Grid Choke
Introduction
There are only two such chokes in the program, but actually these are all you need. The 1667-05mA is a universal genius, but it is also large size. If the size is no issue, this is the one to take. However, the LL1670 is also a very beautiful piece, since the (grid ) current it can take is 800uA which is really more than you need normally, and it has convenient small dimensions still.
Some good reasons for using a grid choke:
- Grid Chokes were originally used in very first radios. They are the perfect remedy against grid current. This is is a small DC current, coming out of the grid. If you have 1uA and a grid resistor of 2mOhm, that means 2 Volt DC. With a 300B that means 10mA (about 12%) more plate current as you intended.
- A good tube has only a few hundred nA grid current only. If a tube has grid current, you may need to throw it away. With a grid choke, you may still be able to to use the tube normally.
- Rumble filtering because very low frequencies can not pass a grid choke easily.
- Electrical safety when using a "no capacitor" input. If the tube grid does shorts internally, lethal DC voltage can appear at the amplifier input. The grid choke will not allow DC voltage over it.
- A not so well known application is the 1:2 step up option with the LL1670. So use it as a auto transformer and as a grid choke at the same time. That gives a noise-free gain of 2x. It reduces the Inductance to 405 Henry, but that is more than you need still. Officially this is not recommended, but it may work. If you need a real step up transformer, you should better use a LL1570-XL. Same as with a grid choke you can ground the grid with it's secondary, but LL1570-XL is not very much DC capable. Unlike LL1670 and LL1667, which can take 0.8 or 5mA. So LL1570-XL can be used at the input of an amplifier only, but should not be used as a step-up grid choke (rather say: ...interstage transformer) for power tubes. Reason being power tubes can sometimes produce quite some grid current, and this damages such transformers, that are not specified for DC.
- Finally, many say a grid choke simply sounds better.
datasheet
|
Idle current |
Henry |
5mA |
810H |
|
0,8mA |
540H |
We have a nice PCB for the LL1670 Grid Choke
For more details, read the application note.
Read here for some measurements I did on LL1670