Absolutely Steve, I should have mentioned that some form of gear reduction
should be used. I just use the 'cheap' Japanese vernier dials which have
adequate torque and backlash reduction to overcome the tension in the
flapper grounding strap. Of course small vacuum variables are extremely
scarce here in the UK. I would perhaps not be so keen on flappers if I was
in the USA. The flapper is very cheap, that can't be denied, a small NOS
vacuum variable at say 7kV would be in excess of $100 over here, even on the
surplus market. There were some cheaper ones around about 2 years ago but I
bought them all :-)
73
Conrad G0RUZ
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
Behalf Of Steve Katz
Sent: 19 February 2003 15:32
To: 'conrad@g0ruz.net'; amps@contesting.com
Subject: RE: [Amps] 2, 3-400Z's on SIX? de VY1JA
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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|