[Novice-Rigs] YTuned / Not-Tuned Input---Final PA

[Brian Carling]

Two tips with "hot" MOPAs, Eddy...

1 - build some kind of shield between the osc and the final.


2 - Use RFC/cap coupling rather than tuned interstage coupling.

3 - If all else fails install a small choke on BOTH the plate AND The 
grid of the RF final.
Make the chokes with 4 turns of wire over a 47 ohm 1 watt resistor.

This will usually fix any self-oscillation problems withouit the use of 
fancy neutralization circuits.

Bry

Ed Swynar wrote:

>Good Afternoon Brian,
>
>Thanks for the directions to your 2E26 rig website...
>
>Wow! You've got a LOT of real estate to play around in with that set-up of
>yours! I think that was my main problem, i.e. the darn thing was TOO compact
>for its (and my!) own good...I simply out-smarted myself, I guess, with
>probably all sorts of feedback paths that simply were unavoidale, resulting
>in the unstable final...
>
>BTW, did I tell you that prior to going to an untuned input, I did manage to
>"dip" the feedthrough power with a neutralizing capacitor...? Alas & alack,
>stable operation was simply not meant to be, because the rascal would
>take-off on me just when I thought it was licked. That rig had a mind of its
>own, I swear...
>
>It's pretty tame now, though---and just in time for the QRN "season" on
>80-meters, too (See...? That rig STILL manages to get the last laugh!)...
>
>~73!~ Eddy VE3CUI - VE3XZ
>
>
>
>----- Original Message -----
>From: "Brian Carling" <bcarling at cfl.rr.com>
>To: "Welcome to the Novice Rigs List!" <novice-rigs at mailman.qth.net>; "MOPA
>GROUP" <MOPAGROUP at yahoogroups.com>
>Sent: Sunday, May 01, 2005 1:20 PM
>Subject: Re: [Novice-Rigs] YTuned / Not-Tuned Input---Final PA
>
>
>  
>
>>Guys - as Eddy has discovered there is nothing quite like a MOPA
>>transmitter!
>>
>>I used almost the same configuration with mine. In my case it is a 6CL6
>>driving the 2E26 final.
>>As stated here I use just the RFC and then capacitive coupling from the
>>plate of the osc to the grid of
>>the 2E26...
>>
>>Try it - you will like it fine! See:
>>http://www.af4k.com/2e26.htm
>>
>>
>>Ed Swynar wrote:
>>
>>    
>>
>>>Hi Gang,
>>>
>>>Anyone who has ever built a MOPA rig for the low bands, & then found that
>>>      
>>>
>the final RF amplifier stage is more T.P.T.G. oscillator than amplifier---as
>I discovered to my chagrin with my 6AG7 driver / 2E26 RF amplifier---pull
>the L/C network right out of the grid stage of the final, & go it alone with
>the 2.5 mh RF choke in the plate lead of the driver...
>  
>
>>>Your input will be untuned by doing this, but on the low bands you should
>>>      
>>>
>have more than enough drive, dependent upon tubes used. I'd be hesitant
>about trying this on, say, 10- or 15-meters, but it seems to work well
>enough for me on 160-, 80-, and 40-meters. Just watch for "double-dipping"
>when tuning the final tank---if your VFO output is untuned, you may well be
>peaking on a harmonically-related band!
>  
>
>>>A lesson learned: when building that first-time GB rig, always remember
>>>      
>>>
>that LESS IS MORE. I started out with an ultra compact set-up here, using a
>6DT6 buffer between the outboard 12AU7A VFO & 6AG7 driver, regulated voltage
>on the 6DT6, selectable cathode / grid block keying, tuned drive, etc. etc.
>What a mistake! I needlessly complicated things with such circuit
>sophistication, & the thing never worked 100%...now I've pulled the 6DT6, VR
>tube, made it cathode keying only, & extracted the L/C after the
>driver---and the result...? It now works like it's supposed to!
>  
>
>>>...AND I have a couple of less controls to fiddle with, & more room under
>>>      
>>>
>the chassis, to boot.
>  
>
>>>~73!~ Eddy VE3CUI - VE3XZ
>>>_______________________________________________
>>>Novice-Rigs mailing list
>>>Novice-Rigs at mailman.qth.net
>>>http://mailman.qth.net/mailman/listinfo/novice-rigs
>>>
>>>
>>>
>>>      
>>>
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>>
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