Ground reference

[mashaffer]

In This circuit how is the grid referenced to ground? I must be missing something vital.

mike

[tubewade]

Greetings Mike,

The link takes me to a page full of information and other links. Which circuit are you referring to?

Wade

[mashaffer]

Sorry, brain fart. This one is the basic circuit. How is it biased with the grid floating like that?

mike

[ak_47_boy]

Rk will bias the grid. Looks like it needs a grid drain resistor from the grid to ground, usually about 500k to keep the grid from floating.

Unless there is an interstage transformer, but then that coupling capacitor would not be needed.

[Tweeker]

Grid leak would bias it (cutting it off if theres no leakage) were Rk not there.

Grid to ground of about 10M is common in grid leak biased circuits. Partial grid leak bias is possible.

[Miles Prower]

Originally posted by mashaffer
Sorry, brain fart. This one is the basic circuit. How is it biased with the grid floating like that?

It's not. Someone was drawing an "anode follower" and simply forgot to add the grid DC return.

There is another possibility, but they were sloppy in the schemo: the grid DC return could possibly be via a second stage grid resistor, since this is a capacitor coupled design. It it's just one stage (as in a line stage) the output end would require a paralleling resistor to provide a DC return.

[mashaffer]

Yes, it is supposed to be a CC stage with local feedback but it seemed to me that we needed a grid resistor from the junction of Rf and Rg to ground. However, since I don't really understand grid leak bias I thought that there might be something there that I was missing.

mike

[tubewade]

Originally posted by mashaffer
Yes, it is supposed to be a CC stage with local feedback but it seemed to me that we needed a grid resistor from the junction of Rf and Rg to ground. However, since I don't really understand grid leak bias I thought that there might be something there that I was missing.

mike

Mike you are right. There needs to be another resistor, either where you stated, or in the following stage. The stage would then be operating in self-bias, where the voltage necessary to bias the valve is developed across the cathode resistor. The grid resistor serves to hold the grid at essentially ground potential. This is not grid leak bias.

For grid leak bias, if the grid of the triode is connected to common through a large resistor, say 5M or 10M ohms, a negative voltage will build up on the grid. This is caused by electrons colliding with the grid wires and giving the grid a negative charge. Grid wires are pretty small, but they are large enough that electrons do accidentally run into them. The large resistor allows this charge to leak off slowly and electrons are constantly striking the grid so a steady DC voltage builds up there. This voltage is enough to bias the valve without any resistor in the cathode. This type of biasing was common in the first stage of microphone inputs on PA amplifiers and in the first audio stage of table radios. Of course, the chief advantage to this setup is economy; a working amplifier stage with very few parts. The chief disadvantage is relative poor bias voltage control and no practical way to measure it.

Hope this helps your understanding a little.

Wade

[Michael Koster]

I'm using this circuit in a microphone preamp and I put the DC reference on the "input end" of Rg to keep it out of the Rg/Rf circuit (I use Rg/Rf as a gain control element to avoid the issues with global loop feedback.)

As mentioned, DC return could be through the following stage Rg or interstage, or input transformer, etc. but it's just not shown in the schematic fragment.