[Amps] Tubes, transistors, and 'abuse'

[Steve London]

It would also be interesting to get Manfred's views on the Hardrock 50:

https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxoYXJkcm9jazUwYmV0YXxneDoxOGMwZTJiMDIzYjZmOWE

This one uses four RD16HHF1's at 12 volts in a parallel/push-pull 
arrangement. Similar design for drain DC voltage and RF output.

5 watts in/50 watts out at a slightly lower price than the hfpacker.

73,
Steve, N2IC

On 04/24/2017 10:54 AM, Steve London wrote:
> 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!
>>