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| | |-+  JTM 45/100 problems- new member
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Author Topic: JTM 45/100 problems- new member  (Read 2988 times)
StuartC
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« on: February 04, 2013, 04:24:10 PM »

Hi, I'm new here, but thought this would be the best place to get advice about my jtm45/100.

I bought the amp already built from its original owner a few months back and am not happy with the sound. For a start the bass has a flubby/farty sound that I've heard described here, so I know the possible mods for that. But on listening to YouTube demos of other jtm45/100's, and in particular those from the guy I bought it from, realise that my amp has none of the 'glassy' or chimey sound that all the others have. The original owner was in an AC/DC band and there's no way I can get the amp to sound a bit like AC/DC!!!
Can anyone suggest what may be wrong???
I'm taking it to my amp tech next week to check valves, bias and voltage etc but any suggestions are welcome.
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wyatt
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« Reply #1 on: March 24, 2013, 12:04:07 PM »

This reply is 6 weeks late, but the forum moves at a glacial pace.

If the original owner got chime form the amp that you aren't, the most possible causes would be speakers and pedals before the amp.

Before anything else, I'm going to address the AC/DC-JTM45 myth.  Angus and Malcolm didn't adopt JTM's until the Ballbreaker album, rather late in their career. At the time, they publicized the JTM extensively in interviews and the band was quickly, and somewhat erroneously, associated with JTM45's.  Malcolm's main amp for his whole career was a Marshall Superbass and then the JTM45/100 was introduced for Ballbreaker and usually he uses both. Angus has gone through a series of amps, but the most famous albums were mostly recorded with '70's JMP's, with Wizards and even Mesa preamps used a lot in the '90's, and then a JTM45 for Ballbreaker; in the studio I believe he favored 50-wat amps so he could crank them easier. So, if you are expecting Hell's Bells and Highway to Hell out of a JTM45/100, you are asking a lot out of a low-gain, high-output amp.

You can certainly tighten it up by swapping some of a large .1 coupling caps for a .022, and can consider the "split-cathode" design of the '68 and later amps, where the first gain stage is treble boosted by a lower value cathode cap instead f the larger cap that boosts everything.
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StuartC
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« Reply #2 on: March 27, 2013, 03:58:27 PM »

Thanks for the reply. Turns out all the valves were crap!!
The pre amp valves were all below 50% efficient (even the new ones that I had put in!), and all the kt66's were losing their vacuum pretty badly.
I've just put new Gold Lion KT66's in it and Sovtek 12ax7's, which has improved it a bit, but not loads.

The glassy tone seems to be there when the master vol is full (and therefore out of the circuit) but not had anywhere suitable to get any overdriven tones like that as its way too loud!!!
It's on eBay and Music Radar classifieds at the moment as I'm not sure I can be bothered to mod it until I get what I want, as I can't do the work myself.
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