Mistake
I made a goof. I'm so used to using a butt set to trouble shoot audio that I forgot its not capable of going very low in frequency.
Yes, I have hum on the output of the pre amp.
The following voltages were taken at the tubes
12AY7
pin 1 120v, pin3 1.9volt pin
6SN7
pin 1 66volt, pin2 243volt pin3 85v
the voltages were taken with respect to ground with zero signal and input unshorted.
any ideas?
Joe
I made a goof. I'm so used to using a butt set to trouble shoot audio that I forgot its not capable of going very low in frequency.
Yes, I have hum on the output of the pre amp.
The following voltages were taken at the tubes
12AY7
pin 1 120v, pin3 1.9volt pin
6SN7
pin 1 66volt, pin2 243volt pin3 85v
the voltages were taken with respect to ground with zero signal and input unshorted.
any ideas?
Joe
Joel
If you substitute a 47K resistor for the 470K value it will bring the voltages to 80 on pin 1 and 87 on pin3 with a supply voltage of 244 volts. If you raise the supply voltage to 284 it will raise the pin1 voltage to 93v and pin3 to 102. Is it possible that the 470K value should have been 47K? Are these operating voltages in line with what a 6SN7 should run? Did I kill my tubes by running too much bias? Help me here someone?
Joe
If you substitute a 47K resistor for the 470K value it will bring the voltages to 80 on pin 1 and 87 on pin3 with a supply voltage of 244 volts. If you raise the supply voltage to 284 it will raise the pin1 voltage to 93v and pin3 to 102. Is it possible that the 470K value should have been 47K? Are these operating voltages in line with what a 6SN7 should run? Did I kill my tubes by running too much bias? Help me here someone?
Joe
burnedfingers said:
1) No, it should not be 47k. 470k is correct. The reason the voltages change when you lower this value is you're making the grid more and more at the same potential as the cathode. You can see where this will lead by shorting the grid to cathode - what is the voltage at both? Why, the same, of course.
2) A 6SN7 can take 400V, so this is nothing to worry about. As close to 250V as you can get it is good in this circuit. Also, note the bias voltages increase when you increase the plate voltage because the current is dropping.
3) You can't "kill your tubes" by overbiasing - that turns them off. You can kill your tubes by UNDERbiasing - that increases current.
Without having it in front of me, it's hard to understand why we're having so much trouble here - but just get the B+ at 250V, and then play with the bias resistor value (470 ohm) until you get a differential of 8V between grid and cathode. Leave all the other values the way they are. Ok?
What voltages should I see?
Since you want me to keep the 470K and the 7.7 values and fudge the 470 Ohm value, what voltage should I see at the cathode and the grid with a given 250 volt plate voltage?
Just making a guess here... since you don't want me to change the 470K value there must be a ballpark level you want to see on the cathode and I must have violated it. It seems to me that changing the 470K to 47K raised the cathode voltage by 2volts and brought the grid voltage more in line with what you wanted me to have.
I'm going to find a very large beer to inhale.
Joe
Since you want me to keep the 470K and the 7.7 values and fudge the 470 Ohm value, what voltage should I see at the cathode and the grid with a given 250 volt plate voltage?
Just making a guess here... since you don't want me to change the 470K value there must be a ballpark level you want to see on the cathode and I must have violated it. It seems to me that changing the 470K to 47K raised the cathode voltage by 2volts and brought the grid voltage more in line with what you wanted me to have.
I'm going to find a very large beer to inhale.
Joe
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.