[Amps] Tubes, transistors, and 'abuse'

[Steve London]

Manfred,

Can you explain what this design is trying to achieve with separate 
transformers for the output (T1) and for supplying DC (T2) ?  Why not 
the more traditional center-tapped transformer that acts as the output 
transformer and the method of suppling DC ?

73,
Steve, N2IC

On 04/23/2017 02:07 PM, Manfred Mornhinweg wrote:

>> http://www.kg9e.net/projects/hfpacker/
>>
>> It may be telling that K5OOR has resigned the amp since my kit was
>> built. The current one being sold appears to be physically larger than
>> mine.
>>
>> Documentation:
>>
>> https://storage.googleapis.com/wzukusers/user-17403798/documents/58c313c4618a4lexQCfw/Construction%20Manual%20miniHFPA%20with%20schematics%20and%20BOM%202017%2003%2010.pdf
>>

> The required tight drain-to-drain coupling can either be implemented via
> a center-tapped output transformer, but it MUST be a real, magnetically
> coupled center tap, _not_ a half-turn tap; or a bifiliar feed choke must
> be used. This amplifier uses the latter option. But unfortunately the
> bifilar choke used isn't up to the task, and judging from the published
> text, the designer hasn't fully understood the requirements for this choke.
>
> The choke he used has 10 bifiliar turns on 2 stacked FT-50-43 cores.
> This must give roughly 100µH per side, which is FAR more inductance than
> needed. This much inductance wouldn't really hurt here, but the problem
> is that along with the high inductance comes a significant leakage
> inductance. And the leakage inductance acts like two independent
> (uncoupled) chokes in series with the (coupled) bifiliar choke,
> destroying the coupling between the FET drains if this leakage
> inductance is too high.
>
> I would have to build that bifiliar choke to measure the actual leakage
> inductance, and I'm too lazy now to do that. But judging from
> experience, I would expect around 0.5µH leakage inductance per side.
> That's an impedance from 6 ohm on 160 meters, up to 91 ohm on 10 meters.
> Meanwhile the drain load impedance in this amplifier is 11 ohm per side.
> So the coupling between drains is modest on 160m, poor on 80m, and gets
> worse on higher bands, to the point that from 30m upwards there is
> essentially no drain-to-drain coupling. And this is the fundamental
> explanation for the poor efficiency of this amplifier! The bifiliar
> choke would need to be re-designed so that it produces tight coupling
> between the two drains, all the way into VHF, to cover at least the
> lower harmonics of 10m. At 11 ohm drain impedance this can be done,
> although it's not really easy. Instead with a high power LDMOSFET amp,
> or a 100W 12V powered amp, both of which have drain load impedances
> around 1 ohm, it cannot be done, to the best of my present knowledge -
> and that's the explanation for the "typical" 45% efficiency of most HF
> push-pull broadband amps we see!
>