Hi All, in a homebrew amplifier with a grounded grid, one can eliminate
most of the heat in the bias circuit by putting the bias between the cathode
and ground. In this case the B- connects directly to the cathode.
An emitter follower circuit or power zener can be connected from B- to
ground. A positive forward ("keep alive") bias should be applied to the B- so
that there is a small current flow through the circuit during standby.
The only current through the circuit is grid current and the small "keep
alive" current. The transmit/receive circuit can be arranged to increase the
bias voltage during receive to cut the tube off. Unlike cathode bias, the
tube can be completely cutoff in standby compared to nearly cutoff in
conventional circuits.
If an emitter follower is used it is easy to add an adjustable bias pot to
allow precise idling current setting.
A PNP power transistor can be used with the collector connected directly to
ground for d.c. and heat sinking. The transistor needs to have a voltage
rating greater than the standby cut off voltage.
I have used this arrangement for several years on 1296MHz tetrode amplifier
with no issues. This amplifier is wired to remove screen voltage in
standby so normal operating grid bias is more than enough to cut the tube off
in
standby. It would be easy enough to arrange for cut off grid bias instead.
73,
Gerald K5GW
In a message dated 2/3/2015 8:39:35 A.M. Pacific Standard Time,
jim.thom@telus.net writes:
Date: Tue, 03 Feb 2015 13:36:23 +0000
From: Manfred Mornhinweg <manfred@ludens.cl>
To: amps@contesting.com
Subject: Re: [Amps] Need Zener for 2KD-3
This solution has a more stable voltage than a series string of diodes,
uses
fewer components, and doesn't need any extra support, because the two
small
parts are simply soldered to the transistor, which is bolted to the
chassis or a
heat sink. It's also suited to replace higher voltage zeners, which is
impractical using diode strings.
Manfred
## Your small zerner + transistor will work, I have used it myself.
A string of diodes like 1N5408 or 6A10 will also work, and are bomb
proof. High power 50 watt zeners are a pita..and run too hot imo.
Come glitch time, hiccup time etc, etc, they will fail shorted on you.
## A 6A10 is rated for 6A CCS, and a 400A surge. Nobody ever blew
a string of 6A10s out , when used for GG bias service. The V regulation
is plenty good enough. Put a 10,000+ uf cap across the entire string,
and it wont
budge. If u have an application where you require say 30-40 volts of
bias, and
cathode currents of 3-4 amps, a string of 6A10s works just fine. Cathode
current
of course being the sum of plate + grid current. 30+ volts with 2+ amps
of cathode
current will cook a small zener + pnp transistor setup. You would require
a big heatsink to
dump the 60-160 watts of heat. Strings of diodes don’t require a heatsink.
## If u really want to go over board, you can also parallel 6A10s..and
the current always
divides 50-50 or 49-51 in the worse case. You also end up with an 800A
surge rating.
## henry radio used strings of diodes, encased in a block, sorta like a
k2aw rectifier block,
for bias on both their 3k ultra and also 8k ultra.
## For whatever reason, when high power zeners are wired in series, the V
regulation
is not good.
## For low current, low bias voltage schemes, the small zener + pnp / npn
work just fine.
Jim VE7RF
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