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 things to know about grid chokes:
- Grid Chokes were originally used in very first radios, in the days where resistors were expensive and unreliable. Also this was the perfect remedy against grid current of historical tubes, which was quite high, for so called low vacuum tubes, from before the invention of the getter.
- Later they were used in HiFi amplifers, to provide low impedance (DC) grounding of the grid, and at the same time provide a high (AC) impedance to the signal.
- Rumble filtering because very low frequencies can not pass a parallel connected choke
- Eelectrical safety when using a no capacitor input. In case the tube grid does short to the anode, or cathode can also habe 100V with some tubes, this DC voltage gets shorted..
- No problems anymore with grid current of power tubes, and no more risk for severe off-bias (and tube damage!) in case of severely overload.
- Grid current 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. But how good is a random tube, and why not use it anymore just because a good working tube has some grid current. Take a grid choke, and you can still use it. The advantage of a grid choke over a grid resistor is the perfect protection of the tube against grid current, coming either out of the tube itself, or it is generated by overdriving the tube. Both ways for getting grid current can happen, and the best protection for expensive tube is a grid choke in that case.
- 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.
- For more details, read the application note.
- As you see, there are only advantages. Some also say a grid choke simply sounds better.
datasheet
|
Idle current |
Henry |
5mA |
810H |
|
0,8mA |
540H |