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HI PA AMP folks
Surprised more LF/MF ops do not use the DECCCA 5501 design, class D mosfets in bridge configuration with series resonant output then matching ferrite transformer to match either 50 or 75 ohms.
I have been using this method now for several years and todate not had a failure.
This design is narrow band but only needs the series resonant cct switched for band change
Ie SMOKE FREE DESIGN
Good luck all with your PA projects and also agree Tube PA,S are hard to beat. I use Dentron PA amps on the HF bands, These amps have a pair of 8875 tubes and live a long time.
73 gl de MAL/G3KEV
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Hi Tom, Wolf and the group,
thanks for feedback, suggestions and nice pictures of the tube TX! :-)
very nice to see!
If I had not room restrictions probably
will ... [I should have hidden in some place some 6146B and PL519s as
spare parts of older rigs ;-) ].
The pushpull FET PA served without problems for at least 5-6 years..
withstanding several kind of insults (mismatchings.. missing antenna..
wspr15 etc..) this time the first FET shorted out after a ceramic
trimmer in the directional coupler arched and it was the first of a
long list: replace/short/replace/..
There is no bias (simply a couple resistances in serie to gate and from
gate to ground) driving the pushpull with a IXD609 via a transformer.
The PA is broadband and is followed from an external LPF for LF or MF.
Hope next FET will survive enough time to check what is going wrong...
73 Marco IK1HSS
----Messaggio originale----
Da: [email protected]
Data: 2-apr-2017 14.29
A: <[email protected]>
Ogg: LF: FETs getting sick - why not use tubes?
Am 02.04.2017 um 13:09 schrieb Dr. Wolf Ostwald:
> hi Marco !
> FETs die in many peculiar ways. Often they just become half
conducting
> on the Source-Gate path. If they are on a common bias source, the
> Source voltage will open up all FETs connected. Leads to immediate
> death of all of them. Use individual bias on each of them. I had
that
> happen a few times b4 i learned my lesson. Of course capacitive
> blocking of DC from the input transformer is essential too. But i
> think these thoughts have been published here b4.
> I am in the process of putting a single tube to use on 472. But its
a
> time consuming enterprise.
>
> 73 de wolf df2py
Hi Wolf and group,
nice to hear that someone else is thinking about this approach! I´m
content with my homemade tube PA for LF and MF which has provided
reliable service since nearly 4 years now. Only some thoughts about
this
concept - I hope not to bore all those hams who are happy with their
semiconductor PAs:
Years ago I had a MOSFET PA for LF, Class B push-pull with 250 W RF.
It
worked well at constant conditions, but when I had to retune the
antenna
due to larger QSY or made antenna experiments there always was the
danger of blowing up these nervous semiconductors. After 4 or 5 times
changing the MOSFETS I decided to build a new PA - with tubes! Looking
a
little bit anachronistic this PA is absolutey good-natured. Designed
for
broadband service on LF and MF it makes no problems when changing the
antenna coars tuning from one band to the other even when the fine
tuning is´nt done yet. With my former MOSFET-PA this would have been
impossible.
I wanted to have a linear PA - this usually means class B. You have to
decide between narrow band and broad band (like an audio-amp) design.
For narrow band you can use a single-ended PA but you have to add a
resonance circuit. For broad band you should use a push-pull PA and
have
to build a suitable output transformer. I opted for broad band design
because it is usable for LF and MF without changes at the PA. With
this
design and sin-driving I reach a total harmonic distortion of about 5
%
at 700 W RF on a pure resistive dummy load. With the usual narrow,
narrow band antennas on LF and MF you don´t need additional filters!
Concerning the tubes: If you take the common TX tubes with plate
voltages of several kV all output circuits have rather high
impedances,
that means large coils for the resonance circuits resp. large
transformer windings and very high voltages - potentially a
construction
problem. This led me to the choice of 2x 4x PL519 in push-pull, a
rugged
colour TV line output tube with low plate voltage and high plate
current. In this way I came down to a plate-to-plate resistance of
about
1 kOhm at 600 V DC plate voltage, where you easily can build a ferrite
broad band output transformer down to 50 Ohms. A disadvantage of this
concept is that you have to give individual bias to each tube, that
means for the first start-up you have to align 8 potentiometers
carefully to nearly equal cathode currents for all the tubes. But
according to my experience this alignment remains stable over a long
time. I have inserted 1-Ohm-resistors in each cathode line and have
brought the voltage drops to 8 cinch connectors, where I can monitor
the
DC component (with external filtering) as well as the real time
current.
With 4 tubes in parallel per branch of course you have to take care
for
self oscillations. The extensive use of bypass capacitors, ferrite
beads
and parasitic chokes in the plate lines is mandatory as well as good
grounding concepts are. The tubes don´t pull control grid current
(this
would even be true in class C!) but you need 3 or 4 W RF input power
due
to all the ohmic loads at the tube´s control grids caused by the
individual bias paths. On the other hand this certainly helps to avoid
oscillations. You can see some pictures of this PA at www.QRZ.com/db/dk1is.
By the way: why not to try these tubes at class D? With DC plate
voltages of perhaps 1200 V you should get a nice QRO-PA ...
Wolf, you are right: building such a PA from scratch is a time
consuming
enterprise. I didn´t count the working hours but according to my lab
log
the whole project took about 9 months - an adequate time for a new
baby!
It was a great experience anyway.
Good luck and 73,
Tom, DK1IS
