Marshall JTM-30

Make it loud here.
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Marshall JTM-30

Post by Racing » Wed Oct 04, 2017 4:20 pm

The infamous later JTM amps.
All plagued by a piss poor design as far as ventilation,which indeed is what kills them. It just escapes me how people living on the british isles to this day can fail in such megamassive ways when it comes to ventilation.

Well. Whatever.

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(Pic stolen online)

So. I picked this busted JTM-30 up for a song. As it turned out,the previous owner had seen the magic smoke come out of it,the powertransformer was shot. For more than one reason sorry to say.
Called my mentor up that has a toroid laying around which could be put to good use,so said and done.

Well. The fuse for the heaters had been upped in a manner you won´t believe (wrapped in foil) and the 250mA fuse for the low voltage part of the amp had been bridged.
Talk about inviting trouble...

Image
(..again..online)

From the onset i bet that the idea was for a more compact Marshall and then one that was in phase with times. These were manufactured in the late -90´s and indeed are children of their time.
As you can see the mother PCB has been installed straight up and that brings that the tubes,all of them,lay down. Now...this is where Marshall fucked up. Big time.
Look up high and to the left of the chassis. There you see sort of stamped louvers and these are there to provide an escape path for the hot air surrounding the powertubes. All good.
Now. Any idiot with a stick realize that hot air rises..and as it does in this case it enters the chassis and cab right. So...question for you. Where does it evacuate the cab?
Answer is that it doesn´t. The JTM-60 is even worse as it 1/ produces more power (read more load on powertubes=more heat) and 2/ is even more filled to the brim with electronics.

Ergo it is a common issue with these that high temp has taken its toll. For the JTM-60 for instance it is not uncommon to see the rear PCB that holds the outputjacks and what not being warped beyond belief.
The 30´s are better off,while still not good. In short the remedy is to cut for a vent in the top of the cab,and many JTM´s has been so modified. For the 60 it might even be needed to install a small computer fan underneath the powertubes to create more flow. Mind you,a hurrican isn´t needed. Just flow..

To a tech the solution of installing the motherboard as have been done is mainly just a nuisance. Ie,it takes longer to implement changes as the board basically needs to be taken out between rounds so to say.
Not the end of the world tho...just cumbersome.

Apart from the above it sports rectified heater voltage for the two preamp tubes and the phaseinverter tube. That rectifier was of course shot too..

So. Came to remedy it all and fire the thing up. It worked alright but as i checked bias i about crapped myself. It was set to 33 watts a pop for them powertubes. A "tad" more than needed to be blunt (read-even more heat)
The bias circuit for them is setup in such a manner that bias voltage is clamped vs ground via zener diodes. To be honest,seing how simple a bias circuit is after all,i really fail to see the reason for this. Yeeeeees...i DO get it,but still fail to see the reason. In short i regard that there as a solution on the hunt for a problem.
So.
Scrapped the damn zeners and set it up with a regular reostat vs ground and so forth and indeed...we had a smooth running stocker.

Played the thing a little and...stockers are booooooooooooooooooooring...so i guess you see where this is headed.

Now. That said i have to say i found it rather loud seing the limited power of 30 watts. From what i´ve gathered Marshall opted for this as the 30 was to be the 60´s little sister i guess and the way they did it was to use a 6k primary impedance output transformer and by limting swing from the phaseinverter by the use of a 47k tailresistor.
A 47k tailresistor might be a good idea with amps sporting EL-84´s or 6V6´s for instance,however the onboard 5881´s DO take more to reach full saturation.
Anyways.
Loud.
So i started looking around the shelves for a replacement when i came across one that lacked markings. Ergo,i needed to measure the conversion number for it.
An OT is really nothing but a fancy gearbox,and at time not even all that fancy. Then it spawned upon me to measure the stocker.
Judge my surprise when it turned out that primary impedance for THIS OT was on the dot 3800 Ohms. Ie;exactly what a pair of for instance EL-34´s ask for if we have power in mind.
No wonder the thing felt powerful!

Back in the stock OT went and as a first mod i replaced that 47k tailresistor with a 15k one and let it rip. Yes Sir. We had liftoff! Difference was right there and then! Cool!
No idea why Marshall states 6k on the schematic and there´s 3,8k ones in there IRL? Beats me!

Image

Well. CAN the JTM be made into a more contemorary sounding,and roaring,combo? Certainly...tag along..

Idea here is to make it "bloom". Ie;let it manage to deliver the power it REALLY can and in turn reset (read-modify) it to a point where we´re in business as far as the 21st century.
That said,it´s been a while since i challenged a Fender/Marshall/Vox tonestack and this little sucker reminded me why. Sure,it sound of Marshall alright (albeit the cathode follower stage before the tonestack is missing) it´s just that this time out the actual dials actually performs something but...compared to what i´m used to from the James networks i use...not even the same ballpark. It´s THAT simple.

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Re: Marshall JTM-30

Post by Racing » Wed Oct 04, 2017 4:38 pm

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Make no mistake. The 30 is WAY less crowded than the 60 within. No joke! Difference is like night and day. No matter,i guess you get an idea of layout. I opted to install fuseholders for both sides of the heaters and in turn that small blue trimpot for adjustable bias voltage.
The toroid PT carried more then one secondary tap so i opted to go high and let it rip. Handed me approx 475VDC@35mA. That coupled with that 3,8k primary OT...well HELLO! Being a JTM-30 that is.

That said i about tossed the entire bag of trix at the thing. It in my opinion surely gave for a difference in behavior that was on the upside. Them damn clipping diodes these amps sport were deleted entirely. There´s no need for them what so ever IMO..

Anyways. Them cooling cutouts above the powertubes,i guess you get an idea right? A really PISS-POOR design if ever!

Image

Then again,that orange wire heading for the small PCB that holds the output jacks is rather nifty as it´s the ground for the powertubes and this heads for the output jacks.
In short,if a speaker is missing the powertubes won´t ground = no chance of burning the powerstage sky high. Smart move in my book.

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A pleasant surprise. The OT. As you can gather it isn´t exactly overly large but...it IS in the game no matter. Does what it should i´d say.

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Then for the toroid. As is i run this thing at a level where the stocker is nowhere near. That brings that some of the hightension electrolytics has been replaced with 500 volt ones too.

So. Up and roaring but to me...rather mundane no matter. Sure,sounded a ton better then the stocker but... as i KNOW what´s there to be had. Hell no!

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Screw Marshall. James time.

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General impression might be that the added controls basically have been tossed in place,but this is not the case. They´re where they´re at for a good reason.

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Uhu. A 6 way switch which varies capacitances inbound for the treble pot. This effectively alters midrange in a manner you won´t believe. Can do anything from a classic "scoop" of the tone to downright midrange HUMP.
This coupled with a simple minitoggle for the bass side of things makes for quite an array of choices. Once you learn how to play around with a James (or a Baxandall for that matter) there really is no return.
The stock midrange pot in turn had been readressed to work as a "separation resistor" between the treble and bass pot,effectively making it into a "bias" pot,thus telling which of the two pots is let to be dominant.
That means that "mid",treble and bass are now all 1M Ohm pots.

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With the chassis in place maybe it becomes a little more clear why the added controls were placed where they were?
Top right is of course a PPIMV pot in turn...

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Yeah.´N then sum!!

Out the other end then? Diffence vs the stocker is MASSIVE. There´s no other way to put it. It can STILL be made to sound Marshalleresk just that now it can do SO much more.

Any flipsides then?
Yeah. I guess? Seing how it performs after the mods i came to question the stock speaker. Really really uncertain if that will stay... We´ll see i guess.

Anyways.
Idea of this post,seing questions onboard this part of the site every now and then,is to prove that if you know to swing a soldering iron an old JTM amp,that can be had for about a song,are contenders when on a budget.

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Re: Marshall JTM-30

Post by NoiseNoiseNoise » Sat Oct 07, 2017 7:25 am

Only you would get a Marshall and think it was boring ;)
It shows you can improve on most things. I can't wait for someone to bring a dead Mesa to your workshop....
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Re: Marshall JTM-30

Post by andy_tchp » Sat Oct 07, 2017 5:56 pm

NoiseNoiseNoise wrote:Only you would get a Marshall and think it was boring
Can confirm he's spot on.

Had the 60 watt version (two of them - the first one invoked the factory fitted self-immolation feature during the warranty period. The replacement repeated this a month after I'd used it as a partial trade on a different amp and someone else had purchased it).

Total pieces of shit.
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Re: Marshall JTM-30

Post by mbene085 » Mon Oct 09, 2017 5:12 pm

I learn so much from your threads every time I open them. Thanks for explaining your process so thoroughly. Glad this fried JTM got some much-needed TLC!

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Re: Marshall JTM-30

Post by Racing » Thu Oct 12, 2017 3:30 pm

https://www.youtube.com/watch?v=9R2LpIE ... e=youtu.be

Shot with the damn cellphone of mine,so don´t expect any culinary fabless. Idea with that clip is mainly to show the width of the amp in modified form. That the James network does it´s fair share i guess is evident to anyone.

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Re: Marshall JTM-30

Post by NoiseNoiseNoise » Fri Oct 13, 2017 3:49 am

I watched that vid. I wish I could speak Swedish (as it is I can barely speak ENGLISH!). Yes, it does sound nice, especially the clean tone! Who knew? You CAN get decent cleans from a Marshall!
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Re: Marshall JTM-30

Post by bmorjamn » Sat Dec 05, 2020 12:05 pm

Upon my search to see if there is a way to salvage JTM 30 I replaced some pots , knobs and effects jacks.
My first idea is maybe build something all tube with a turret board. My problem is if I just mod the existing pc board I don't know which diodes can be removed and what to do make it sound right. I wish had seen this thread when is current. Pictures would be nice ! I did read through and find it very interesting as well as the video.

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Re: Marshall JTM-30

Post by andy_tchp » Sat Dec 05, 2020 5:22 pm

The PCBs can be modified by carefully removing them from the amp and then carefully throwing them in the bin, along with all of the other inevitably blown up/overheated components (ie transformers).

Even when working/sounding 'right' they're uninspiring, dull sounding amps.
bmorjamn wrote:
Sat Dec 05, 2020 12:05 pm
My first idea is maybe build something all tube with a turret board.
Yes. Do this reusing the chassis/cabinet.
"I don't know why we asked him to join the band 'cause the rest of us don't like country music all that much; we just like Graham Lee."
David McComb, 1987.

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Re: Marshall JTM-30

Post by bmorjamn » Sat Dec 05, 2020 5:53 pm

It is a bit uninspiring , I will figure something out then . Thanks

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Re: Marshall JTM-30

Post by andy_tchp » Sat Dec 05, 2020 8:51 pm

Ha, sorry to be such a downer, I'm still bummed out by my experience with these amps nearly 20 years on.
"I don't know why we asked him to join the band 'cause the rest of us don't like country music all that much; we just like Graham Lee."
David McComb, 1987.

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