JTM45 presence squeal

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casey73
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JTM45 presence squeal

Post by casey73 » Thu Sep 14, 2017 12:13 am

I recently completed a JTM45 build on a pre-made JTM45 style chassis and JTM45 pre-made turret board. It's fairly true to the original but I used 1K 5W screen resistors instead of 470 ohms. and 5.6K grid stoppers on the KT66 output tubes so I can run EL34s also. I took my time and it came to life when powered up. Used premium parts and was meticulous about layout, etc.. I've built 20+ tube amps over the years but this was my first JTM45. I ran my leads from the turret board to the pots as direct as possible, not dropping them down from the board to the chassis and up to the pots. It sounds like the classic JTM45 I've heard on records and read about.

However, when I use the high treble channel or jumper it with the normal channel, I start to get a high frequency oscillation when I turn the Presence up past about 7. On another forum a few JTM45 owners (not builders, but owners of old JTM45's) said this was not unheard of with JTM45s.

I found that moving the lead from the 250pf cap to the treble pot lessened the squeal. At that point I stepped away from it. I worked on it for 4 months in the evenings and have found stepping away from a troubleshooting situation for a few days tends to help when I return to it.

Since this is a fairly JTM45 specific site I thought I might find some tips specific to this amp to help me resolve this issue. Any insight is appreciated.

Roe
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Re: JTM45 presence squeal

Post by Roe » Fri Sep 15, 2017 11:24 am

Try shielding the bottom of the cab and to reflow the soldering joints
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danman
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Re: JTM45 presence squeal

Post by danman » Sat Sep 16, 2017 9:12 pm

How did you route the wire from the nfb resistor to the presence pot? I know George was very specific in his kit instructions about how to route this wire to avoid noise. It should come out from under the end of the board before heading to the presence pot. If ran to close to the other tone pots, it can introduce noise into the circuit.

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