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Why do I have DC on my cathode-biased input tube's grid?

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Good morning everyone, here is my conundrum:
I just fired up a build last night, first preamp stage is a paralleled 12AX7, 165 volts B+, 90 on the plate, 120K plate resistor, 680 ohm cathode resistor, 39K grid stopper, 1 meg from grid to ground.
So... it works, I guess, but there is about -.5 volts on the grid, consequently, -.5 volts on ehatever I plug into it. What would cause this? It is very annoying, there is also a good deal of hum coming through, and reduced gain as a result of this weird unwanted negative bias.
Any suggestions? All my wiring seems to check out... what gives?
Thanks!

Follow Ups:

Thanks everyone, - Alexo 13:12:37 04/27/07 (0)
I will call this one solved, DC is down to -.17 with the proper bias and I suspect it will drop much further once I replace the grid-leak resistor, have a great weekend, I really appreciate the help!
Re: Why do I have DC on my cathode-biased input tube's grid? - mikeyb 11:03:35 04/27/07 (4)
This means that the grid of your 12AX7 is drawing .5 uA of current. This seems rather high - have you tried a different 12AX7?
Take a look at the data sheet for a 12AX7 (page 3, below). Ralph is correct, the spec sheet (page 3) shows a recommended Rk value of 1800 Ohms with a supply voltage 200 volts and a plate resistance of 100K. Look at the distortion level - 4.8% at 20 VRMS. 'It would be nice' if you had a higher supply voltage - 300 Volts or so. That would bring the distortion down.
39K for a 'grid stopper' seems really high - they are usually in the 100 Ohm region.
- 12AX7 data sheet (Open in New Window)
- Thanks for the link, but... - Alexo 11:29:36 04/27/07 (3)
  This is a paralleled triode, so having a 120K plate resistor and 680 cathode resistor amounts to 60K plate load and 1360 ohm Rk's.
  So looking at the datasheet you posted, closest design is Vp 200, Rp 47K, Rk 1K5. Whopping 8.5% distortion! Yikes!
  So I had .4 volts on the tied cathodes, which works out to .58 ma, or .29 ma per triode, which is about a third of the .89 listed on the datasheet. Is this another indication of a bad tube?
  ...fwiw, a priest told me the tube tested good, yes, a priest, who sells tubes on ebay in his spare time... hmm, maybe I will try another when I get home tonight!
  - Hmm actually... - Alexo 11:39:22 04/27/07 (2)
    Isn't having a 120K Rp ion a parallel triode like having 240K on each one? I've been thinking about that backwards... no wonder it's biasing so cold!
    - Actually, actually... - Alexo 11:42:52 04/27/07 (1)
      Looking further down the datasheet, if you use a 220K Rp at 200 Volts, you need a 3.3K Rk, and bias to .36ma, so I am actually right on target for an "appropriate" bias.
      - And put in a 200K Grid Leak Resistor (nt) - Chris O 13:02:57 04/27/07 (0)
        
        In Reply to: Actually, actually... posted by Alexo on April 27, 2007 at 11:42:52:
        
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        Long Live Dr.Gizmo
Thanks... - Alexo 10:53:27 04/27/07 (0)
I went ahead and replaced the 620 ohm resistor for 1K8 and now DC is down to -.17 from -.4, really can't fathom where a negative voltage is coming from.... alleviated the problem a bit but bias is a bit cold now, getting 125 at the anode and 178 at the other end of the 120K plate resistor.
I agree that the hum is probably unrelated, yet equally mysterious since I stuck to good grounding practices, but I will get that sorted later.
Re: Why do I have DC on my cathode-biased input tube's grid? - Chris O 10:53:20 04/27/07 (3)
What voltage do you have on the tubes cathode? Or in other words, what is your tube's plate draw?
Have you tried replacing the 12AX7 with another sample?
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Long Live Dr.Gizmo
- Re: Why do I have DC on my cathode-biased input tube's grid? - Alexo 11:05:43 04/27/07 (1)
  With the 620 ohm resistor:
  Rail voltage: 163
  Plate Voltage: -.6 (think I put -.4 in my other post, it was -.6)
  Cathode: .4 volts
  With 1K8:
  Rail: 178 (rose due to an unrelated circuit change)
  Grid: -.17
  Cathode: 1.06
  - Gah! Errors! See the corrections here: - Alexo 11:07:21 04/27/07 (0)
    With the 620 ohm resistor:
    Rail voltage: 163
    Plate Voltage: 92
    Grid Voltage: -.6 (think I put -.4 in my other post, it was -.6)
    Cathode: .4 volts
    With 1K8:
    Rail: 178 (rose due to an unrelated circuit change)
    Plate: 106
    Grid: -.17
    Cathode: 1.06
- Re: Why do I have DC on my cathode-biased input tube's grid? - danlaudionut 11:04:44 04/27/07 (0)
  Chris
  If you run the numbers you'll get .625mA.
  I agree the tube is suspect.
  DanL
Re: Why do I have DC on my cathode-biased input tube's grid? - Triode_Kingdom 10:18:07 04/27/07 (0)
The bias voltage is the (normal) result of grid leak. If it's a problem, replace the 1M with 270K-390K. You might also want to replace the grid stopper with a value closer to 5K. Incidentally, the hum and gain issues may not be related to this.
Re: 680 ohm resistor is wrong! - Ralph 09:34:23 04/27/07 (0)
Years ago Audio Amateur published an article called the 'Last PAS'. The update article was about doing an over-the-top update to the PAS 3, which uses 12AX7s in the line stage.
The article had a 680 ohm resistor in the cathode- a mile off from the correct value. Given a 120K plate resistor, I would suspect a 2.21K to 2.74K cathode resistor will work much better!
If you want the exact value, obtain a 5K pot and install it in the cathode circuit. Then run a sinewave into the circuit and observe the output with an oscilloscope. Vary the resistance until the sine wave peaks. Then increase the value slightly to deal with tube variance. remove the pot carefully and measure it. Obtain a fixed value and install it- Bob's your uncle.

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