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@contesting.com [mailto:amps-bounces@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@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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