Hi Bob,
His output impedance will be slightly above 3 kohms.
So 300pf are plenty on 80m, also on 160m when his Q is not higher then 10 it
will be fine as strays add to it.
But with high impedance a higher Q will be needed for sufficient harmonic
supression, so 500pf is the better choice.
A padder of 500pf for 160m loading should be sufficient for proper loading
from 25-150 ohms.
73
Peter
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Robert B. Bonner
Sent: Montag, 6. November 2006 20:20
To: 'PE1E'; amps@contesting.com
Subject: Re: [Amps] pi filter QBL 5/3500
Maybe I'm missing something here.
I didn't take the time to calculate your actual plate impedance. It will be
high, which will lower the C requirements. Even said with my experience,
that 300 pf tuner will just make 80 meters. You'll need all of 500pf
sometimes slightly more on 160. If you do click down the high voltage at
all, you'll instantly run out of cap.
The 1000 pf max of your loader will not be enough for even 75 meters that
needs around 1350 or so let alone the better part of 3000 you'll need on 160
meters.
What you don't want is not enough loader on 160. I've had amps in the past
(Alpha 77S) that tuned to massive power input with low grid current and the
plates meshed all the way on the loader trying to get power out.
I'm afraid you will be switching in a handful of big padder caps to make
those components work. Plus you will not have much extra range for impedance
matching.
When I purchase 80-10 meter components I generally buy 500 pf tuners and
1500 pf loaders they are plentiful on the surplus market. The 500 tuner
usually will make 160 and I use with a 3000 pf loader.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of PE1E
Sent: Monday, November 06, 2006 6:16 AM
To: amps@contesting.com
Subject: [Amps] pi filter QBL 5/3500
1.
If I build a plate output pi filter ( > Cin 5-300 pf 25Kv var.vacuum >
tapped coil .3 " copper pipe > Cout 15-1000 pf 5 Kv var.vacuum ) for my QBL
5/.3500 HF amp ( Va 6 Kv , Ia 1.1 A ) in statu nascendi ( 160 mtr - 10 mtr
), what 'd be the output Z matching range , i.s. is a separate tuner still
required in order to tune out the antenna reactance or could the output pi
filter does this job ?
2.
Could I replace the pi filter .3" copper pipe coil by a roller induictor ( 3
mm copper pipe, close wound, 32 uH ) i.e. could this ( very heavy ) roller
inductor withstand > 4 Kw RF out ?
What is the impact of ( it's a long time ago I learnt ) a close wound
inductor instead af of spaced wound conductor on the pi filter's performance
?
Which one has the least loss in a pi filter ?
( In some commercial amps I see a close wound 3 mm roller inductor, in other
types I see free hanging fixed spaced wound 5 mm tapped inductor ).
3.
I'd build the amp cf the G2DAF concept.
Does this have an impact on the values of the pi filter components ?
( I intend, as stated above, to use Cin 5-300 pf and Cout 15-1000 pf ).
Thanks for any help or reference.
Peter, PE1E
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