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

[rlm]

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 at 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/