|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
192.94.94.105
In Reply to: Why do I have DC on my cathode-biased input tube's grid? posted by Alexo on April 27, 2007 at 08:39:54:
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.
Follow Ups:
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!
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!
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.
_______________________________
Long Live Dr.Gizmo
This post is made possible by the generous support of people like you and our sponsors: