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Hey guys, i've finally finished a matchless lightning build and I'm not getting any sound. I did notice that after some voltage checks on the pi3&8 of the first preamp tube I get a reading of 517vdc? On pin 3 of the second preamp tube the voltage is high as well,560vdc? pin 8 of the second preamp tube is -17.5 vdc which seems better to me. so what can be causin g the first tthree readings to be so high and what should i be lookin at in terms of voltage on the cathodes here?
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I am having a real hard time believing those voltages. That this little amp has 560v anywhere sounds fishy, and then -17? There should be no negative anywhere in it. It has no bias supply, so where would it come from?
So I first ask, where did you ground your meter for these readings? Then I wonder if you did not include all the ground connections in the circuit, so voltages can float.
You mention 560v on pin 3 of the second tube and -17 on pin 8, but those two pins are wired together!!!
Look at your power transformer wiring. The high voltage wires go to pins 4 and 6 of the rectifier tube and the center tap is grounded. If you grounded one of the ends instead and then connected the other end and the center tap to the pins 4 and 6, it would result in double the voltage on the B+. Your caps might last a little while, but...Education is what you're left with after you have forgotten what you have learned. -
First, are these readings with no tubes in the amp? Then:
How does your amp differ from the schematic?
Are you using a diode rectifier?
What are the specs for your PT?"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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pins 3 & 8 on the first preamp tube are connected together. They are not on the second tube,where pin 3 * 8 are wired seperately. I have the ht connected to my rectifier correctly and the ct grounded. I may as you mentioned , have missed a ground connection in the circuit. I will re-check alll wiring. I took my voltages from the preamp tubes with the ground probe grounded on the chassis and the positive probe to each pin. i will re-check everything again ans report back. I have built about eight of these amps and never had this kinda problem before. I'm getting 400vdc on the B+ and 356v on the output trany's leads going to pins 9 of the power tubes.Originally posted by Enzo View PostComment
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Aside from the weird voltage readings, make sure resistance from pin 3 & 8 of first tube measures 1.5K to ground.Originally posted by Enzo
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lightning
Well guys, I think i may have found my problem, an open choke. I don't have another on hand so what size of power resistor can i use in it's place?Originally posted by chuckb View PostComment
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i do have 1.5k to ground on p3&8.Comment
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An open choke should give you a zero volt reading, not a higher volt reading. It's possible that whatever is causing your high voltage opened the choke.Originally posted by chuckb View Post
Is your amp different from the schematic?
Are you using a diode rectifier?
What are your PT specs?"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." HelmholtzComment
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No 500V or anything even remotely close anywhere on that amp so you have made some GROSS wiring error.
Sorry.
Start by checking zero ohms between chassis/transformer center taps (both filaments and HV)/22&33uF HV filter cap negatives (all 6 of them)/grounded end of all resistors and capacitors.
Save your .pdf schematic as a .gif so you can edit it adding labels to all parts (R1, R2, .... V1A ... etc.) and repost it here, to avoid confusions.
No precise suggestions can be done without that.Juan Manuel FaheyComment
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Maybe try a 470 or 1K, 5W.Originally posted by chuckb View PostOriginally posted by Enzo
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SOrry, somehow I saw the PI as the second tube.
400v on the B+ makes it extra hard to get 500v anywhere. Did we scope this to make sure it isn't cranking out RF?Education is what you're left with after you have forgotten what you have learned.Comment
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If the choke is open, then all those weird DC readings were just some kind of measurement glitch.Originally posted by Enzo
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