[Amps] push pull or parallel ?

[Carl]

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 at xs4all.nl>
Sent: Monday, January 26, 2015 1:40 PM
To: <amps at contesting.com>
Subject: Re: [Amps] push pull or parallel ?

> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Thomson
> Sent: maandag 26 januari 2015 17:40
> To: amps at contesting.com
> Subject: [Amps] push pull or parallel ?
>
> 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
>
> -------------------------------------------------------------------------
>
> 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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