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Title: Strange noises from my JTM45 Post by: Damian9645 on June 16, 2008, 05:39:43 AM My new JTM 45 is, what I consider, fairly noisy. That is, a lot of hiss over 1/3 to 1/2 volume.
I have experimented with several preamp valves with little sucess. (My other amps are not noisy with the same guitars, plugged into the same outlets etc.). I can make the amp generate a high pitched whistle if I turn the Presence knob up full. Not just hiss and not feed back type squeal, I mean a whistle. The Presence pot is a little scratchy too. Any thoughts? Title: Re: Strange noises from my JTM45 Post by: cmoore on June 17, 2008, 12:15:19 AM Who built the amp? If not Ceriatone, can you gut shoot the chassis? A picture is worth a thousand words. And four pictures would be worth....
Title: Re: Strange noises from my JTM45 Post by: Damian9645 on June 17, 2008, 12:27:22 AM I got it made and mounted in a cab by Ceriatone. The only option they fitted was an external bias pot. It is very touchy too.
(The power valves are biased to 38mA and 40mA.) I can take some photos, what are we looking for? Title: Re: Strange noises from my JTM45 Post by: bloodedge on June 18, 2008, 03:58:44 AM take some clear picture of the board/internal wiring.
i think that is what he is looking for :) so he can see if there is anything wrong. Title: Re: Strange noises from my JTM45 Post by: 72sg on June 19, 2008, 08:46:32 PM The JTM45 "presence squeal" issue is well known over at the Metro forum. IIR, it's a shielding solution and many folks over there report good results from using a layer of flashing material under the chassis. My Bluesbreaker squeals if presence and volume are both over about 8; I don't have the additional shielding.
The presence is much stronger than other amps I've used, and it can really add sizzle, chime, etc, and can dramatically increase perceived volume. The preamp stage seems VERY sensitive. With presence and mids off, it's surprisingly Bassman-like. Conversely, with mids at 10 and treble off it's pure Marshall. As for noise, mine seems pretty well behaved. I don't get noise as early as what you describe. Not even close! Even with 6db of clean boost I don't get excessive noise. Title: Re: Strange noises from my JTM45 Post by: cmoore on June 20, 2008, 06:15:45 PM 72sq may be right on the money. We have a Wreck clone that had a Presence problem (among others). You might want to just try increasing the value of the Presence cap and see if that will give you some relief.
Good Luck Title: Re: Strange noises from my JTM45 Post by: Damian9645 on June 30, 2008, 04:15:42 AM I replaced the noisy Presence pot, did some additional shielding, moved some internal wiring and the problem is all but gone. The squeal is only there at way higher levels of presence than I ever use. Thanks for everyones help.
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