[Amps] 10 KW CCS ON 6M...USING THE 3CX-6000A7... PART 12

Thu Nov 24 17:35:24 EST 2022

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 at 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 at 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 at 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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>>
> 