• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

grid chokes

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Joel

Joel

Member
Joined 2002
joeltunnah.com
Tim, there are actually some benefits to using a grid choke. All tubes have a fractional amount of leakage current in the "grid circuit". The big old triodes have a bit more, and so you'll often see a limitation on grid DCR - and you'll see two numbers, one for self bias, and one for fixed - the value for fixed being a fifth lower usually. This is because in the self bias situation the tube can compensate for any change in it's grid-cathode voltage relationship, whereas in the fixed situation - it can't.
You might ask, what's the big deal? Well, even a small leakage across 1 meg can shift the bias enough to cause problems.

So.... we can use a choke between the grid and ground instead, and those teeny tiny currents will only see a few thousand ohms, instead of 500k, etc. Of course the AC signal sees the inductance of the choke as a high resistance, and away we go!
Make sense?

A Hammond 156C is a cheap $10 way to try them out for yourself.:nod:
 
T

Technical

Member
Joined 2004
The Hammond 156C as a grid choke?
Most applications I've seen use a billion billion henrys. Overkill?
I've been too lazy to do the design work required to become familiar with grid chokes, but I have some 156C's and it would be great if I can use them with some type 50's in S.E.

Joel said:
Tim, there are actually some benefits to using a grid choke. All tubes have a fractional amount of leakage current in the "grid circuit". The big old triodes have a bit more, and so you'll often see a limitation on grid DCR - and you'll see two numbers, one for self bias, and one for fixed - the value for fixed being a fifth lower usually. This is because in the self bias situation the tube can compensate for any change in it's grid-cathode voltage relationship, whereas in the fixed situation - it can't.
You might ask, what's the big deal? Well, even a small leakage across 1 meg can shift the bias enough to cause problems.

So.... we can use a choke between the grid and ground instead, and those teeny tiny currents will only see a few thousand ohms, instead of 500k, etc. Of course the AC signal sees the inductance of the choke as a high resistance, and away we go!
Make sense?

A Hammond 156C is a cheap $10 way to try them out for yourself.:nod:
Click to expand...
 
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