Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DanielG on February 25, 2020, 08:18:26 pm
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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?
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yes
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fwiw;
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
read it many times, always find another useful nugget :icon_biggrin:
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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.
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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!
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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.