[Amps] push pull or parallel ?

[Jim Thomson]

Date: Mon, 26 Jan 2015 16:41:48 +0100
From: "Carel, pc5m" <pc5m at xs4all.nl>
To: <amps at 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