>R. Measures wrote:
>
>>>you can safely operate *any* tetrode in class AB1,
>>>cathode-driven. The requirement is that the control grid has the correct
>>>negative bias, and the screen has the correct positive voltage. These
>>>voltages are exactly as recommended by the manufacturer for class AB1,
>>>grid-driven.
>>>
>>>For cathode drive, both grids must be grounded to RF. But in order to
>>>apply different DC voltages, at least one grid cannot be connected
>>>direct to chassis - it has to be RF-grounded through a bypass capacitor.
>>>Because the recommended voltages are measured relative to the cathode,
>>>all three power supplies (anode, screen and control-grid bias) generally
>>>have to 'float' relative to the chassis potential.
>>>
>>>The correct DC bias voltages ensure that all the DC "grid" current goes
>>>to the screen, and *none* to the control grid... in other words, correct
>>>class-AB1 operation.
>>
>>** Ian -- Is the power gain with cathode-driven and no g1 current the
>>same as with 'super-cathode' drive -- i.e., c. 5?
>
>No.
>
>In 'Single Sideband Principles & Circuits' by Pappenfus, Bruene and
>Schoenike (aka the Collins company) they first describe GG triode
>operation, and quote typical power gains of 7 to 20.
>
>They then continue: "Tetrode tubes can also be used in cathode-driven
>operation. The power gain is considerably higher, and is in the 20-to-50
>region."
** Ian -- I too once thought this was the case, but a friend proved it
is not by experimenting with a 4-1000A tetrode amplifier. The reason the
power gain is not: "considerably higher, and is in the 20-to-50
region."
is simply that when the grids are RF-grounded, and the cathode is driven,
the driver must be able to supply the relatively large cathode current on
the negative half of the driving cycle. However, when the grid of a
tetrode is driven, the current is less because the only current flowing
is in the grid terminator.
- For example, the grid-driven tetrode amplifier I am currently
collecting parts for: The driving current needed to supply the grid
terminator-R is (160w/50-ohms) ^0.5 = 1.8A. However, if the cathode of
this tube were driven instead, the driver would need to supply c. 5 A to
the cathode. Since P = I^2 x R, the difference in driving power needed
is nothing to sneeze at.
- Humans make mistakes. I make mistakes and even some blunders. Had
Pappenfus, Bruene and Schoenike tried driving a tetrode via the cathode -
like my friend did, they would have no doubt discovered that when a
tetrode is not grid-driven, it acts like a g-g triode. The reason is
that g1/g2 amplification does not occur unless driving potential exists
between the grid and the screen, with the screen and cathode at the same
RF potential -- i.e., screen-cathode RF-grounded.
>
>They then consider semi-cathode-driven, super-cathode-driven and the RF
>feedback arrangement used on the 30S-1.
>
>It's well worth reading.
** I have a schematic diagram. The power gain of a stock 30S-1 is
reportedly c. 9.5. In AB1 grid-driven configuration, instead of
cathode-driven configuration, the power gain of its 4cx1000A could be 50x.
>
>>- If g1 is never to be driven more positive than the cathode potential,
>>why not connect it to the cathode?
>
>It's all in that book.
>
** Indeed.
cheers
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