues electric!

I've never heard a better explanation!


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## al3d

Jean..come and try it in a Greeback Marshall...THAT..will make it sing for real..


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## HumanJHawkins

*Beautifully Done.*

That's a beautiful build. Without considering the countless hours you must have spent (and the cost of tools, etc.), would you mind sharing about how much all the parts cost? I'm trying to figure out if I could afford to start on a similar endeavor.

Thanks in advance.


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## Jean GODBOUT

Well, about $1,000.00 including parts, shipping, broker fees and taxes.

Cheers, Jean
Princeton Reverb clone (DIY)


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