new jtm45 build trouble

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new jtm45 build trouble

Postby tooloud » Sat Oct 13, 2012 11:00 am

PT 1202-55 Marstran
OT 784-103 Marstran
getting B+ 365 standby switch
getting 388 at pin 3 of power tubes
bias current at 1 ohm resistors 180 power tubes will start to
replate after short time.

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Re: new jtm45 build trouble

Postby David B » Sat Oct 13, 2012 12:24 pm

Are those caps in the bias circuit backwards?
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Re: new jtm45 build trouble

Postby tooloud » Sat Oct 13, 2012 1:55 pm

yes David they were in backwards forgot those are in that way......problem solved........... Thanks
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Re: new jtm45 build trouble

Postby David B » Mon Oct 15, 2012 4:45 pm

Easy fix, good news…
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Re: new jtm45 build trouble

Postby dai h. » Mon Oct 15, 2012 7:56 pm

nice solder joints! 8)

FWIW, might want to rearrange the green grid wires (for the first two tubes) a bit so they are in more of a "T" relationship relative to the heaters (upper part of the 'T' being the heaters along the back, and the center of the 'T' the grid wires in the air and heater wires flat against the chassis to maximize capacitive coupling). (If you don't have a hum/buzz problem then please disregard.) Also, the heaters for the preamp tubes don't require as much current as the power tubes, so a narrower gauge should be fine (and this is easier to twist and shape as well). Also, a wire (or wires) across the back of the pots makes sense IMO from the perpective of preventing hum and instability in case a pot works a bit loose (i.e. provides a redundant connection that doesn't leave a big piece of metal around the signal floating). Additionally, a separate AC safety ground connection to the chassis (as seemingly is usual practice in commercial equipment) may help to lessen the chance that you (or some other serviceperson) forgets to reinstall if say the rectifier socket is removed or replaced for whatever reason. (My nitpicking aside,) it's a very nice build!
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Re: new jtm45 build trouble

Postby David B » Mon Oct 15, 2012 8:21 pm

Dai,

Great recommendations particularly the separate ground to chassis from the AC port.

WTZ up?

David
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Re: new jtm45 build trouble

Postby dai h. » Tue Oct 16, 2012 12:53 am

hey David, not much going on, esp. on the amp front. Still leading a boring life. No major quakes (fingers crossed). :mrgreen: :lol:
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Re: new jtm45 build trouble

Postby David B » Tue Oct 16, 2012 7:02 pm

Been slow here as well, good to hear the ground isn’t shaking much..
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