Re: [Amps] Re: Future availability of 3cx1200a7 and 3cx1500a7?

[rlm <r@somis.org>]

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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