High voltage at PI in 5E3 **SOLVED** thanks sluckey

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stelligan
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High voltage at PI in 5E3 **SOLVED** thanks sluckey

Post by stelligan »

Have an old Deluxe head(not vintage) I was revisiting and voltage on one of the PI plates is about 90 volts over expected. Pin 1 on V2 reads at 251v while all other voltages in the amp appear in range. I have checked the 100k there and it read fine but replaced it anyway. Have swapped the tube and no change. Next stop is tube socket. Amp sounds OK but nothing that couldn't be gutted or modified :) . The 100k doesn't seem to be doing much to voltage there - only 2 volts drop. Any ideas y'all?

The B+ feeding the 56k and 100k there is 253v.
Values are:

pin 1 251v
pin 6 210v
pin 7 16v
pin 8 44v

Pic for reference....
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Last edited by stelligan on Thu Aug 17, 2017 10:57 am, edited 1 time in total.
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Re: High voltage at PI in 5E3

Post by sluckey »

Measure resistance from pin 3 (probe directly on socket) to chassis. Is it the expected 1500Ω?
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Re: High voltage at PI in 5E3

Post by stelligan »

sluckey wrote: Wed Aug 16, 2017 3:29 pm Measure resistance from pin 3 (probe directly on socket) to chassis. Is it the expected 1500Ω?
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Re: High voltage at PI in 5E3

Post by Stevem »

I don't know that a 200 ohm difference in the Cathode value will make a 90 volt difference , but it's easy enough to change out !
How good the tube is will make a big difference even though you have tryed another tube in that socket the question is how good is that tube?
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Re: High voltage at PI in 5E3

Post by stelligan »

Stevem wrote: Wed Aug 16, 2017 5:44 pm I don't know that a 200 ohm difference in the Cathode value will make a 90 volt difference , but it's easy enough to change out !
How good the tube is will make a big difference even though you have tryed another tube in that socket the question is how good is that tube?
Will try yet another tube. Swapped in a resistor that measured 1.47k and made no significant difference.
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Re: High voltage at PI in 5E3

Post by sluckey »

What dc voltage do you measure on pin 3?
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Re: High voltage at PI in 5E3

Post by stelligan »

sluckey wrote: Wed Aug 16, 2017 6:20 pm What dc voltage do you measure on pin 3?
Whoa, Nada! low mV range..... And now I am silent and was playing it last night before the solder storm...
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Re: High voltage at PI in 5E3

Post by sluckey »

That proves there is really no current flowing through the tube. Are you sure that 1500Ω resistance on pin 3 to ground is 1.73K and not 173K or even 1.73Meg? And you didn't cheat by just measuring across the resistor leads did you?

Same goes for the 100K resistor on pin 1. Measure resistance directly from pin 1 to the B+ side of that 100K. Don't cheat by measuring directly across the resistor leads. Measuring from the actual tube pin will tell if a wire is broken between the resistor and the socket.

If your readings are correct for pin 1 and pin 3 resistance checks then pull the tube and check resistance from the top side of the socket for pin 1 to B+ and pin 8 to ground.

If all the above read correctly then measure resistance from pin 2 to ground. Should be about 500K (two volume pots in parallel). If open or very high check ground connection to the pots.

If it ain't fixed by now post some good pics showing the wires between the socket and the board.
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Re: High voltage at PI in 5E3

Post by stelligan »

sluckey wrote: Wed Aug 16, 2017 8:21 pm That proves there is really no current flowing through the tube. Are you sure that 1500Ω resistance on pin 3 to ground is 1.73K and not 173K or even 1.73Meg? And you didn't cheat by just measuring across the resistor leads did you?

Same goes for the 100K resistor on pin 1. Measure resistance directly from pin 1 to the B+ side of that 100K. Don't cheat by measuring directly across the resistor leads. Measuring from the actual tube pin will tell if a wire is broken between the resistor and the socket.

If your readings are correct for pin 1 and pin 3 resistance checks then pull the tube and check resistance from the top side of the socket for pin 1 to B+ and pin 8 to ground.

If all the above read correctly then measure resistance from pin 2 to ground. Should be about 500K (two volume pots in parallel). If open or very high check ground connection to the pots.

If it ain't fixed by now post some good pics showing the wires between the socket and the board.
Thanks for all the help! Greatly appreciated. I didn't cheat on pin 3 but, did on pin 1 - plate. Power tubes and chassis were becoming hot so - I called it for the night/day.....
I work nights so, had to get to sleep. Now breakfast and work. Will recheck when I get home in the morning.

Will post after 7am CST
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Re: High voltage at PI in 5E3

Post by stelligan »

Home early and back to fixing this thing. PROBLEM SOLVED!! Special thanks to sluckey. Bad ground at volume pots was the issue. Voltages back to normal and all is well. Funny how I tend to focus on the real estate near the problem and stay there flogging the dead horse. As always, you guys are the greatest. I still have a lot to learn and that's the reason I hang around. :roll:
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