On Friday, October 24, 2003, at 09:33 AM, Dave Haupt wrote:
Rich,
Feedback capacitance is certainly a worthy
consideration. To accurately model the potential
trouble spots, it needs to be made a part of the tube
model which includes the transmission line
characteristics of the various concentric internal
geometries. These dimensions can be obtained readily
if a good relationship with the tube manufacturer is
maintained, or by sectioning a tube. I think
capacitance, and nothing else, is specified simply
because when tubes were "new" they only knew how to
measure capacitance. Today, with even relatively
simple tools, like the MFJ-259B, one can measure the
transmission line characteristics of the tube, and use
free "student version" software to rapidly and
accurately predict instability potentials in
amplifiers.
*** So why not write an article about parasitic oscillation for QST or
Communications Quarterly?
I would be interested in a scan of the pages in
catalog 175 you reference. My yellow Eimac tube books
are buried in storage, and I extracted my info on the
tubes from their website.
*** Catalog 175 does not list the 3cx1200A7 but it does list the
3-1000Z which uses the same filament and grid.
I've seen several comment on the 3CX1200A7 versus
3CX1200Z7, indicating that the Z7 is easier to tame.
However, the Z7, per Eimac's website, has 0.8pF of
anode-to-cathode capacitance, roughly 10 times that of
the A7.
*** yet another typo. The ...Z7 was reportedly designed to have less
tendency toward regeneration. I have an Eimac spec sheet for the
YU-181/3cx1200Z7 and the stated feedthrough C = 0.08pF. (JPEG e-mail
available)
>
If the Z7 is easier to keep from oscillating,
it would seem to be something other than just the
difference in capacitance.
*** both tubes seemingly have similar grid collet L, although the
...Z7's SK410 socket has less grid lead L than the A7's ancient-design
socket. // The ...D7 also uses an SK410 socket.//
Probably an assessment of the differences between the
tubes, focusing on lead length, would reveal that the
A7 has excess series inductance in the grid lead.
Best,
W8NF
--- rlm <r@somis.org> wrote:
On Thursday, October 23, 2003, at 05:06 PM, Dave
Haupt wrote:
Ed,
The difference between the '1200 and '1500 in
feedback
capacitance is only 2:1,
*** There is apparently a typo in Eimac Catalog
175. The correct
anode-cathode C for a g-g 3cx1200A7 or 3-1000Z is c.
0.2pF, so Dave
Haupt is correct. The anode-cathode C is 0.1pF for
a g-g 8877 (Eimac,
5/15/1977). The anode-cathode C (g-g) for a
3cx1200Z7 is 0.075pF
(Eimac #2209).
and both are very small,
indeed. While there are the occasional reports of
oscillations with the '1200, it is more likely a
result of the long element leads inside the tube,
and
not a result of the 0.2pF feedback capacitance.
**** Can 0.2pF of feedback be dismissed as
insignificant? '
- At a freq of 110MHz, 0.2pF has c. 7k-ohms of XC.
With an anode
supply potential of 4000VDC, the AC current through
C-feedback is
0.385A. In my opinion, it would not be good
engineering practice to
ignore this.
The
3CX1200A7 data sheet shows the in/out/feedback
capacitance when operated in grounded cathode, and
Rich appears to have not converted that data into
grid-referenced data.
However, the feedback capacitance information has
absolutely nothing to do with your original
question.
Were it my amplifier, I would choose the 3CX1500A7
immediately. I have seen the 3CX1500A7/8877 used
as a
driver in many broadcast applications, and as a
final
in many scientific applications. I have never
seen a
3CX1200A7 outside of a ham amplifier, ever. The
8877
is popular enough that, at least at one time, it
was
available from Chinese sources.
The 8877 is available frequently as a medical
pull
for $300 or so. WA8WZG usually has them offered
on
his website.
73,
Dave W8NF
Rich wrote:
**** The problem with the 3cx1200A7 (and the
3-1000Z)
is that it has
roughly 7x
**** this should read 2x as much ... ...
as much feedback-C (Anode-cathode C) as an
8877. Although
more feedback-C is beneficial in VHF oscillators,
it
can be a problem
in HF amplifier applications. To reduce
feedback-C,
Eimac came up with
the 3cx1200Z7. It has about 1/10 as much feedback
C
as the ...A7. The
cost is c. $20 more than an ...A7, it uses a 6,3v
filament, and it
requires a better-design socket, so it is not a
plug&play substitute..
cheers
On Tuesday, October 21, 2003, at 06:09 AM, Ed
Briggs
wrote:
I was wondering if it is possible to predict how
long the 3cx1200a7
will continue to be available from Eimac. I'm
considering buying a
new
amp, and pondering the future availability of the
3cx1200a7,
3cx1500a7, and of course the Svetlana 4cx800.
Is the 3cx1200 widely used? I know of two amateur
amps in production
with this tube, and I wonder if there are
non-amateur applications.
It seems the 3cx1500a7 is more widely used in
non-amateur
applications
and thus might be a longer survivor. Also, it
seems
to have a second
source.
Anyway, if anyone has guidance, particularly wrt.
the 3cx1200, I'd be
very grateful indeed.
Ed
N1TS
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Rich Measures, AG6K, 805.386.3734. www.somis.org/
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