Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Heavenamps on June 16, 2018, 08:28:59 pm
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I built a little single ended 5F1 circuit and noticed that the plate voltage on the second stage of the preamp was 325V. After probing around a bit I noticed there was only about a 5v drop across the 100k plate resistor. I couldn't spot any short anywhere which leads me to believe that the second stage of the preamp tube isn't drawing the current it needs. I've tried multiple tubes with the same result. I've checked my connections and everything looks ok. Any ideas?
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with power off and V1 out;
ohm pin 3 and 8 (tube side) to chassis
post
ohm 2 and 7 to chassis (tube side), (Volume fully CW)
post
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3 and 8 both show 1.5k each, and pin 2 shows 1Meg and pin 7shows an open circuit no matter where the volume and tone knobs are. I should've mentioned that I added a tone knob.
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this is the schematic I'm looking at;
try this, power OFF, caps discharged AND gator clipped + to Ground;
measure pin 1 and 6 to ground (tubeside)
DON'T forget to un-ground the caps before power-up !!!!!!
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Yes that is a more accurate schematic. Pin 1 measures ~134k and pin 6 measures ~138k
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pin 2 shows 1Meg and pin 7shows an open
Pin 2 should measure about 33K with nothing plugged into the input jacks ***IF*** the jacks are wired like the schematic. Pin 7 should measure zero ohms with volume on zero or 1meg with volume at max. Check the ground on the volume pot.
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So I plugged everything in again and measured it, and for whatever reason it measured normal. I plugged in a guitar to try it out and it made kind of a loud crackly chirp and the output got quieter. I measured the plate voltage again and it's back up to 325V
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The second preamp triode cannot work properly as long as pin 7 remains open. Fix that and the plate voltage will probably become normal.
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Thanks both of you for your wisdom! Turns out I mixed up my tone and volume pots and wired them backwards. I straitened that out, and the voltage does now indeed appear to be normal. Thank you again!