Lower the Q for wider bandwidth and less need of a retune, you should be
able to use a 6 Q and followed by coax stubs for 2nd and 3rd harmonics.
Carl
KM1H
--------------------------------------------------
From: "Carel, pc5m" <pc5m@xs4all.nl>
Sent: Monday, January 26, 2015 1:40 PM
To: <amps@contesting.com>
Subject: Re: [Amps] push pull or parallel ?
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
Sent: maandag 26 januari 2015 17:40
To: amps@contesting.com
Subject: [Amps] push pull or parallel ?
Date: Mon, 26 Jan 2015 16:41:48 +0100
From: "Carel, pc5m" <pc5m@xs4all.nl>
To: <amps@contesting.com>
Subject: [Amps] push pull or parallel ?
Hi,
160mtrs contest past and we need more power... Plan is to add 3 dB by
throwing in another tube (2.5kw anode dissipation Tetrode, grid driven).
I could go for push-pull of just parallel the tube's. I think push-pull
would be nice to get less even harmonics, but seems to be a big hassle and
just put the tube's in parallel is much easier. Or am I missing something
?
73's Carel, pc5m
## is this a monoband 160M amp...or multi band ? How much drive do you
have available ?
The problem with doubling the power output is, you double the plate
current, and the plate load Z
will drop in half. You now require twice as much tune and load C....and
half as much tank coil,
assuming a PI net output. IF it’s a monoband 160m amp, no big deal, just
pad both caps, and
replace the coil with one that has half as much uh. If it’s a multiband
amp, you can still pad
both caps on 160m, via vac relays, but you will have to watch the peak V
across the vac relay contacts on the tune padder, when on 80-10m, esp the
high bands. vac relay peak V rating drop,
as freq increases. Coil will be one half as big on each band, but with
double the rf current through it. .
## I think push-pull will work on a monoband amp...dunno about multi
band. With 5 kw
anode diss, maxed out, and assuming 65% eff on 160m, you should be able
to do 14 kw dc input,
and 9.1 kw output. (= 4.9 kw anode diss).
## 160m is pretty forgiving for the most part, with long leads etc. Is
this a CW or SSB amp...
or both ? If its cw or for a cw contest, you can always increase the TX
bias, and reduce idle
current quite a bit.... and increase the tank eff.
## 3db is not a huge amount. If instead you tripled the po, then you
get 4.77db...which is
a well worthwhile increase. 3db would be enough to get you through in
most cases, and is a good starting point. 4 X = 6db. Now you have a
substantial increase.
You can only do so much with 160m TX ants.....after that, more PO is
required. You cant
work them...if they cant hear you.
Jim VE7RF
-------------------------------------------------------------------------
Hi Jim (and others who did reply)
It is a monoband 160mtrs amp. Output power is at the moment limited by
screen current, so surely not running it to the limit. Reason: we don’t
want to retune as we change QRG (1810 - 1900).... Screen current will
increase of course when Zload is not optimum. Also running the PI-L match
already at rather low loaded Q (8 or so). Normally we see the amplifier
twice each contest...at the start and at the end...located in other room.
Would be interesting to learn if in that respect parallel operation is
behaving different than push-pull ?
73's Carel.
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