George,
Converting the capacitance numbers from grounded-grid
to grounded-cathode is relatively easy because Eimac
made the measurements in a shielded fixture, and
simply measured from tube element to tube element.
In a grounded-cathode measurement, Feedthrough
capacitance is anode-to-grid. In a GG amp, that
becomes output capacitance.
In a grounded-cathode measurement, output capacitance
is anode-to-cathode. That becomes feedthrough in a GG
amp.
In a grounded-cathode measurement, input capacitance
is grid-to-cathode. In a GG measurement, it is still
input capacitance.
So, after doing the conversions, the various flavors
of 3CX1200 end up with the following values:
A7: Input, 20pF; output 12pF, feedthrough 0.2pF
D7: Input, 17.6pF; output, 9.7pF; feedthrough 0.57pF
Z7: Input, 17.6pF; output, 9.7pf; feedthrough 0.8pF
On first blush, the A7 has the least feedback
capacitance. However, that's almost irrelevant.
What's more important is at what frequency does the
grid structure cease to effectively shield the output
from the input. That is a function largely of the
grid geometry inside the tube, and the low-frequency
capacitance measurement tells us almost nothing useful
about that structure.
There is a fair amount of anecdotal evidence
suggesting that the Z7 is superior to the A7 in terms
of keeping it from oscillating at VHF. However, that
results is obviously not due to reduced feedback
capacitance.
Whether a given tube oscillates in a given circuit
depends on a large number of factors. The mu of the
tube, the dimensions of the elements inside the tube
(which we are never given), the method of grounding
the grid and how the cathode is terminated out-of-band
are probably more important than the anode-to-cathode
capacitance. Input and output capacitances are
largely subsumed into the matching networks, so
they're relevant only to the extent that we can design
matching networks to accommodate them.
In engineering, it's always a big challenge to sort
out which items are important and which are not. And
it varies with what, exactly, we're trying to design.
73,
Dave W8NF
K6GT wrote:
Hi,
Rich always points out that the 3cx1200z7 has much
less feedback
capacitance than the 3cx1200a7, yet the (CPI) Eimac
reference data
pages say (and something here doesn't seem right!!)
a7 grounded cathode input=20pf, output=0.2pf,
feedthrough=12pf
z7 grounded grid input=18pf, output=9.7pf,
feedthrough=0.8pf
Does anybody have an apples/apples comparison? My
archives don't
have data for any of these "new" tubes.
73,
George T. Daughters, K6GT
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