CHAMP ELECTRONICS -" THE VINTAGE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

 

REPAIR / RESTORATION OF MARSHALL JTM 45 AMPLIFIER HEAD

 

.................saved from a fate worst than death!

 

 

This 1960’s, aluminium chassis, Marshall JTM 45 came through the workshop in a very sorry state. In the past, it had been operated on by a very incompetent surgeon!

This person had replaced the power transformer (the original was probably long gone?) with a Drake upright mounted type, he had drilled new holes in the chassis, and the main smoothing capacitor had been remounted on the underside end of the chassis. The replacement power transformer that he had used didn’t have the 5 volt heater winding for the GZ34 rectifier, so this had been omitted and silicone rectifiers had been soldered across the tube base instead.

There were some circuit modifications which had been done which had required some extra chassis holes to be drilled. Needless to say, the  metalwork/drilling was very crude! Even the original Bulgin power socket had been replaced with an IEC connector.

 

 

All of the wiring on this transformer and the modifications were absolutely dreadful. It looked as though someone had just grabbed a handful of components and wire and thrown it all in! The soldering was also pretty disgusting.

Even though this amplifier was in a bad state, it would however have worked to some extent.......at sometime. Nevertheless, its final death had been a massive arc across one of the output tubes and its base, carbonizing both; probably the result of attempting to use the amplifier without a speaker connected.

So, the first job was to rip out all the disgusting wiring, incorrect components, and the wrong power transformer (see photo below). Next was straighten out the bent chassis, clean and de-burr all the badly drilled holes and then fill these unused/unneeded holes with s/h original 4BA brass nuts & bolts. This at least makes it look tidier from an aesthetics’ point of view.

 

 

When I do this type of repair/service, I appreciated that the amp needs to be returned as close as possible to being original. I therefore  have a stock of old nuts/bolts general hardware along with used-but-good components for such occasions, even though I have good stock of new components and hardware. I wouldn’t jeopardise a job by using unreliable s/h components of course. If I had been worried on that score, then I would have used new stock as necessary.

The tube base that had arced across was actually not an original; this had obviously been changed at some point. I removed both bases, cleaned all the soot (from the arcing) off the under-chassis and tag board as best as possible without leaving things looking “too clean”. I found an identical used tube base, cleaned them both off, and re-fitted the pair.

 

 

As can be seen in the photos I have stuck to the old hardware. I have replaced the IEC connector with a proper Bulgin. The original smoothing capacitor was tired and dried out; this has been replaced with a used one that is perfectly OK. This used one is discoloured and tarnished through use and is perfect for keeping in looks with the amplifiers age.

One thing that I’ve never been able to do is the lacing on wiring, as all the girls used to do on hand-wired amplifiers back there in the 60’s & 70’s. I unfortunately have to rely on the modern equivalent……cable ties!! Nevertheless I can do everything else well.

As I didn’t have the original power transformer I simply had to design and make one; this is now of the correct drop-through type. I couldn’t do much about having to use new laminations. However I did find a used top end shroud that really looks the part.

 

 

After many hours of painstakingly installing the new transformer and redoing all the necessary wiring, fitting of a new pair of Mesa 5881 tubes (of which this amplifier was originally designed for) plus a new JJ GZ34 rectifier valve, I carried out the final check-up and then the test. (By the way, the JJ GZ34 is the best I have found of today’s tubes and the closest to the original Mullard GZ34 in performance.)

She burst into life, no problems at all, and is as good now as the day she was made........and now worth quite a bit too!

 

 

 

 

 

Cheers for looking, John.

 

 


 

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