JTM45 Inductance

Get support and show off your MetroAmp JTM 45 kit builds.

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chopperferrari
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Joined: Mon Mar 04, 2013 6:35 am
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JTM45 Inductance

Post by chopperferrari » Wed Apr 27, 2016 4:40 pm

Hi Chaps,

So after wrestling with my JMT45 build for the last few years to solve the overwhelming bass problem ( just turning the bass pot down, with any of the cap mods i've tried just doesn't get me there) I stumbled across a post on another forum indicating that the real culprit is the transformer not providing inductance; with a suggestion to try KT88s or swapping out the transformer for a 50watt model and move to EL34s. Do either of these ideas hold water with you guys?

Here is the post "The main problem is the output transformer for this amp never had enough inuctance..original Drake or aftermarket...The inductance dictates the low frequency response... The amount of inductance you need coresponds with the plate resistance of the tubes you use in parallel with the plate load of 3.4K.... The higher the plate resisitance and load resistance, then the higher the inductance you need to mainatin low frequency response, Since you can not vary the inductance of the transformer, you can however lower the plate resistance to obtain better low frequency repsonse... This requires change of tubes with lower plate resistance... Try KT88 or 6550 Valves..... This is exactly what a PARK 75 is...a 50W Marshall with KT88 valves and no screen resisitors....same circuit, same transformers....But mucho nice and more controled low frequency response..."

If I go down the transformer route i've found that David Bray Amps seems to do a very similar mod JTM 45/50 Hybrid Mod/Rework I http://davidbrayamps.com/modifications.html. Has anyone embarked upon this journey? I'm guess it would be getting the 50watt OT, EL34s and change the screen resistors to 1k...

Anyway a bit a bit of rambling above - i'd love to get your thoughts

Thanks,
Ben

chopperferrari
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Posts: 37
Joined: Mon Mar 04, 2013 6:35 am
Just the numbers in order: 13492

Re: JTM45 Inductance

Post by chopperferrari » Thu Apr 28, 2016 1:06 pm

Anyone? :D

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