Suppressors wont hurt anything and can protect the amp. Unless you are
running matched tubes one will have higher gain and just might decide to
become unstable.
Carl
KM1H
> Great information, very helpful! This is so much better than trying to
> figure this out
> while soldering taps on the coil, cutting tubing, etc.
>
> I am also wondering whether I will need parasitic suppressors at all. Many
> 813 amplifiers
> I've seen (GE SB handbook, ARRL HB's, etc.) do not use them. I plan to
> mount the sockets
> slightly below the chassis and bring bypass leads to the top of the
> chassis with very
> short leads to the same ground point used for the tuning capacitor. Also I
> am going to use
> parallel 0.01 and 0.001 uf for the bypasses. This should improve
> stability.
>
> On 4/7/2010 5:30 AM, Jim Thomson wrote:
>> Date: Mon, 05 Apr 2010 23:03:33 -0700
>> From: Vic K2VCO<vic@rakefet.com>
>>
>> pi-net spreadsheet question
>>
>> I'm using the GM3SEK pi-net design
>> spreadsheet<http://www.ifwtech.co.uk/g3sek/>. It
>> requires that you enter the inductance and resistance used in the
>> parasitic suppressor.
>> What I'm wondering is how to do this for two tubes in parallel, where
>> there are two
>> suppressors.
>>
>> Do you enter the value of one coil and resistor, or do you treat it as
>> though these
>> components are in parallel?
>>
>> ## excellent question.. and I have been using his pi sheet for yrs
>> now... but only
>> 1 x tube amps... or 2 x GS35bs... where no suppressor is used at all.
>>
>> ## I'd say to parallel the values of the 2 x suppressors. If it's a GG
>> amp, the grids
>> are in parallel at one end. The plate block cap is where the 2 x
>> suppressors come
>> together at the other end.
>>
>>
>>
>>
>> This makes a big difference at 28 and 21 MHz.
>>
>> I am finding this exercise very interesting, playing with the value of
>> "lead inductance".
>> I'm trying to add a small "l network" before the main pi network which
>> will cancel out
>> some of the large amount of output capacity of a pair of 813's so they
>> will be reasonably
>> efficient on 10 and 15 meters. The parasitic suppressors become part of
>> this, too.
>>
>> ## that feature is the BEST part of the spread sheet ! Start with
>> just the paralleled
>> suppressors, NO lead uh. The tune cap may end up neg value.. or very
>> low.. like
>> just a few pf. [say for a Q=14-18] Take what ever value it spits out
>> for the main coil..
>> and then divide by 2. EG: spits out say .. '.8 uh' .8/2 = .4uh
>> Ok, then make the
>> lead uh .4 uh. Main coil value may well change. Juggle the lead Uh
>> value, till BOTH the
>> lead uh and main coil are identical.
>>
>> ## Ok, now you can probably DEcrease the loaded Q. This all depends on
>> Min C u have
>> and whether a vac cap is used, etc.
>> The idea here is to obtain the lowest Q you can, and still not have the
>> tune cap totally
>> unmeshed.. and keep the main coil and 'lead uh' the same value.
>>
>> ## I call the result a ' L-PI' and it works very well ! On my
>> 160-15m [ no 10+12m]
>> 3CX-3000A7 amp, sure, it would tune 15m.. with out the extra .66 uh
>> b4 the main
>> pi-net.. but loaded Q was WAY too high for my liking. Also, my 15 M
>> position is also
>> used on 17m. So loaded Q on 17M is through the roof. The 3x3 tube
>> has 24 pf of
>> anode to grid C... and rises to 33 pf.. when tube in the socket. Min C
>> of the vac cap is
>> 12 pf... so I have 45 pf.. with the vac cap unmeshed !
>>
>> ## with .66uh b4 the pi -net..... plate load Z drops way down. Now I
>> can get the loaded Q
>> on 15m down to just 8....... which rises to Q=12 when on 17M.
>>
>> ## In this case.. a .66 uh coil was made from 7 x turns of 3/8" od
>> tubing... on a 1.5" ID.
>> The .66 uh coil is at right angles to main tank coil. I THINK the
>> harmonic suppression increases
>> having the 2 x coils at right angles. Main tank coil was 3" OD.. also
>> made of 3/8" OD tubing.
>>
>> ## so the extra coil doesn't have to be the same diam as main coil. In
>> my case, I wanted it
>> long and skinny, to bridge the gap between Plate block cap assy.. and
>> vac tune cap.
>>
>> ## on another amp, the vac tune cap was tapped one turn into the main
>> coil. In this case, it's
>> all one coil. So both coil orientation schemes have been tried, and
>> both work very well.
>>
>> ## On older desk top amps, where loaded Q is sky high on 10M... adding
>> a tiny bit of uh between
>> plate block cap and C1 cap does wonders. The extra uh can also go
>> anywhere between the anode
>> and C1 cap. Then, using the stock value 10m tank coil... the loaded
>> Q drops like a rock, way less
>> circulating current, more eff, and main tank coil and the poor band
>> switch don't get cooked! But the
>> extra L coil has to be same OD as main tank coil.. or bigger, if main
>> 10m tank coil is marginal to start
>> with.
>>
>> ## I designed a 6m 8877 tank coil setup for a fellow 2 yrs ago. It
>> used a .5 uh coil... wound with
>> 1/2" wide flat strap... and tapped dead center in the middle of the
>> coil.. with the tune cap. The strap
>> coil is wound like a suppressor.. with the advantage that there is zip
>> C between turns.. since they
>> look like knife edges facing each other. 3/8" wide strap is the eq
>> of 1/4" tubing, since the
>> circumference of both is aprx the same. 1/2" wide strap = .318" OD
>> tubing, etc. He ends up
>> with .25uh on either side of the tune cap. This works very well.
>> Two separate .25 uh coils
>> can also be wound... and at right angles, or anything from in line to 90
>> degs between em.
>>
>> ## That extra L coil gets installed on all my amps, since I
>> discovered the spread sheet, 10-12
>> yrs ago. b4 that.. the maths by hand was tedious.
>>
>> later....... Jim VE7RF
>>
>
>
> --
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
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