>
>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.
? Not if one wants to build a linear amplifier.
>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."
>
? However, tetrodes that are connected this way require large amounts of
grid current in order to draw moderate plate/anode current, are
virtually certain to oscillate without neutralization, and the results
are by no means linear. . . When the screen and grid are tied
together, one has little choice but to ground them and drive the cathode
>Now for some questions:
>
>Can other tetrodes be used the same way? i.e. 4-1000, 4CX3000,
>4CX5000,4CX10000
? No tetrode can be used in such a way to produce a practical linear
amplifier. Tetrodes are designed to be grid driven using a regulated
screen supply. They work best when neutralized and not driven into the
grid current region. However, only tetrodes that are designed for Class
AB1 will work well. The 4-400A and 4-250A were designed for Class AB!.
The 304TH and 833A were designed for Class C operation -- i.e., they
will only draw the rated current with substantial grid current, and that
makes ssb doo-doo in AB1.
>
>How do you determine Bias voltage for a triode connected tetrode?
>
>In what range would you except the IMD to be?
>
? My guess is. about -15db total IMD.
>Is there any problems with this type of amplifier?
>
? None -- if you don't mind taking a chance on getting tarred and
feathered at ham conventions.
>
- later, Art
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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