Should a 12ax7 with no bias resistor pass signal?

[DanielG]

I was using a resistor box to find a new bias value for my sunn model t build, in an effort to diagnose a weird noise, and the behavior was not at all how i expected.

As i increased the value of the bias resistor up to like 400k or more (insanely high) the signal would get quieter and quieter, which makes sense.
However, as I decrease the bias resistor close to 0, the signal would not really change significantly.

So, should the tube even be able to pass a signal with a bias of essentially an ohm or two?

[sluckey]

yes

[shooter]

fwiw;

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

read it many times, always find another useful nugget 

[tubeswell]

So, should the tube even be able to pass a signal with a bias of essentially an ohm or two?

In the attached circuit the cathode is grounded. There is a small charge on the grid - indicated here by the battery symbol, although the presence of a battery isn't necessary to achieve this if there is a grid leak/load resistor present (or if there is a -ve bias supply voltage present). A grid leak resistor will (to some extent) provide bias voltage up to about 1V, due to the presence of a small amount (a few nano amps) of grid current through the resistor.

[DanielG]

Thanks for the replies!

I forgot all about grid leak, and even as I was writing the question, the idea of a tube diode was running though my head.

Diagnosing the weird issue with the amp has had my head spinning off into all kinds of crazy theories!

[Colas LeGrippa]

No.


If the grid has no ref to gnd, signal won t pass.
Measure the k voltage. If 0 volt even with k resistor, signal won t pass. The cure is a grid leak to the corresponding grid.