For sure the stub (looking like a short to even harmonics) works best
when combined with some series inductance to work against. Hopefully the
load cap is already a low impedance to harmonics so the short circuit of
stub won't do so much when added across it - but it shouldn't do any
harm either, and appeals to my laziness compared with having to
manufacture a nice choke like you did.
Nice job on your build, and yes, the ASI amps are just gorgeous.
73, Steve G8GSQ
On 24/11/2022 14:32, jim.thom jim.thom@telus.net wrote:
I designed the entire RF deck. That was my original plan, to use a 1/4 wave
shorted stub.... and provide a DC short, + kill even harmonics. That
SDT-600 teflon coax has a .70 VF. The issue is, it has to be wired to the
vac load cap / NO contact of the pair of vac relays. It can't be wired to
the output connector. Works out to 3.38' in length (51 mhz).
The story I got from K9YC is..... IF it's a PI-L network, the 1/4 wave stub
is usually installed at the output connector...(on outside of amp). IF
it's a PI network, the 1/4 wave stub is ineffective as an even
harmonic trap. For PI networks, the 1/4 wave stub has to be located an
electrical 1/8 wave on 51 mhz... downstream from the load cap itself.
Since the load cap is mounted to sub front panel, and vac relays a 1/2"
away.... we have to factor in the length of SFT-600 from load cap...to
rear panel.... then some more required on outside of amp to make up the
electrical 1/8 wavelength on 51 mhz. 1/8 wave on 51 mhz = 1/4 wave on
102 mhz ( 2nd harmonic). THEN the T connector installed, with 1/4 wave
shorted stub hanging off the T connector.
So we went to a conventional safety choke. The end user can always install
the T connector + shorted 1/4 wave stub if he thinks it's required.
Another method is to use a series harmonic trap, tuned to the 2nd
harmonic. ( L in series with a NPO cap). High L, low C provides for the
deepest notch. But that only takes out the 2nd harmonic. A 2nd trap
could be used for the 3rd harmonic.
K9YC's rationale was based on exhaustive testing. Turns out it's based on
whether the last component is a coil (PI-L) OR a cap (load cap) in a PI.
Now I have some ham in indonesia who wants me to build him a monoband 20 kw
pep out amp for 7.1 mhz . He has the new 3x10 tube, and also the
fil xfmr..and also the completed B+ supply. . Complicated a bit, since
he has 400 vac 3 phase power and 230 vac from phase to grnd...and 50 hz.
60 hz rated blowers run a lot slower on 50 hz. Bear-el in Japan no longer
builds tube amps..and charlie asamoto ( ASI) is now retired..and moved to
Ore. Nobody left that builds custom one off amplifiers. Charlie built the
last big 6M amp, using a 4x10, with screen V..and cathode driven...like a
30S1 on steroids. He still has his ASI website up....the pix are
fabulous. http://www.ampsystems.com/
Jim VE7RF
On Thu, Nov 24, 2022 at 4:14 AM Steve Thompson <g8gsq@gmx.com> wrote:
You could also try a shorted quarterwave line across the output, gives
you a solid dc short and helps with harmonics.
Steve G8GSQ
On 23/11/2022 22:34, jim.thom jim.thom@telus.net wrote:
Here, the 15 uh safety choke is installed across the ceramic vac load cap
on the output side. This DC grounds the entire PI output tank circuit at
all times. IF any of the 4 x plate block caps ever failed shorted, or
semi
shorted, you don't want 5.5 kv getting onto the tune + load caps, vac TR
relay's, coax between TR relay's and output connector, nor going up the
coax to the ant array. You want the entire tank DC grounded anyway, so
the
plate block caps act like plate block caps...IE: they will still have a
tiny leakage due to high internal leakage resistance.
18 ga magnet wire used for the safety choke, wound on 1" solid Teflon
rod.
Mounted to sidewall with a 1/4-20 screw. Cold end of choke is bonded to
the
sidewall. Teflon wire used to wire hot side of safety choke...to the NO
contact of the vac output relay assy. The NO of the output relay is
directly bonded to the vac load cap.
I see in a lot of HF / 6m amplifiers, both commercially built, and also
HB, they will use a pie-wound safety choke...like a 2.5 mh type. The
problem with that is.... the DC resistance of the choke is typ 25-40
ohms. IF the plate block cap assy fails shorted, the pie-wound safety
choke will not handle the entire fault current...and simply explodes. I
have seen them blown to bits a few times over the years.
The safety choke is installed at the 50 ohm end of the tank circuit. It's
XL does not have to be high. 50 uh is ample on any 160-10m amp. 10-15
uh
is ample on a 6M amp.
This latest video also shows the coax wiring to the input ceramic vac
relay, which also includes the bypass leg, and also the input going back
towards the SO-239 input coax connector.
https://www.youtube.com/watch?v=r_jxiYM7bcs
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