Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Snake5150 on November 24, 2024, 05:55:51 pm
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Hello all.
I have started my jtm 45 build working thru rob rinettes startup instructions using a variac and lightbulb limiter . I have the rectifier tube installed and the pre amp tubes. My questions is , in the start up instructions he states
f the light bulb dims with all the preamp tubes installed then measure the voltages at all the preamp pin tubes. Nine pin preamp tubes' pins 4 & 9 and 5 & 9 will have 5.9 to 7v AC heater voltage and the pins 1, 3, 6, 8 will have DC. Normal preamp gain stages should have voltage on their plates and cathodes but not on their grids. Phase inverters will have voltage on their grids. Cathode followers may have high voltage on their plates, grids and cathodes.
Well my lightbulb has never lit at any point , and I know it works because I created a short and checked it.
Tubes lit, but seem dim. Although I’m not super familiar with 12ax7s.
I have heater voltage as indicated, but some of the other voltages were weird. Which I know a limiter can cause. When measuring pins 1368 am measuring between the pins, or the pins to ground?
I have approx correct voltage at all the caps and the ot secondary’s to ground.
I do have a lack of bias range tho if measuring between the pot legs .
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what watt bulb are you using?
is it incandescent or halogen, or God forbid an LED?
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what watt bulb are you using?
is it incandescent or halogen, or God forbid an LED?
I google good enough to not do that. lol I started with a 150 and now am using a 100 watt incandescent. It’s a 45 watt, jtm 45. I read approx double the wattage is usually right .
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the amp shouldn't draw much current if wired properly, an amp maybe with heaters.
what are the PS (power supply) VDC values?
what is your VAC reading pin 4 to 5 Pins 5 to 9 and pins 4 to 9
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the amp shouldn't draw much current if wired properly, an amp maybe with heaters.
what are the PS (power supply) VDC values?
what is your VAC reading pin 4 to 5 Pins 5 to 9 and pins 4 to 9
My amp has pins 4 and 5 tied together on the preamps. All those measures between pin 9 are 6.28 vac. Idk we’re to measure power supply vdc. It’s a pacific audio magnetics 345-0-345 , 690v ct 150 ma.
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on this schematic, measure "A" to chassis ground.
you'll find "A" in the upper right, or middle bottom.
if it's about what you expect, remove the lightbulb limiter from the circuit, re-measure heaters VAC and "A" again, post the numbers.
*****KEEP power tubes OUT til bias is dialed in*********
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_readable.pdf
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on this schematic, measure "A" to chassis ground.
you'll find "A" in the upper right, or middle bottom.
if it's about what you expect, remove the lightbulb limiter from the circuit, re-measure heaters VAC and "A" again, post the numbers.
*****KEEP power tubes OUT til bias is dialed in*********
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_readable.pdf
My caps are wired a lil different. I was advised to wire the resivor cap before standby.
I have referenced the schematic you sent for most of the build .the altered cap layout and original layout attached for reference .
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on this schematic, measure "A" to chassis ground.
you'll find "A" in the upper right, or middle bottom.
if it's about what you expect, remove the lightbulb limiter from the circuit, re-measure heaters VAC and "A" again, post the numbers.
*****KEEP power tubes OUT til bias is dialed in*********
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_readable.pdf
440 vdc at A
6.28 vac at all pin 4,5-9
Idk if that’s too high or not to continue the test without the limiter.