JTM 45 Clone - PI Cap Error????

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sixstringesq
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JTM 45 Clone - PI Cap Error????

Post by sixstringesq » Sat Feb 20, 2016 8:57 pm

So, I purchased a JTM 45 clone from a great builder who will remain nameless because his amps are usually flawless (I have several others).

However, this amp has such a boomy bottom. Now, I know this circuit is known for its absurd low end, but there is so much boom even with the bass dialed down to 0.

I opened her up to check out the circuit and I everything was as it should be except the PI cap. It's a black dipped silver mica cap and the value says 100!!! (which I'm surmising means 100pf). That's seems absurdly high given that stock should be 47pf.

Could this be to blame for my boomy low end?

I've attached pics. This is a wrong value, right? I'm not crazy, right?

Image
Image

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bill bokey
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Re: JTM 45 Clone - PI Cap Error????

Post by bill bokey » Sat Feb 20, 2016 9:37 pm

sixstringesq wrote: Could this be to blame for my boomy low end?
No.
I doubt you'd hear the difference between 47pF and 100pF fizz cap but it only has to do with (extreme) high frequencies.

sixstringesq
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Re: JTM 45 Clone - PI Cap Error????

Post by sixstringesq » Sat Feb 20, 2016 10:53 pm

Thanks for the reply Bill.

I guess I'll have to experiment with the various mods to try and reduce the boom. I know there are a lot of threads on this topic, but has anyone come up with a good solution without comprising the tone too much?

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Re: JTM 45 Clone - PI Cap Error????

Post by Roe » Sun Feb 21, 2016 4:46 am

reduce coupling caps AND increase filtering slightly. Try ca 5nf on v1 and a 1nf phase inverter intro cap (instead of the 22nf connected to the treble pot)
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neikeel
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Re: JTM 45 Clone - PI Cap Error????

Post by neikeel » Sun Feb 21, 2016 8:34 am

Replace the 330uF on V1 with 4n7, that will keep the flavour but be less boomy.
Neil

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