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
>
>
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