Date: Wed, 11 Aug 2010 10:00:35 -0700
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
Subject: Re: [Amps] 50 watt stud mount zener diodes
On Wed, 11 Aug 2010 10:45:42 -0400, "Carl" <km1h@jeremy.mv.com> wrote:
>
>Just another reason for 1N5408 or 6A10 strings. A tube arc will take out a
>50W zener as fast as a 10W version.
REPLY:
6A10s are a good choice, but for a high bias tube like a GS-35b you
need a lot of them - about 40. (24 VDC x .6 V per diode).
You may get tired of soldering.
73, Bill W6WRT
## I use 6A10 diodes for bias.. on both 3x3 and 3x6 tubes. I tested the
perf boards in a test jig with a variable supply + series resistor. With 200
ma of current flowing
[typ ZSAC].. they appear to be aprx .71 vdc across each diode. With 2-3 Amps
flowing, the V drop rises by 12%. [.79vdc].
## I install a 5000-10,000 or bigger.. lytic across the entire string of
diodes. This
improves the dynamic bias regulation. On GS35B's and YC-156's, 3x3 etc... a
20 x pos rotary switch is used
to tap off every 2nd or 3rd diode..starting with diode #10-25. Call that
the coarse adj.
For ultra fine adj... a small spst toggle is used across diode #1. Sure,
20-60 x diodes for bias is a lot...it's a good
project for ur kids..or if you want to do something simple one eve.... on a
par with punching holes in doughnuts. It also
won't blow up this century. You can suck 2-3 A CCS through a 6A10 all day
and not blow it up either.
## You can also use a 3agc type fuse in series with the bias diodes [ with a
paralleled pair of 100 K - 3 watt mof
resistors ACROSS the FAST 3agc fuse.] This is a cathode fuse..and has the
grid + plate current flowing through it.
Suck too much cathode current, fuse opens up, and now you have 50 k in series
with the CT/bias diodes asap.
## On all the HB amps.. I use a small SPST VAC relay.... or just the COM
and NC contacts of a SPDT VAC relay
to short out the 50 k resistor used for the RX cut off bias. [Cut off bias
resistor also consist of a paralleled pair
of 100K-3 watt mof resistors] . I use a SPDT vac relay on the RF input to
the amp... and a 3rd spdt vac relay for the
RF output..... and speed all 3 of em up. That, and use a 50 ohm glitch R /
cathode fuse / B+ fuse / 6A10 bias diodes
.. and you will never blow up any bias diodes, power supply diodes [also
6A10's] nor weld the contacts of the
RX cut off relay. I can repeatedly crowbar any of the 2-8 kv B+ supplies...
and all that happens is the B+ fuse blows open.
## On my buddies single GS35B 6M amp, he used W4ZT's bias board. Something
went wrong, with sparks,and a huge
arc across the OUTSIDE of the vac relay used for the cut off bias. Relay
survived just fine. [ Gigavac GH-1]
## amp rebuilt... but using a pair of GS35B's on 6m. This time 2 of the
w4zt bias boards used. Separate, switchable
metering also used. Then adjustable bias to each tube is used. 3 x GH1 vac
relay's used..all sped up. Input, output,bias.
Flat swr on bypass on 6m.
later... Jim VE7RF
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