How to Fix Marshall JTM45 Super 100 Hum Plus Recap

This started out as a basic service and change a few caps. Ended up chasing a VERY intractible hum loop problem.

[Skman]

I learn so much by watching you systematically diagnose these issues. Im learning it's all about a process of elimination. Please keep these videos coming. Thank you.

[leecaruso]

You said you hoped this video was interesting, and you hoped that it wasn’t boring. Mr. Smith, all of your videos are educational as far as I am concerned. Tracking down a hum source or multiple sources is educational and I learn more times than not. I am not formally educated in elections, I learned what I know from online courses and from people like yourself. It is nice to have the benefit of someone like yourself and your experience to fill in the gaps of the knowledge that I have. If I was younger I would go back to school for electronics. I’m not what you would call old but I’m not young anymore either. I am retired in my late fifties, my son has a decent sized and reputable recording studio in Pittsburgh, PA. He is a professional touring musician as well. I do a lot of work for him as well as other musicians. I appreciate what you do, and please never think that it’s boring or too long. I use many diagnostic approaches from you. Thank you sir!! Lee

[kengevers]

It's fascinating, extraordinary, now look, no hum.
I'm poking a little humor here, but I've learned when building amplifiers, it has been quite a lot better using aluminum chassis. Crazy stuff. Great video. Thanks, Cheers 😀

[neilverdon]

i nearly wet myself when the Zen music came on!

Your humour is so very British keep it up old chap.

[gib]

I always enjoy your videos but this one was especially informative for me with regards to grounding (earthing) schemes. I have built a couple of early Marshall 100 watt clones over the past few years; and while they do not exhibit a great deal of hum I think they can be better. This video has given me some pointers in the right direction I believe.

[petercornell]

That is remarkable! Great video, thank you.

[simonkormendy]

I took notes and wrote-down how to calculate the output power of an amp, thank you so much for

[tjsogmc]

Excellent job on the amp. I just repaired my Marshall DSL for a humming / crackling noise. Long story short, the new power tubes I put in were defective and it took a week of headscratching to sort it out. The problem was the tubes as I originally thought.

Sometimes that's how it goes🤷‍♂️

[jameslane]

Very good video.I think the best one on hum I have ever seen.Jimmy USA

[pgw]

Great video as always Stuart! on a classic amp from Marshalls early 100W history

[hookydoo]

This hum issue can be so frustrating! I remember an instructor in an electrical class I was in years ago speak about how just the most minute length of wire +/- can cause EM crosstalk, Rf transmission, receiving, etc.. I have also in my own experience, been able to cut a few inches off of a guitar cable to stop it receiving radio signal. I have never seen something like you are dealing with here though. Wow! the same ground lug producing a different potential from one side to the other. That’s a head scratcher for sure. Great job getting that to a reasonable level.

[edwardhannigan]

Another excellent repair..Hum is definitely a pain in the !!!..Great explanation, and info...Thanks for sharing..Ed..Herts..uk..😀

[ILTXGunslinger]

That amp is in incredible condition (physically) for its age!

[MreViewer]

What a beauty, a mint '65 Plexi, worth a small fortune! Good work as usual Stuart

[robertg]

This channel is so great and interesting! Thank you for sharing. Wish you were in the states. Have the same amp but reissue with the same hum 😂

[ferraridinoman]

Another great informative video young Stuart!! (God! you must sleep at night dreaming of all these damn rotten amp problems!!) Thanks!

[jedishaw]

Very enjoyable video!! I am an amplifier nerd!!!

[DrWatts-bijv]

I can tell you from experience that a poor connection between the pots and the chassis will cause an earth hum.
Simply undoing the pots and doing them up again can shut a hummy sixties Marshall up.
Sounds like nonsense, but it's a fact - and I do an awful lot of work on these old things.
Beautiful amplifier though Stuart.
I also always change the Bias caps because as you've said, if the voltage goes down, the Bias goes up and all hell breaks loose lol
Original valves would've been 6L6 or EL34.
You can tell what valves it should have in it by the value of the screen R's.

[LPCustom]

Hard to tell, but if it’s an early Aluminium chassis, they were dodgy! Glad you sorted it! Cheers Stuart! Those had KT66 valves in them. That’s probably from 1965, in 1966 they went to EL34’s..

[silasfatchett]

An easy way to determine the value of resistors in parallel using a calculator. Clear the memory, enter the value of R1, take its reciprocal, and add it to memory. Enter the value of R2, take its reciprocal, and add it to memory. Recall the memory, take its reciprocal, which is your answer. This works for any number of resistors in parallel.
For two resistors in parallel, the formula simplifies to Rtot = R1 x R2 / (R1 + R2), known as the ‘product over sum rule’. Incidentally, both the above methods result in 45.9 Ohms.