[Amps] 2, 3-400Z's on SIX? de VY1JA

Steve Katz stevek at jmr.com
Wed Feb 19 14:21:29 EST 2003 

Conrad,

If you ever visit the U.S., come to Los Angeles and I'll take you to the
local monthly ham radio swap meet (flea market).  You will think you've died
and gone to heaven....all sorts of vacuum variables available, including the
little 2-30pF glass and ceramic ones about 1" diameter, for typically $35 or
so ($USD) each.  Maybe less, if there's a lot of competition and several
sellers have them.

This may be because they were all manufactured in California, and thousands
have "fallen off the truck" over the years.....

73!

Steve, 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 at g0ruz.net]
> Sent:	Wednesday, February 19, 2003 2:01 PM
> To:	amps at contesting.com
> Subject:	RE: [Amps] 2,  3-400Z's on SIX?  de VY1JA
> 
> 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 at contesting.com [mailto:amps-bounces at contesting.com]On
> Behalf Of Steve Katz
> Sent: 19 February 2003 15:32
> To: 'conrad at g0ruz.net'; amps at 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 at g0ruz.net]
> > Sent:	Tuesday, February 18, 2003 2:55 PM
> > To:	amps at 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 at contesting.com [mailto:amps-bounces at contesting.com]On
> > Behalf Of Steve Katz
> > Sent: 18 February 2003 17:23
> > To: 'J. Allen'; amps at 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 at internorth.com]
> > > Sent:	Sunday, February 16, 2003 11:08 PM
> > > To:	amps at 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 at internorth.com
> > >
> > > J.,
> > > VY1JA
> > >
> > >
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