Hi all

The Decca PA seems to be scattered with spike limiting diode from memory 
Transils / Transorb TVS diodes. (don't know the values) I usually put one in 
the drain set above the peak drain wave voltage seen on the scope and below the 
peak operating value of the FET  So far so good.

Jim M0MBU did a short treatise see http://www.wireless.org.uk/guard.htm.  

The Decca TX "guard circuit" by Jim Moritz, M0BMU.

73 petefmt




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On Mon, 3/4/17,  <[email protected]> wrote:

 Subject: FW: LF: FETs getting sick - why not use tubes?
 To: "[email protected]" <[email protected]>, 
"[email protected]" <[email protected]>
 Date: Monday, 3 April, 2017, 17:39
 
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 #yiv8987743228    Sent from Mail
 for Windows 10
  From: [email protected]
 Sent: 02 April 2017 15:28
 To: [email protected]
 Subject: RE: LF: FETs getting sick - why
 not use tubes?
  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
 changeIe 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    Sent from Mail
 for Windows 10
  From: [email protected]
 Sent: 02 April 2017 14:34
 To: [email protected]
 Subject: R: LF: FETs getting sick - why
 not use tubes?
  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]: 2-apr-2017 14.29A:
 <[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
 df2pyHi 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