[Moon] RE: [Moon-net] Amplifier technical questions

[Russ K2TXB]

Oene, If the amplifier tuning is so sharp them the tubes are not loaded
heavily enough. Increase the coupling and retune.  Continue doing this until
the tuning gets broad and the power output falls off slightly.  Then reduce
the coupling just until you can get full power out but the tuning is still
broad.

Hope this helps,
73 - Russ K2TXB 

> -----Original Message-----
> From: moon-net-bounces at list-serv.davidv.net 
> [mailto:moon-net-bounces at list-serv.davidv.net] On Behalf Of 
> Oene Spanjer
> Sent: Saturday, February 10, 2007 5:31 AM
> To: moon-net at list-serv.davidv.net; moon at moonbounce.info
> Subject: [Moon-net] Amplifier technical questions
> 
> Hello All (amplifierbuilders)
> 
> Recently I have build a 2 m amplifier with 2 x GI7b but 
> experience some troubles.
> I have build this amplifier with separate bias control, so I 
> can adjust each tube separately and therefore the cathode 's 
> are not connected as seen in all the 2 tubes designs but each 
> tube get's it's input RF via a capacitor.
> 
> First I build an amplifier with stripline according to G4DCV 
> his design, but got only few Watts out. 
> Then I rebuild the design to the 'lazy builder' design of 
> YU1AW to see if the tubes (NOS) were still 'alive'  and yes 
> they were; I am able to get somewhat 700 Watts out at abt 40 
> - 50 W input, so the tubes are doing as expected.
> 
> But with this design consisting of  2nF / 30kV from anode to 
> coil and tune cap to earth and output coupling by another 
> variable capacitor,  I experience that is is extremely sharp 
> and when the amp starts getting warm I have to retune every 
> time I am on the air again.  Output  drops each sequence to 
> the 50W level and this is in practice not working.
> Also at some points of chancing the output coupling C it 
> starts sparking and that is really bad. 1 C has already got 
> black and those RF voltages must be huge !!
> 
> Now I am thinking of rebuilding this to a version with stripline.
> My anode chamber is not that big so I guess have to play with 
> the dimensions.
> 
> My questions: Does anyone have had those experiences and does 
> a stripline gives me a 'flatter' resonance so I have not to 
> retune that often ?
> My single GI7b has also a stripline ( DJ9HO design ) and when 
> it has warmed up I only have to retune very little, so I 
> guess a stripline is much better ( it even gives some extra 
> cooling surface ) And how are the stripline dimension 
> influenced by the dimensions of the anode chamber ?  I read 
> that the distance from bottom to stripline and also from 
> stripline to top are important for the dimensions.
> 
> I hope anyone has some answers because it's sad having the 
> capability of medium power but still can't use it :-(
> 
> Thanks all for reading and GL
> 
> 73 Oene
> PA3CWN
> 
> 
> 
> 
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