Date: Thu, 2 Jun 2011 15:11:39 -0600
From: "Jim Garland" <4cx250b@muohio.edu>
Subject: [Amps] Should a blower run after amp shutdown?
To: <amps@contesting.com>
Message-ID: <AEA880D1565E4930BDDA3599FE9CA146@JimsOffice>
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Like many homebrewers, I've always built a timer into my amps that powers
the blower for a couple of minutes after killing power to the amplifier.
Today, however, in a conversation with another builder, I began to question
whether this is actually a useful feature. (Obviously, exhausting hot air
from the enclosure after powering off an amplifier does no harm, but the
issue is whether any presumed benefit is worth the effort building in this
feature.) Here's my reasoning.
### On big external anode tubes like the 3CX-3000A7 / 3CX-6000A7, 3x10 etc...
all with thoriated tungsten fils, Eimac sez to run the blower for 15 mins
after the
amp is shut off...and that includes the fil being shut off as well.
## here's the procedure I use on both a 3CX-3000A7 [ 7.5v @ 50A]...
and also a 3CX-6000A7 [7.0V @ 80A] . Separate 240 vac feed into the
input of a 1kva Sola constant voltage xfmr. Output of Sola xfmr is fed into
the
input of a 0-240 vac @ 5A variac. Output of variac feeds the input of the
dahl fil xfmr. Variac has a cam + microswitch on it's shaft. Microswitch
contacts
activate the coil of a dpdt relay.
## 1 x toggle switch for blower, and a 2nd toggle switch for the fil. Input
of 2nd toggle gets it's power from the
output of the 1st toggle. It's then a simple interlock. I can have the
blower on and no fil, but not the other way around.
The variac is used to slowly ramp up the fil V to desired level. When
done for the night... the amp is placed into standby, B+
is shut down, and then the fil is slowly ramped down to zero V. Blower is
left on for 15 mins..then shut off.
## The 15 min blower over run is all automatic, done with an adjustable 'delay
on' type timer. If I forget to ramp the variac down
at 3 AM, I don't have to worry about fil inrush the next night. The cam +
microswitch on the shaft of the variac takes care of that problem.
The fil CAN'T come on until the variac shaft is rotated fully CCW,..and
engages the cam / microswitch! Once the variac shaft is
rotated fully CCW, the microswitch engages, and then feeds the juice to the
coil of a dpdt relay.... whose contacts pass the regulated 240 vac
into the input of the variac. It sounds more complicated than it actually is.
It's all quite simple really.
## Cold resistance of a light bulb is only 1/10th it's hot resistance.
Thoriated tungsten tubes are worse, only 1/12th.
## One day I had just the fil + blower only [ no B+] running on my
3CX-3000A7 amp. I shut off both the fil + blower
at the same time..and removed the tube from socket 1 min later. I had the
tube between my legs, while on a chair... adjusting
a hose clamp around the top of the tube [ which held several layers of silicone
rubber, used to shim the oversized chimney].
## The coaxial fil on the bottom of the 3x3 tube was warm... THEN started
getting hotter and hotter and hotter. The fil
was abt to burn my stomach ! My theory is/was... the heat inside the fil
had migrated back out onto the coaxial fil.
## and the amp was in stand-by and NO B+ on at all...just fil + blower.
From then on in...... blower over run was on for 15 mins
as Eimac suggested..and with the fil OFF too. No point in having blower over
run, with amp on standby..and fil still on.....not
with 375/560 watt type fils.
later.... Jim VE7RF
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