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.
(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...
(..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!
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.
