Advice needed: triode grid becomes negative by itself

[MohiZ]

Hey!

I've been trying to get a tube overdrive of my own design working with an ECC83 (12AX7) tube. I've been reading the articles from http://www.freewebs.com/valvewizard/

Let's focus on just one stage. The problem is that even with the most basic triode stage possible the grid becomes negative by itself although I have an input cap and a grid leak resistor going to ground. When using cathode biasing, the grid should be at zero volts, shouldn't it? So there seems to be some grid current.

I've tried different resistors on both the cathode and the plate. The only way I can get the grid closer to zero is to use a smaller grid leak resistor (like 100k or even smaller), but that lowers the input signal. I can't even use a cathode resistor because that would make the grid-cathode voltage even more negative. The circuit does basically work but it just annoys the hell out of me that I can't get the grid at zero volts. Could it be because of the low power supply voltage?

My supply voltage is 38 volts DC, plate resistor for instance 100k, cathode connected straight to ground and a grid leak of max. 1M. I've checked the voltages at the cathode (0V) and plate (about 28 volts with a 100k plate resistor).

PS. Damn English is hard, I know the words and I know the grammar but it's just really hard to naturally phrase my thoughts into sentences. 

[Mike Burgundy]

I don't think you should worry about it, actually, I think it's normal.
Lets see if I get this right - the Cathode is heated and starts to boil off electrons. These are drawn towards the positively charged Anode. This current is regulated by a grid in between, charged negatively compared to the cathode. It basically goes "shoo" to more or less of the electrons trying to get through to the anode, depending on the -voltage of the grid. This changes the current between anode and cathode (or flow of electrons the other way) and we have an amplifier.
Most electrons repelled by the grid are hovering  in a sort of cloud between cathode and grid. SOME however were on their way to make it through but hit grid thus giving it an extra minute negative charge. This is neglectable at high plate voltages since the anode pulls really hard, and the flow is comparatively massive (LOADS of electrons) plus the electrons do NOT want to go there because of the negative charge.
If I remember correctly, with starved plate conditions it's a LOT easier for electrons to actually hit the grid and get trapped, so the grid will get a negative charge all on its own.

Edit: ...and your English is fine

[Ripthorn]

Also, I seem to recall hearing that the 12ax7 doesn't like a starved plate as much as, say, a 12au7.  I don't have a reference, it just seems that I read it or heard it someplace.  I could be way off there though.  Also, I don't think you have to worry about the grid voltage all too much, and I think Mark is right.

[MohiZ]

Thanks for the answers! I'm going to try it with high voltage sooner or later anyway, so I'll see what kind of an effect that has. Is it a good idea to use a cathode resistor? The output is the highest without one, at least with this power supply. I'm guessing having a higher voltage power supply would create the need for a cathode resistor.

[Ripthorn]

You should probably have a cathode resistor, but it can be small.  Values like 470R or 820R give very high gain while still maintaining a good bias.  Not having one gives (in my opinion) a little mushy of a sound and can cause some other tonal and response issues I think.  I just know that for a tube boost I designed, I have my gain pot on the cathode and have a 470R resistor in series, so that max gain is a 470R cathode resistor because without it, the sound goes bad at max gain (0 ohms).

[Mike Burgundy]

Yes, a cathode resistor is definitely needed for "normal" voltage operation. What value depends on the voltages and desired behaviour. Aikenamps.com has brilliant info on designing triode stages.
At low voltages, the starved plate apparently self-biases through the negative grid effect. This won't work for higher voltages, it will run *very* hot (100V will exceed max anode current if I'm not mistaking) and badly - full cutoff for one half of the signal cycle.
For starved plate having no cathode resistor is fine, don't do it on higher voltages. There's a pretty defined range of values for a given set of voltages in which the tube will behave nicely, see Randall Aiken's page for details. You can make the tube misbhave quite excessively, take care not to exceed any max values (plate current and such) and experiment away.
You can bypass the cathode resistor with a cap to get some more gain out of the stage, if that helps.

[yertle]

You could always put a bypass cap parallel to the cathode resistor to have max AC gain and you can also shape the freq. response with different values for that cap. BTW, why do you wan't the grid voltage to be zero? That would almost make a half-wave rectifier of your gain stage, or is that what you're aiming for? I just started with tubes a while ago and came across the same 'problem', I just couldn't understand how the valvecaster (and other circuits with a cathode directly to ground) could amplify a signal without completely cutting of the negative side of the waveform. After I read the articles on the valvewizard side alot of these things became clear to me.

[MohiZ]

Thanks again, so it seems the negative grid effect is indeed enough to bias the valve at such low plate voltages. I don't necessarily need to make the grid voltage zero, I just thought that there must always be a cathode resistor, and with a resistor the grid was getting too negative compared to the cathode. But if I'll leave the cathode resistor out or use just a very small value, it'll be fine. I think the 12ax7 tube would sound better with higher plate voltage though. I'll also have to check the Aiken amps web page!

[MohiZ]

Sorry for bringing this up again, I just wanted to say that I found the problem I was having. Maybe this'll help someone someday. 

By putting in a cathode resistor the grid voltage actually rose very close to zero. So it seems the tube was trying to bias itself without the cathode resistor and that's why the grid was negative. With a larger cathode resistor I was able to get the output signal nice 'n clean, with more headroom, but smaller output. Thanks to everyone for the help!