[Amps] pi-net spreadsheet question
Carl
km1h at jeremy.mv.com
Wed Apr 7 11:45:20 PDT 2010
I dont know the parasitic frequency but a new 813 will work fine on 6M as
will a 811 and 572B.
Carl
KM1H
> Vic,
>
> Ground the grids at the bottom, direct to the chassis as close to the tube
> sockets as manageable. The chassis serves as a large low inductance ground
> plane for grounding the grids and the plate tune cap.
>
> Where parasitics are usually a problem is when tubes have long internal
> grid
> leads. Such tubes as the 811A, 572B etc have these problems.
>
> Tubes like the 8877, YC156 are much less prone to problems because of
> their
> internal structure where internal leads are very short.
>
> The 813 may not need suppressors as its gain above 30 mhz drops pretty
> low.
>
> It is the internal plate to cathode capacitance that causes the coupling
> that lets high frequency parasitics to exist. Nothing that you can do
> about
> that other than to kill the gain in the plate circuit at the parasitic
> frequency.
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
>> On Behalf Of Vic K2VCO
>> Sent: Wednesday, April 07, 2010 11:10 AM
>> To: Jim Thomson
>> Cc: Amps reflector
>> Subject: Re: [Amps] pi-net spreadsheet question
>>
>> 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 at 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/
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
More information about the Amps
mailing list