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Old 20th August 2012, 05:14 AM   #11
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by aristonuser View Post
You show the 100k resistor to ground before the 1K series resistor to the grid. ChrisA (in a subsequent comment) seems to suggest that a 1 meg resistor is connected direct from the grid to ground and a10k resistor is wired in series - between the input jack and the grid/100k resistor junction. So, I have 2 questions.

Why do you appear to wire your 2 resistors in reverse order from what ChrisA suggests? And, why are your values so different from his?

Just trying to understand.

(Sorry about the 1st messed attempt to communicate this question!)

Pete
It does not really matter. OK well maybe it does. There are two things we need to do

1) Connect the grid to ground. (called a grid leak resistor) To do this the order is not important at all. If the charge first flows through a 10K resistor then the 1M it is like a 1.01meg resister. And the value is not critital at all anything from about 470K to 2M is good. We use 1M mostly by convention. Some time we see volue control pots serving double duty as grid leak resistors. Any path to ground is fine as long as it is less than 2meg. A handy place to mount the grid leak resistor is directly across the input jack. This looks odd at first. Why wire a resistor across an RCA jack? You do it because it is easy and it fits there

2) We need series resistance for a "grid stopper" resistor. Electrically, in theory this can be any place but in real-world practice the best place is soldered directly to the tube socket with as short of a lead as possible. The reason is that wire acts like an antenna and the wire between the grid stop resister and the tube is "un-protected" by the resister. Soldering to the tube socket makes the antenna as short as possible.

So it does not matter except for making the "antenna" small.
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Old 20th August 2012, 05:15 AM   #12
dgta is offline dgta  United States
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Agreed. The important thing is to place the series resistor (grid stopper) as close to the tube as possible, usually right at the socket.

To elaborate on what Kevin said, when placing the stopper "before" the grid leak (resistor to ground), you are forming a voltage divider and you must consider the values. You will note that Kevin and Chris proposed different values, but in each case they are in the ratio of 100:1. Therefore there is negligible loss of signal either way. If, on the other hand, you chose your resistors as 10k and 100k, you would now lose 10% of the signal when placing the stopper before the leak. So you must pay attention to those implications.
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Old 20th August 2012, 05:25 AM   #13
ChrisA is offline ChrisA  United States
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Here are two good articles on the subject. The 2nd one at aikensamps talks about the order of the resisters and why we need them. the first on has some lite math. Read both to understand this. Note the grid stoppers are used on all stages, even the power tubes typically have them.

Another reason for them is to prevent oscillation. this is why you see them used in non-input stages too.

The Valve Wizard

Grid Resistors
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