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Old 23rd December 2004, 07:54 AM   #1
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Default Knowledge needed. Need answer to basic yet puzzling question.

In searching for the best method of emulating a triode power tube using a inexpensive (widely percieved to be inferior) pentode or tetrode I found myself trying to think of a good answer to a question that occured to me. Why do you need to connect the screen and suppressor grids to anything. Why not just connect the tube as a triode using only the signal grid, the plate and the cathode. I know that there is a very practical and logical reason for not doing it but it is eluding me. Anyone in the mood to answer a basic tube theory question and help me reduce my acute case of ignorance and bafflement to a merely moderate one?
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Old 23rd December 2004, 08:42 AM   #2
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If you leave electrodes disconnected, they will gently charge up to a potential simply because of the capacitance between themselves and nearby (connected) electrodes. The thing is, that potential may not be the potential you want, so you define it by connecting to electrodes of your choice, thus avoiding undefined potentials from adversely affecting the valve's operation.
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Old 23rd December 2004, 08:45 AM   #3
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I see. Since they are going to have a potential whether you want them to or not you make the best of it by deciding and controlling what the value of the potential is. Thanks.

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Old 23rd December 2004, 08:51 AM   #4
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Indeed,

and added to the post of EC8010, do not forget that in a lot of amps the penthode is connected as a triode by connecting the screen grid to the anode (with a resisitor).
The suppressor is often internally connected to the cathode, so leave it as it is.

Dick.
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Old 23rd December 2004, 09:23 AM   #5
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Quote:
Originally posted by djmiddelkoop
Indeed,

and added to the post of EC8010, do not forget that in a lot of amps the penthode is connected as a triode by connecting the screen grid to the anode (with a resisitor).
The suppressor is often internally connected to the cathode, so leave it as it is.

Dick.
Thanks for the reply and the info. Unfortunately I found that out the hard way when I installed a set of EH6CA7s in my amp. No harm done just a redundant conductor between the suppressor and the cathode. I haven't noticed any overtly bad results.
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Old 23rd December 2004, 10:43 AM   #6
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The screen grid voltage affects the cathode current just like the control grid, only to not such a great extent. If the screen is was disconnected I would imagine the cathode current would drop to a very low level.
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Old 25th December 2004, 06:17 PM   #7
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Quote:
in a lot of amps the penthode is connected as a triode by connecting the screen grid to the anode (with a resistor).
I regulary do this in my amp driver stages....in push pull with a common cathode resistor......but watchout with resistor value doesn't exceed the screen grid current. The g2 will only handle a few mA. It's a damned good way to get a linearized stage......from unmatched tubes.

<pentode gain is too high and unpredictable and decent triodes too low>.
Okay one can use anode to g1 feedback to tame a penny but then one lowers the input Z and reduces thd......... reducing input Z isn't aways my desire as the previous stage then has greater loading and increases the thd.

Anything else ?
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