Date: Mon, 27 Feb 2017 11:02:06 -0700
From: Steve London <n2icarrl@gmail.com>
To: amps@contesting.com
Subject: Re: [Amps] Maintenance-free
On 02/27/2017 10:55 AM, Jim Brown wrote:
> Yes, the control circuitry is quite complex, and I wouldn't want
> to work on it. But that control circuitry can prevent a lot of damage,
> and can make tubes last longer by going into protect mode when grid
> current rises.
There are very simple ways to automatically bypass the amp when the grid
current becomes excessive. Here's an example :
http://www.k6xx.com/radio/76a_prot.pdf
73,
Steve, N2IC
## On tubes like 4CX-1000A with its ZERO watt grid diss, and the 4CX-1500B,
with its 1 watt of grid diss, and 3CX-800A7, with its 4 watt grid diss, grid
over current
protection is required. You dont require a processor to pull this off either.
Once it trips,
you can set it up to either auto reset after XXX secs, or kick the amp
offline till a manual
re-set is done. They can also be set up to require 2-6 occurrences of
overcurrent, then kick offline, requiring a manual re-set.
In some cases, you can just open off the ley line, or activate the TX inhibit
lead... or apply a few volts negative, to the xcvrs ALC buss. Or a combo of
those 3 items.
## I spent some time recently helping a fellow with a tetrode amp, which
includes grid over current,
screen over current, and plate over current. Straight forward simple circuit
for each item. If you really
want to go crazy, you can also add B+ below a certain threshold voltage, and
ditto with screen voltage.
## High swr can be done either exceeding XXX threshold for rvs power, or
exceeding a specific swr value.
The problem with the high rvs power method, like alphas pre-set 300 watt of
rvs power is.... the 300 watt
threshold equates to aprx 2.5:1 swr. But if you run the amp at say 600
watts out, the 300w rvs threshold now becomes
50% ref power, = sky high swr.
## at least with the array solutions watt meters, they trip on swr, and
anything from 1.5:1 to 2:1 to 2.5:1 or 3:1
Or if you dont want any swr protection, that function can be shut off all
together. Again you can have it either auto
reset after XXX secs.....or do an instant lock out, requiring a manual re-set.
And again, the amp can be kicked offline,
either by opening off the key line, or applying a dew volts negative, to xcvrs
alc buss, or inhibiting TX inhibit lead on the xcvr.
## On bigger metal triodes, a simple rear mount, fast 3agc grid fuse will
suffice. That also works with 3-500Z amps as well.
If grid fuse opens off, no path for dc grid current, amp cant be driven, PO
drops to zero, input swr rises to infinity, xcvr shuts down asap.
## I typ use a B+ fuse in the B+ lead, inboard of the 50 ohm glitch R. Be
careful with this scheme, on any GG amp, if no B+ present, and drive
applied, you will peg the grid current meter. A low B+ voltage alarm is
easily incorporated. The array solutions wattmeters also have low and high PO
alarms..with various pre-set thresholds.
## as far as alarms in general go, you can divide them up into soft and hard
faults. For catastrophic events, you want it shut down asap. For
soft faults, like wind induced swr variation on long eles, an auto reset is
more inline. A buddy set up the grid over current protection on his hb
YC-156 so the green led is normal. Yellow led is slightly excess grid
current. Red led lights, and it trips asap, requiring a manual re-set.
A lot of us will use airflow switchs in either the metal tube exhaust, or on
the output side of the blower used. Blower failure is catastrophic.
Vane closes a micro switch via its sail, then the open contacts... open off the
coil of a small relay, kills power to the fil xfmr asap, and kicks amp off
line.
## dead simple to trbl shoot, no processor and lap top required. The 87A is
a superb amp, but most protection can easily be added to an existing amp, or a
hb
amp. Since I had spare aux contacts on some of my contactors, and time delay
relay modules, I added other simple protection, like the key line is opened
off,
till the step start sequence is completed. Tubes are too expensive to damage
from pilot error, esp with guest ops etc.
Jim VE7RF
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