"Frank Gilliland" wrote in message
...
On Thu, 21 Oct 2004 17:37:04 -0500, "DR. Death"
wrote in
:
snip
The bulletin uses a 1N4997 diode for D2 and a 33 ohm for R4. The
construction hints uses a MJE243 Transistor for D2 as they state the
1N4997
is no longer available and R4 as 82 ohms. The 63R resistor parts set has
it
at 47 ohms. So the choice I think would be either the 47 or 82 ohm.
It looks like the resistors were chosen based on the characteristics
of the respective device, so keeping the match would probably be the
best choice. IOW, use the 82 ohm resistor with the MJE243 and the 33
ohm resistor with the 1N4997.
I was able to find a 1N4997 replacement NTE5800 and might try it with the
33
ohm as the MJE243 will be a bit harder to install as it goes between the
PCB
and heat sink and has to have a spacer.
Avoid generic replacements. Unless it's reverse-referenced as the
original component, it may not be a drop-in replacement and is not
likely to have the same specifications as the original. Besides, using
the MJE243 is, IMPO, a better choice than the 1N4997 for a couple
reasons: First, as a diode it will have forward voltage drop (bias
regulation) similar to the emitter-base characteristics of the power
transistors (mainly because it -is- a power transistor); and second,
because it will have a better thermal connection with the heat-sink
providing bias regulation with improved thermal tracking. That is, of
course, assuming you are able to mount it physically close to the
power transistors. And when you mount it, use just a very thin film of
heat-sink grease -- avoid the "more-is-better" mentality.
The MJE243 mounts close to the power transistors. The hints say to use a
mica washer and coumpound. I was certainly a little concerned with using
replacement 1N4997.
Thanks for your input, if the 1N4997
doesn't work like expected I'll give the series parallel a try.
Using multiple resistors is an excellent idea. If the 33 ohm resistor
is used the power dissipated by the resistor will be slightly greater
than the 5 watt rating specified -- it would be much better to have a
resistor (or resistors) rated for 10 watts or more. Also, mount that
resistor (as well as the base resistors R5 and R6) slightly above the
board (1/8" to 1/4") or the heat will cook the board and the foil will
fall off. The extra wire also helps dissipate heat.
Glad you mentioned that. I would have mounted them against the board.
There was also the suggestion to use a Dale, but that's really just a
waste of money in this case -- the input impedance of the transistors
is so low that feedback from stray RF isn't going to be a problem.
Another issue: While looking at the schematic I was suprised to see
that there isn't a choke between the bias supply and the input
transformer -- there -really- needs to be one there! At the very
least, add a few beads on the wire from the center-tap of the input
transformer. A nice torroid or hash choke would be even better.
When I bought the transformer set it came with the beads and wire and the
hints tell where to put the choke.
Anyway, while you are building this thing, keep in mind that it is a
very basic circuit (i.e, primitive, uninproved, bare-bones), so don't
expect much. And yes, it -will- need a filter.
LOL, no doubt. I have a pretty good low pass in line filter that I will use.
Thanks for the info Frank.
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet
News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000
Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---