Jtm45 choke getting pretty hot

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menger66
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Jtm45 choke getting pretty hot

Post by menger66 »

I Got some new kt66 for my jtm45 build today after a good 2 or 3 hours of playing and tweeking I felt the tranys for heat PT and OT were fine just warm to the touch but the choke was very hot not burn the skin but definitely to hot to hold in my hands, it's a CT jtm45 clone, I haven't had a lot of exsperance with choke in bigger amps don't seem right that the chokes so hot is this commen? thanks Mark
Stevem
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Re: Jtm45 choke getting pretty hot

Post by Stevem »

They must be strong 66s that are pulling more screen current then what you had in there before, and unfortunately the only way to adjust for it is by increasing the valve of each screen resistor' or a Meater choke.
If it's getting that hot, it will die and while running hotter will be dropping more voltage than it should .
If your inclined to I would up the screen resistor by 50 to 100 ohms.
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Littlewyan
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Re: Jtm45 choke getting pretty hot

Post by Littlewyan »

Which choke is it? 3H or 20H? The 3H Choke can take 250mA and the 20H can only take 70mA.

What Screen resistors have you got at the moment? Some JTM45s had 1K resistor shared by both KT66s but then a 470Ohm Resistor for each KT66 as well. So each valve would see 1.5K coming off the Choke. So as Steve said it would be worth upping the screen resistors if you've only got 470Ohm resistors in there and no 1K resistor. If you can't add the 1K in then I would just up the 470Ohm resistors to either 1K or 1.5K.

But before you do all that it would also be worth measuring the voltage drop across the choke when the amp is running flat out and then working out the current draw. Then measure it across the screen resistors as it could just be one KT66 is a bit dodgy and is drawing too much. In which case you need to send them back.
menger66
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Re: Jtm45 choke getting pretty hot

Post by menger66 »

In using the 20h choke screen r are 1.5k on each socket, may have the other CT choke on hand I know I have a extra trainwrek choke and a ac30 choke that are both rated higher than 70ma, , amp sounds fricken fantastic now kt66 are Chinese and sound really good too my ears. Amp is a little bass heavy but easily adjusted with bass control.
menger66
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Re: Jtm45 choke getting pretty hot

Post by menger66 »

With lower screen voltage is 20h 70ma choke rated hi enough for the jtm45? I thought this choke
Was specific for this circuit.
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M Fowler
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Re: Jtm45 choke getting pretty hot

Post by M Fowler »

For all my Marshall JTM45, JCM800 2204/2203 and my Hot Rod Plexi I use Magnetic Components 40-18058 choke.
JTM45 Choke.pdf
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tictac
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Re: Jtm45 choke getting pretty hot

Post by tictac »

Screens shouldn't have the current draw to make a properly spec'ed choke hot....

Maybe you should check your wiring to make sure the KT66 plates are on the "upstream" side of the choke....

TT
menger66
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Re: Jtm45 choke getting pretty hot

Post by menger66 »

Found issue I forgot the 15l R that goes along bottom of bias supply between the two reversed 10 100 e caps effectively losing 15k of dropping for the screens, also forgot the 47 mica thought I amp sounds great not sure I truly need it.
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