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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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26th Jul 2004, 12:00 pm | #1 |
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What is grid base (NOT grid bias).
In descriptions of valve circuits reference is often made to "grid base" such as short, long or restricted grid base. I cannot find grid base defined anywhere though. Does anyone know what it is? Some circuit descriptions link it to using a high value screen dropper resistor, so that small variations in grid voltage result in large changes of screen voltage. On the other hand triodes seem to have grid base as well.
Can anyone enlighten me? Thanks Graham Last edited by Paul Stenning; 26th Dec 2004 at 9:34 pm. Reason: Fix link or code for vBulletin |
26th Jul 2004, 12:31 pm | #2 |
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Re: What is grid base (NOT grid bias).
Hi Graham, I've understood it to mean the range of GB the valve was designed to operate over.
That is between saturation ( at any particular anode/screen conditions) and cut off. For audio use it may be more restricted to define the low distortion, linear region only. What does everyone else think? Ed |
26th Jul 2004, 2:36 pm | #3 |
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Re: What is grid base (NOT grid bias).
I think the same as Ed. I seem to remember something about it from the dim and distant past when doing the City and Guilds course. We set up experiments with signal valves and power valves and measured the different range of bias voltages that they operated over.
Rich.
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26th Jul 2004, 6:43 pm | #4 |
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Re: What is grid base (NOT grid bias).
It's the grid bias voltage at which the anode current starts to fall. A " short grid base " valve will start to become biased off at only a very few volts (the opposite of what is needed for an AGC controlled stage, where a variable mu valve would be used)
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27th Jul 2004, 12:05 pm | #5 |
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Re: What is grid base (NOT grid bias).
This explains a lot for me. Reading various books on WWII radar one mentioned an "obsolete valve with a short grid base" as being ideal for pulse circuits. Now I know why
The valve mentioned was CV1116 Cheers
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Chris C G8TJR Last edited by Paul Stenning; 26th Dec 2004 at 9:34 pm. Reason: Fix link or code for vBulletin |
29th Jul 2004, 4:19 pm | #6 |
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Re: What is grid base (NOT grid bias).
If you Google for "short grid base" it turns up just 4 hits including one to a radar theory pub and one somewhere in a PDF about Colossus. I must admit I'd not come across the term.
Last edited by Paul Stenning; 26th Dec 2004 at 9:35 pm. Reason: Fix link or code for vBulletin |
30th Jul 2004, 7:45 am | #7 |
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Re: What is grid base (NOT grid bias).
Hi all.
I knew I'd seen a reference to this so I consulted my 'bible' when such matters arise (The RSGB Radio Communications Handbook) and page 2.6 quotes: 'Sometimes valves are merely divided into two varieties -short grid base and long grid base- corresponding to the relative grid voltage required for cut-off, the name originating from the appearance of the Ia/Vg characteristics. So there you have it. Rich.
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30th Jul 2004, 8:41 am | #8 | |
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Re: What is grid base (NOT grid bias).
Quote:
The previous paragraph in the RSGB Book gives a very precise and understandable definition of Semi-remote cut-off and Remote cut-off valves in terms of grid voltage/screen voltage expressed as a percentage. This obviously refers to tetrodes and pentodes, not triodes. The definition of grid base refers to relative grid voltages required for cutoff. Relative to what? Are we talking about 1V versus 3V, 1V versus 12V or the difference between the cutoff and saturation voltages? There is also a reference to the appearance of the Ia/Vg or gm curve. Does this refer to the straight part of the curve, where gm remains constant, the curved part of the curve where gm varies with grid voltage or the whole of the curve? The mention of screen voltage is interesting. Text books seldom mention the fact that screen current and hence screen voltage varies with grid bias. The higher the value of the screen dropper resistor the greater the change in screen voltage. In fact a high value screen dropper resistor is recommended for a valve to which agc is applied. Presumably the grid base lengthens as the agc voltage and screen voltage change? Graham.
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30th Jul 2004, 5:47 pm | #9 |
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Re: What is grid base (NOT grid bias).
Hmmmm.....
Jeffrey (ppppenguin) where are you? Can you clear up the loose ends on this debate? Rich.
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30th Jul 2004, 6:47 pm | #10 |
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Re: What is grid base (NOT grid bias).
Sorry, valve theory is not my speciality. Need somebody like Rob Burman or Keith Thrower for a really authoritative view.
I think G4ILN and Leon Crampin have got it about right. |
31st Jul 2004, 12:03 am | #11 |
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Re: What is grid base (NOT grid bias).
I am pleased to say that I am now able to answer my own question.
From "A Dictionary of Electronics" 1962:- "GRID BASE. The value of grid bias at which cut-off occurs in a valve." From this I deduce that "short grid base" refers to a sharp cut-off valve and "long grid base" refers to a variable mu/remote cut-off valve. The curved part of the Ia/Vg line will obviously be longer for a variable mu tube. A higher screen voltage results in a longer grid base for a given valve. Using a high value screen resistor means that as the control grid voltage goes more negative due to the application of agc, the screen current falls, the screen voltage rises and the grid base lengthens. I'm not sure how this differs from keeping the screen grid voltage at a constant high value. It would be interesting to plot the two curves. Graham. Last edited by Paul Stenning; 26th Dec 2004 at 9:36 pm. Reason: Fix link or code for vBulletin |
31st Jul 2004, 6:48 am | #12 |
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Re: What is grid base (NOT grid bias).
Hi Graham.
Well that seems to be the end of the debate at last!! Very interesting thread. Seems to me that the Grid Base characteristic is probably only important when designing circuits or making subs of valves that are no longer obtainable. For normal repairs/servicing it's one of those things you don't need to worry about. It also explains why some valves work well in a particular position and others do not. I would presume that this also has something to do with the term 'High Slope' as in 'High Slope RF Pentode'. Rich.
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