I agree the "flapper" tuning cap works quite well in lieu of a small value
vacuum variable; but its adjustment is quite critical, and if a flapper is
used it should be with a gear reduction or some other method of making very
small adjustments easily. The beauty of a 2-30pF vacuum cap, of course, is
that it adjusts in very small increments easily without any further gear
reduction or drive mechanism...usually a few shaft rotations only changes
capacitance by 1-2 pF when the cap is used in its nearly "open" position.
-WB2WIK/6
"Success is the ability to go from failure to failure with no loss of
enthusiasm." -Winston Churchill
> -----Original Message-----
> From: Conrad G0RUZ [SMTP:conrad@g0ruz.net]
> Sent: Tuesday, February 18, 2003 2:55 PM
> To: amps@contesting.com
> Subject: RE: [Amps] 2, 3-400Z's on SIX? de VY1JA
>
> On 6m it becomes critical for efficiency to keep the loaded Q down. With 2
> tube amplifiers the output C is significant and it is difficult to get
> loaded Q's of below about 30. So I have found it useful on 6m to use
> 'flapper' type capacitors for C1, these need only be about 1-5pf , which
> helps keep the loaded Q as low as possible. You need to have the ratio of
> L
> to C as high as possible. Steve's values for C1 and C2 are spot on in my
> experience. These flapper type capacitors are very cheap indeed and if you
> put some PTFE between the plates will stand very high voltages, you can
> see
> examples here http://wd7s.home.att.net/gallery-2.htm and here
> http://www.nd2x.net/EA3AXV.html, two slightly different approaches but
> both
> work well. I prefer the WD7S method personally because I always feel
> uneasy
> about RF grounds going through standard metric brass threads, I like the
> big
> wide copper strap approach. Also because of the relatively high loaded Q I
> would use 10mm wide silver plated strap for L1, certainly nothing smaller
> than 6mm copper tube. I have built quite a few amps like this, mainly
> using
> the GS35B and it has worked very well for me. Not original but cheap and
> easily made.
>
> Regards
>
> Conrad G0RUZ
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
> Behalf Of Steve Katz
> Sent: 18 February 2003 17:23
> To: 'J. Allen'; amps@contesting.com
> Subject: RE: [Amps] 2, 3-400Z's on SIX? de VY1JA
>
>
> VY1JA: I'd use 3-500ZGs and not 3-400Zs in a new construction amp, since
> 3-400Zs are obsolete (unless you just happen to have some new ones). Even
> if building for the zero-bias 3-400s, leave sufficient space above the
> sockets for the full height of the 3-500 envelope and anode heat
> dissipating
> cap, plus some clearance, because eventually when 3-400s go the way of
> dinosaurs and only fossils are found, 3-500s will still be in use, and
> they're a taller tube.
>
> I've built several dual 3-500Z amps on six meters, they're easy to work
> with, stable and provide 10dB gain, or a tad more depending on plate
> voltage
> and circuit efficiencies. Using a vacuum variable 2-30pf plate tuning cap
> and a 200pf air variable plate loading cap, and 3 turns of 3/16" copper
> tubing wound on a 1-1/2" diameter form, the whole plate tank occupies
> little
> space and costs almost nothing to build except for the cost of the vacuum
> variable. I'd highly recommend the vacuum cap for tuning as almost
> anything
> else becomes finnicky to work with. (Mica compression trimmers for the
> input circuit work fine and aren't finnicky at all)
>
> 73 & happy building!
>
> Steve, WB2WIK/6
>
> "Success is the ability to go from failure to failure with no loss of
> enthusiasm." -Winston Churchill
>
> > -----Original Message-----
> > From: J. Allen [SMTP:jallen@internorth.com]
> > Sent: Sunday, February 16, 2003 11:08 PM
> > To: amps@contesting.com
> > Subject: [Amps] 2, 3-400Z's on SIX? de VY1JA
> >
> > Hi, All,
> >
> > I have been offered the opportunity to clean out a local's garage of amp
> > parts and good tubes. Since I have the AL-80A, I was considering
> > construction of a six meter amp with a pair of 3-400Z's.
> >
> > Most amps I see on six use small ceramic tubes. I checked the
> > inter-electrode capacitance ratings and it looks like the 3-400Z's are
> > actually lower in capacitance than the 400 Watt ceramic triodes (8874).
> >
> > Am I missing something here, or is it likely that I can build this amp
> > with
> > some time in the books, or possibly by finding a design out there
> > somewhere?
> >
> > I have built HF amps before and been successful, but VHF techniques will
> > be
> > new to me.
> >
> > Thanks... Please respond directly to:
> > jallen@internorth.com
> >
> > J.,
> > VY1JA
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
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