Date: Thu, 12 Feb 2015 14:40:14 -0600
From: Bill Turini <mailing_lists@hamanuals.com>
To: amps@contesting.com
Subject: [Amps] Need Zener for 2KD-3 - Part Deux
### You are wasting your time, and overthinking all of this. Forget the
small zener + transistor route. That combo is non adjustable, and requires
a heat sink etc. Put 10-14 x 1N5408s or better yet, 10-14 6A10s in series,
on a perf board. Since this is real low voltage application, the diodes can
be physically mounted in parallel, all in a row, yet wired in series. Ok, now
you can EASILY adjust the exact idle current u desire, simply by tapping
down on the diode string. Each diode of course is aprx .72 volt, so u can
adjust
in .72 volt increments if u want to fine tune it. You don’t need to be
sucking
180ma of idle current on a pair of 3-500Zs. 100-150 ma is more than ample.
## I run 10 x 1N5408s on my drake L4B 2 x 3-500z amps....and end up with
100ma idle when on ssb / 2650 vdc position.....and 40ma when on CW / 1900 vdc
position. IMD is good in either position on ssb. Your xcvr driving the amp
will have
way worse imd than the linear ever did... by about 10 db.
## I installed a spdt-CENTER OFF miniature toggle switch on the rear apron,
which allows
me to obtain 3 x bias positions on the string of diodes. CENTER OFF means all
the
diodes in the string are in the circuit. The other 2 x positions are my
taps. So tapped
for 3 diodes, or 7 diodes...or all 10 of em. I did that since I also use
eimac 3-400Zs and
also 8163s.....which have a higher mu..like 200...vs the 160 of the eimac
3-500Z.
The higher mu tubes will idle a lot lower, and require less bias.
## You are not after a specific bias voltage per se.... but a range of idle
current. Crank up
the bias V till u get the desired idle current....and don’t worry about how
much V is required
to do the job.
## If u operate CW or FM, u can reduce the idle current even more..like down
to 10-20 ma very
easily. On any GG amp, on CW, you cant operate the tubes at 0 idle current,
or u will get key clix.
## A 1N5408 is rated for 3A CCS.... but runs hot with 3A CCS in test jig.
( 3 x .7 = 2.1 watts) .
It will easily handle 1 A CCS. It also has a 200A surge rating.
A 6A10 is rated at 6A CCS, but it too runs hot, ( 6 x .7 =4.2 watts).....but
is fine at 2A CCS.
6A10 has a 400A surge rating.
## Neither the 1N5408 or 6A10 requires a heatsink either. The leads ARE the
heatssink, so leave the leads
long. All these other various schemes require big heatsinks, and are not
adjustable in most cases. A string
of diodes will never over heat and never blow up. Henry radio used strings
of diodes on both its 3 k ultra
and 8 k ultra. 15-20 used on the 8 k ultra.
## another scheme I have used is a small 12 position rotary switch, to tap
every 2nd or 3rd diode, but that is on
bigger hb amps.
## If u want 100% rock solid bias V.... u can always install a 10,000 uf
electrolytic across the entire string, it wont budge.
The diode string..sans lytic is plenty good enough.
## BTW, the only place the bias can go is in series with the CT of the xfmr.
Either b4 or after the resistor used for
RX cutoff bias.... which is usually a 10-100k resistor. TR relay contacts
just short the 10-100k resistor.
Jim VE7RF
Folks:
On 2/2/2015 2:36 PM, Bill Turini wrote:
> I need a zener for my 2KD-3. It's a 1N4561. It appears that they are
> really hard to find and exorbitantly expensive when you do. Has anyone
> found a source for a replacement at a reasonable cost?
>
This request generated many wonderful replies and suggestions to fix my
problem. Last night I was RTFM when I noticed something very strange -
the zener diode I had removed from the amp was not the same as
identified in the schematic, and it was connected totally differently
from the schematic. Remembering the wise post from the Henry web site,
I decided not to trust my eyes or the schematic and asked the learned
group for their opinion.
First, the zener diode I removed from the amp was a 1N4561, a 5.6v 50
watt device. The schematic calls for a 1N2804, a 6.8v 50 watt device.
Since the removed device was not wired in any way close to the
schematic, I assume that this was an attempt at a repair or
"improvement". So, what should the voltage be? 5.6 or 6.8volts? Or
should it be something else? Since most of the other Henry 3-500z amps
have 10 volt zeners, I would vote for the higher voltage 6.8v device.
Second, as for the wiring in the amp, I think it is just wrong, since
all the other Henry GG amps have the cutoff circuitry the same. So, I'm
going with the schematic.
And, in case anyone is wondering, the 1N2804 costs just about the same
ridiculous price as the 1N4561, so I'll probably opt for the zener and
transistor route.
73's and thanks in advance.
Bill
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