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causes of low voltage on preamp tube plate???

3/26/2006 7:43 PM
John L.
causes of low voltage on preamp tube plate???
I'm having probs with an ampeg jet II. I posted below but no one has offered help yet. I was wondering what some possibles might cause low dc voltage on 12ax7 preamp tube plate. I switched out the filter caps, to no avail. Any Ideas??? Thanks.  
John L.
3/26/2006 8:37 PM
Billy R

One cause can be that the tube is drawing too much curent, which causes a higher than normal voltage drop through the power supply or plate resistor. Measure the voltage at the plate supply, and the plate (both sides of the plate resistor) with the tube out of the socket, and in the socket. One cause of the tube drawing too much current can be the grid is not connected to ground through a resistor. Do you know what the voltages are supposed to be, and how much lower than that is it?Does the amp work? What is the cathode voltage?
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3/26/2006 8:46 PM
Rick Erickson

To add to what Billy R said:  
Another cause of low voltage is one or more resistor(s) that have drifted out of tolerance. Check the power supply dropping resistors and plate resistors for upward drift, and cathode resistors for downward drift. Also check cathode bypass caps for leakage or shorts.  
3/27/2006 8:17 PM
John L.

Hi Billy,  
Thanks for your response. I do know what the voltage should be, and it is currently about half of what it should be. The amp does work. All the controls function as they should. The amp just isn't putting out as it should. It sounds like maybe a 3 watt amp instead of a 15 watt. I will try what you said and check.
3/27/2006 9:05 PM
Don Symes

Another part of a thought is that the Cathode cap may be shorted, which would also act like a loss of grid bias.
3/26/2006 9:32 PM
John D

Another idea. I had that kind of problem trying to fix a Traynor. What I finally figured out was that the pins were not making solid contact. Very gently I bent each pin in slightly and voila the voltages were where they belonged.
3/28/2006 7:35 AM
Wild Bill

What is the plate voltage?  
What's the voltage drop across the plate resistor?  
What's the voltage at the cathode pin?  
---Wild Bill
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