Since the parasitic debate has died down, maybe we can get it going again!
I offer the following two experiences to the group for comments and/or
discussion.
In the past year, I have put together two amps with *NO* parasitic
suppression of any kind.
Case #1: 4 x 813's in TCGG, started out to be 160 through 40. Had a unique
layout problem that fried (arced) band switches which had hi-pot tests that
exceeded twice the anode voltage. Turned out not to be any kind of an
oscillation at all. Temporarily fixed by removing band switch and fixed on
160. Details on request, but how about you guys dry-labbing this one and
see if you can guess what was going on? This amp was deliberately built
with no parasitic suppression of any kind. I knew going in that the 813's
wouldn't be bad boys, and I was right. Easy 1500 out with a muffin fan for
cooling and only 20 bux for the tubes, not too bad.
Case #2: 4CX1000A amp converted to a YC-156. The sheer physical size of
this jug with handles made this a mechanical nightmare. Tank is an air
variable and roller coil. Anode "C" plus strays, etc., make it barely able
to tune on 20 mtrs. That's OK, don't need more then one band per amplifier
anyway! I fix tuned the cathode network for 40 just to get it going. I was
scared that the high gain/bandwidth of this tube would make it be one of
the "bad boys". I did not put any parasitic suppression in at first. My
thought was to see what it did, then design an appropriate fix. The
parasitic anode resonance dipped at 80 Mhz. I turned it on the first time
expecting to use the fire extinguisher. Was I wrong! It is absolutely
stable under any conditions I could come up with. Of course that may change
if and when I run the anode supply up. My present limit is about 3100v. The
tube is capable of 6.5kv operation.
Here is my take on all this. Every 160-10 amp project I have undertaken has
been a nightmare if not a downright disaster. Every single band amp has
been an unqualified success. Two or three band jobs are a little harder to
tweak to perfection, but still fun to do.
Most, if not all, parasitic problems ( VHF, HF, and even VLF ) can be
traced to a component selection and layout that is compromised to allow
alleged operation over 4 octaves. Yes it can be done. This is one of
Peter's "engineering challenges"! Hey, I'm doing this for fun, not profit.
I don't have to bend to some insane manager or marketer's idea of a
saleable amp. A room full of single band amps is a lot more emotionally
satisfying then one little bitty thing that needs a fire extinguisher
nearby. Besides, look at the incredible physiological advantage that gives
you over the locals in a pile up!
73 & ENJOY!
Larry - W7IUV
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|