'89s JTM45 without choke - why ?

TheRedDevil · Post by **TheRedDevil** » Fri Jan 12, 2007 2:33 pm

Hello.

Hope you can help me. I've got a '89s JTM45 Reissue amp. I bought it used some years ago. Now I really think about to upgrade my JTM45 at first with a Metroamp JTM 45 Retrograde Kit (+Sozo Mustards) and KT66 tubes.

I looked at the circuit diagrams and I found out, that my amp does not have a choke. I only can find a special resistor under my board (WH2.5 100R Ohms 5%) that is connected to the 10K/1W resistor near bias regulator and is also connected to 50uF/500V capacitor (see attachment).

What do you think: Does it make sense to remove the resistor and to integrate a Shinrock SCH-RS20H choke from Germany (I can't get a MM choke in Germany).

Thanx a lot for your help.

Regards,
Johannes

wadeberglund · Post by **wadeberglund** » Fri Jan 12, 2007 2:50 pm

I'm not familiar with that particular RI, I'm guessing they used a resistor there instead of a choke to reduce the cost of the amp. The 100ohm resistor emulates the DC resistance of most chokes. It's cutting corners if you ask me and putting a choke in there will make it closer to an original. What you can expect if you make this change is more power supply regulation. This implies less 120Hz buzz in the output and a tighter response. It is a very cheap and easy change, so I would recommend you give it a try. If you like the loose response and a little more sag, feel free to experiment with a lower valued choke (7H or 10H).

wadeberglund · Post by **wadeberglund** » Fri Jan 12, 2007 2:54 pm

Forget what I said about no 120Hz ripple. It does exist in a JTM45.

TheRedDevil · Post by **TheRedDevil** » Fri Jan 12, 2007 3:19 pm

wadeberglund wrote:I'm not familiar with that particular RI, I'm guessing they used a resistor there instead of a choke to reduce the cost of the amp. The 100ohm resistor emulates the DC resistance of most chokes. It's cutting corners if you ask me and putting a choke in there will make it closer to an original. What you can expect if you make this change is more power supply regulation. This implies less 60Hz buzz in the output and a tighter response. It is a very cheap and easy change, so I would recommend you give it a try. If you like the loose response and a little more sag, feel free to experiment with a lower valued choke (7H or 10H).

At first thanx a lot for your help. For me JTM45 tuning is a totally new thing. Nice that you help such a german guy like me!

Oh, the resistor was stock. I thought it was made after buy. Very interesting.

You talked about 7H or 10H chokes. My english is not the best (response/sag?). Is it better to use a 20H choke to get the AC/DC Angus Young Powerage album rock sound out of the JTM45 or should I use a 7H/10H choke? What are the sound differences ? Hope you can explain it for me.

Thanx a lot,
Johannes

wadeberglund · Post by **wadeberglund** » Fri Jan 12, 2007 3:55 pm

Sure, no problem! Welcome to the forum by the way...

First, I'm not 100% sure that the resistor is stock. It looks like there are chassis holes drilled for mounting a choke and there is a hole/grommet for the leads to come through also. It may very well be that the previous owner wanted the 100 ohm resistor there instead. Chokes are a pretty cheap part, so it would be pretty weak for Marshall to not include one for cost reasons.

As for the value of the choke, let me say this: the power supply can either be described as 'hard' or 'soft'. A hard power supply is really tight, and a soft supply is loose. Sorry, those words are really subjective, but think about the difference between someone playing metal and someone playing blues. It all has to do with the feel of the amp and it is something you will be able to hear and feel in your playing once you can identify the style of power supply in an amp and how it responds when you play. The power supply on a JTM45 definitely drifts towards being a soft power supply because it has a tube rectifier and smaller filter caps. That being said, you would expect a smaller choke, but the typical value is 20H, which is high.

I would start with the stock 20H choke and see what you think. If the amp feels stiff, you may want to go lower. I'm not familiar at all with any mods Young might have done to his amps, but a stock JTM45 is an excellent start.

45auto · Post by **45auto** » Fri Jan 12, 2007 5:12 pm

take a look at this thread:

http://forum.metroamp.com/viewtopic.php?t=6479

MacGaden · Post by **MacGaden** » Fri Jan 12, 2007 5:52 pm

Hi Devil:

Wellcome to the Forum.

Check this: http://www.tubeampdoctor.com/product_in ... anguage=de

And this: http://www.tubetown.de

Both in German and in Germany !

neikeel · Post by **neikeel** » Sat Jan 13, 2007 8:15 am

The Marshall JTM45RI that I attacked/modified was exactly the same as yours and was stock for that first series of RIs.

I used 16/16 and 32/32 filters and initially a 10H MM choke. I found it too hi-fi and too stiff with a MM OT and NOS GEC KT66s

I have since modded it again to JTM50 spec with different OT 3H choke and 32/32 and 32/32 filter cap and using Mullard xf4 EL34s - much better for me.

I suggest you get a stock Marshall 1987X choke (chassis is already drilled for it!) and lower the filter caps to 16/16 and 32/32.

I would use a Heyboer or Marstran OT.

Just my 2 Euros

TheRedDevil · Post by **TheRedDevil** » Sat Jan 13, 2007 5:12 pm

neikeel wrote:The Marshall JTM45RI that I attacked/modified was exactly the same as yours and was stock for that first series of RIs.

I used 16/16 and 32/32 filters and initially a 10H MM choke. I found it too hi-fi and too stiff with a MM OT and NOS GEC KT66s

I have since modded it again to JTM50 spec with different OT 3H choke and 32/32 and 32/32 filter cap and using Mullard xf4 EL34s - much better for me.

I suggest you get a stock Marshall 1987X choke (chassis is already drilled for it!) and lower the filter caps to 16/16 and 32/32.

I would use a Heyboer or Marstran OT.

Just my 2 Euros

Hi. Thanx for your answer.

Are you also an AC/DC sound fanatic ?

I already have got a 1987x and I like that sound. It's more a AC/DC Live sound. But to get the studio "blues" sound of Angus which you can hear e.g. on the Powerage album I want to "tune" my JTM45. Does your JTM45 (JTM50 rebuilt) sound like that studio sound ?

At first I don't want to change the OT (output transformer), but the cheap choke. Do you really think a standard 1987x choke is better than a custom vintage 20H/10H/7H choke for the JTM45 to get the studio sound ? Where are the differences in sound ?

Thanx you very much !!!

Regards,
Johannes

monsterwalley · Post by **monsterwalley** » Sat Jan 13, 2007 5:47 pm

Cool thread, "Powerage" my favorite ac/dc albums, I look forward to
finding out what you figure out.

BTW.... welcome aboard "reddevil" good to have a fellow angus nut here!

neikeel · Post by **neikeel** » Sun Jan 14, 2007 7:36 am

TheRedDevil wrote:
neikeel wrote:The Marshall JTM45RI that I attacked/modified was exactly the same as yours and was stock for that first series of RIs.

I used 16/16 and 32/32 filters and initially a 10H MM choke. I found it too hi-fi and too stiff with a MM OT and NOS GEC KT66s

I have since modded it again to JTM50 spec with different OT 3H choke and 32/32 and 32/32 filter cap and using Mullard xf4 EL34s - much better for me.

I suggest you get a stock Marshall 1987X choke (chassis is already drilled for it!) and lower the filter caps to 16/16 and 32/32.

I would use a Heyboer or Marstran OT.

Just my 2 Euros
Hi. Thanx for your answer.

Are you also an AC/DC sound fanatic ?

I already have got a 1987x and I like that sound. It's more a AC/DC Live sound. But to get the studio "blues" sound of Angus which you can hear e.g. on the Powerage album I want to "tune" my JTM45. Does your JTM45 (JTM50 rebuilt) sound like that studio sound ?

At first I don't want to change the OT (output transformer), but the cheap choke. Do you really think a standard 1987x choke is better than a custom vintage 20H/10H/7H choke for the JTM45 to get the studio sound ? Where are the differences in sound ?

Thanx you very much !!!

Regards,
Johannes

I love Angus Youngs tone generally but confess I cannot comment on the differences in tone on all the albums except for I prefer his studio sound (although Donington DVD is pretty good).

I think that the JTM50 (as per layout on the 50watter forum) is the nearest (using EL34s) although I have seen somewhere - probably here

that he does use a KT66 powered 45 too. It has plenty of grind and 'gives it up' at sensible volumes too. Not as in your face as a metal panel 1987 (my gigging amp is a '76 tweaked to '72 spec (old trannies and different NFB, 1000pF bright cap and Metro PPIMV) not dissimilar to your 1987X.

For me the Marshall choke was cheap, off the shelf, fitted without hassle and does the job - when it comes to the OT I am much more picky

.

mightymike · Post by **mightymike** » Tue Jan 16, 2007 7:28 am

I'm tuning a 50 watter sent to me by another forum member, who has a tonal goal of the first couple bars of Shoot To Thrill.

The amp has a JMP50 board on the JTM chassis. (power section)

I fixed the bias circuit
And Heyboaer sent the guy an 18 watt PT instead of the JTM 45 PT.
So he only had 295 on his PT secondaries, and 378 on the plates, if I use the Diode Plugin, (even less if I use a Tube Rectifier)

So a replacement PT is on the way.
Every Voltage chart for a JTM 45 in my MArshall book shows 330 on the scondaries, but varys between 430 to 440 on the plates (with tube rectification)
I'm sure with a diode rectifier that would be even more, so we went with the standar 330 secondaries. But I see where guys like Brian Wallace use the 350 secondaries, which would put you at 450 with tube rectifiers.

I've also been trying some different boost caps. I see the that they used .68uf on the 1987, but I also tried a 330uf (like a 12000). I also uped the filter cap next to the rectifier, to a 32+32 (it had a 16+16) The increased filtering helped with the buzz, but made it sound thicker and tighter bottom, but less open.

There's some buzzyness isues, so I upped the fizz to a 100pf.
Which helped.

I think once the right plate voltages are acheived, that the buzzyness will be much improved, and mayve with the higher plate voltage that increased filtering will sound better.

I've also heard of choke being replaced by resistor with ACDC, and will look into that as well.

45auto · Post by **45auto** » Tue Jan 16, 2007 5:10 pm

that's the link i posted above. wasn't there discussion of the no-choke acdc mod?

mightymike · Post by **mightymike** » Wed Jan 17, 2007 9:51 am

OK now I remember.
Yeah That sounds like a fun experiment worth trying.

I like Larry's design so you can find where is the sweet spot, but I also would like it to be bypassable to the choke.

SO all you'd need a 1k/50w reostat, and the 20uf after the choke in line with the screens, and a nice heavy duty switch.

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 3:50 pm

Metropoulos Forum

'89s JTM45 without choke - why ?

'89s JTM45 without choke - why ?

Re: '89s JTM45 without choke - why ?