JTM30 Misconceptions & Confusions!

Discussion in 'Amps and Cabs' started by The Ballzz, Apr 26, 2015.

  1. The Ballzz

    The Ballzz Member

    Mar 7, 2015
    This thread is intended as an attempt to consolidate info concerning this well loved by many, and hated by some, amp. Sorry in advance for the length.

    FIRST) Many discussions lump the JTM30 & JTM60/601 together as nearly the same amp. This is not really the case. The design was quite different in many ways and each has it's own set of potential issues and/or problems. While the cosmetics, some components and monicker are shared, the similarities really end there.

    SECOND) The User's Manual, schematic and speaker output labeling are somewhat in disagreement and/or conflict or confusion with each other. It may even be that, depending on year of manufacture, the labelling on the rear of the amp may be a little different along with a possible different arrangement of how the "speaker out" jacks are wired. It is my considered opinion that this confusion, along with replacing power tubes with improper units, have largely contributed to failures and issues that have given these glorious amps a bad reputation. I will elaborate, imagine that!

    A) The schematic
    shows that there are actually separate taps from the output transformer for 16 ohm & 8 ohm operation. It alludes to being able to use the extension speaker output jack for a single 8 ohm speaker load, but (at least on my very early unit) this is not actually the case. You get no output at all when plugged only out of the extension speaker jack. This means that the only way to truly access the "8 ohm" tap is to have a 16 ohm load plugged into each of the two jacks! This leads to the problem that when you use only an 8 ohm external speaker, plugged out of the internal speaker jack, you are actually connecting an 8 ohm load to the 16 ohm tap of the output transformer! Not really a good thing! (Please note that the schematic linked above is DEFINITELY NOT the best one available for this amp. Notice where one of the leads to "CON5" on the PRE-AMP BOARD DIAGRAM comes from? That's just one example of the errors. There "IS" a better one out there , with the "invisible" lines filled in, I just don't remember where at the moment!

    B) The labelling on the rear of the amp (at least the pictured unit, as well as mine)
    along with the info in the "User Manual"
    would lead you to believe that you are OKAY plugging an 8 ohm or 16 ohm load out of the only jack that works by itself. This does not truly appear to be the case. I believe that this "piss poor" lack of clarification by Marshall has been a major contributing factor in many "perceived" difficulties with this amp model.

    In light of the two issues mentioned above, it seems that this particular version of this amp should "NEVER" be plugged into a single 8 ohm speaker load out of just one speaker output jack. I have never put a meter on mine to test, but it may be that the proper way (maybe the only way) to access the 8 ohm OT tap would be to plug an unwired "dummy" plug into the unused extension jack. As I said, I have not confirmed this "possible" work around yet, so don't take it as gospel truth!

    Now on to the power tube, lack of bias adjustment control issue:

    1) These amps were designed and factory, pre-set biased surrounding a specific power tube. This tube is only available as the Sovtek 5881/6L6WGC and has somewhat different specifications than others in both the 6L6 and 5881 families.

    2) The only way to keep these amps happy is to either use well matched pairs of those somewhat crappy Sovtek tubes or have the amp modified to allow bias adjustment for use of other tubes. The caveat here is that the output transformer is not very robust and by no means over specced and may or may not get over stressed by some other tube choices.

    My observations, as a 20+ year owner, user & lover of this amp, leads me to say that these are fantastic little amps when used and cared for properly! Most, if not all overheating issues experienced by many have been contributed to/caused by the points mentioned above. While many attempts to minimize the overheating have been posted/documented, (fans/venting/etc) they are all simply putting bandaids on a severed arm to control the bleeding! In other words, these "fixes" only address the symptoms and not the root causes of the overheating.

    Certainly, the addition of a fan or extra venting won't hurt the amp in any way, but they serve only to postpone inevitable failures when using improper speaker loads and/or power tubes! These amps are what they are and while they require some very specific usage guidelines to be followed, when those guidelines are strictly adhered to, they are fantastic little (Did I say little? Crankin' 30+ watts into a nice 4x12!) amps!

    Just My & Likely Worth Even Less,

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