Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: clyde on May 22, 2020, 02:39:53 pm
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Princeton AA964 build, my first in awhile due to relocation and health issues. On a Filmosound chassis, I had this issue previously and left it for awhile, redid the power supply and it fired up this morning, beautiful. I changed out tubes and now I have 498v throughout the amp, plates, screens preamp everywhere. I'm thinking something isn't grounding, internal to the PT? The PT is Filmosound original 360-0-360, but with a 5Y3 it was almost dead-on to factory AA964 voltages earlier today.
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Are you sure that when you changed the tubes that you didn't stick in a 5AR4 by mistake?
Dave
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That number would indicate that none of the tubes are conducting. It is as though you have no tubes installed.
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Princeton A964 schematic show PT 340-0-340 volts. You have 20 volts over on both side.
And what is the current (power ) of this PT ? For Princeton you need no more than 70 ma
You PT could be too powerfull and may fit with Tremolux or Vibrolux who need same voltage with 180 ma ?
Or 6V6 bias is too cold ?
I often use zener diodes to reduce B+ voltage when I built amp from scratch.
Zener diodes for reducing B+
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage
https://el34world.com/Forum/index.php?topic=25996.0
https://el34world.com/Forum/index.php?topic=17595.0
https://www.tedweber.com/v-dump
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That number would indicate that none of the tubes are conducting. It is as though you have no tubes installed.
+1
Or Power supply see no tubes. ?
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Are the heaters lighting up?
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And what heater voltage are you reading?
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Heater volts are fine, bias voltage is fine. 500v more or less at every B+ node, dropping resistors do nothing.
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Heater volts are fine, bias voltage is fine. 500v more or less at every B+ node, dropping resistors do nothing.
Then suspect you’ve missed a critical signal ground connection. With your R-meter, check for DC-continuity between each (lead of every component* with a) ground return lead, and the chassis. You’re looking for open readings
*including all the filter caps, grid leak resistors, cathode resistors and volume pots etc
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What Tubeswell said..what is your grounding scheme? First check I would make would be from the cathode pin of each tube to ground.to the chassis, not your ground point connection. Do with the power turned off to the amp.
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Thanks guys, I suspected it was grounding somewhere. This is not my first rodeo by far. I'll ground every connection by clip lead and get back to you. Darn!
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Heater wire was floating around in one of the pins loose, made contact just often enough to piss me off. After a long layoff I've lost my edge. Thanks everyone!