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
.
--------------------------------------------
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
#yiv8987743228
#yiv8987743228 --
_filtered #yiv8987743228 {panose-1:2 4 5 3 5 4 6 3 2 4;}
_filtered #yiv8987743228 {font-family:Calibri;panose-1:2 15
5 2 2 2 4 3 2 4;}
#yiv8987743228
#yiv8987743228 p.yiv8987743228MsoNormal, #yiv8987743228
li.yiv8987743228MsoNormal, #yiv8987743228
div.yiv8987743228MsoNormal
{margin:0cm;margin-bottom:.0001pt;font-size:11.0pt;}
#yiv8987743228 a:link, #yiv8987743228
span.yiv8987743228MsoHyperlink
{color:blue;text-decoration:underline;}
#yiv8987743228 a:visited, #yiv8987743228
span.yiv8987743228MsoHyperlinkFollowed
{color:#954F72;text-decoration:underline;}
#yiv8987743228 .yiv8987743228MsoChpDefault
{}
_filtered #yiv8987743228 {margin:72.0pt 72.0pt 72.0pt
72.0pt;}
#yiv8987743228 div.yiv8987743228WordSection1
{}
#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
|
