[AMPS] Triode conected Tetrodes

[km1h@juno.com]

On Tue, 07 Jan 1997 12:30:42 -0800 Arthur Moe <kb7ww@chatusa.com> writes:
>
>While looking through the ARRL hand book (1979) I came across an
>article for an 833a tri band amplifier. In the first part of the
>article the author says 
>
>"Pi-network value are given for various surplus tubes: 4-400A, 
>4-250A,
>304TH and 833A.
>
>THE  CIRCUIT
>
>The power tube is used in the grounded-cathode configuration. If the
>tube to be used is a tetrode, the screen and grid should be tied 
>together to simulate a triode. The 0.01-u F disk capacitors from each
>cathode terminal to ground provide a low-impedance rf path,
>effectively by-passing the cathode to ground. Grid voltage is 
>developed across a low resistance, R1, which coincidentally maintains
>a constant load impedance for the exciter and prevents instability."


That is real mumbo jumbo Art. First of all the 4-250/400 is a completely
different tube than the 304TH/833A and simple Pi-Network tables can not
be interchangably used between tubes designed for different classes of
operation when the end goal is a Class AB1 linear for SSB.

For reasonably clean IMD a tetrode may be run grid driven, grounded grid
or grounded screen. There are pros and cons galore with all three methods
but they are useable.



>
>Now for some questions:
>
>Can other tetrodes be used the same way? i.e. 4-1000, 4CX3000,
>4CX5000,
>4CX10000


Sure, with equally poor results.


>
>How do you determine Bias voltage for a triode connected tetrode? 


Most popular tubes have been covered for years in articles. A good rule
of thumb is to set the idle current at 50-66% of rated dissapation to get
in the ballpark. A simple way to do that in GG service is a reverse
connected string of 1N5408 diodes in place of the ubiquitous zener and
tap off at  0.6V intervals until it works.
In grid driven service an adjustable bias supply is required. 


>
>In what range would you except the IMD to be?


With that ARRL idea?  maybe -15 to -20 dB 3rd


>
>Is there any problems with this type of amplifier?

MANY

>
>Thanks for any and all answers.

Your welcome

73  Carl  KM1H




>
>Art
>KB7WW
>
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>

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