Does running the blower after shut down apply to a tube like the
4CX1500A [5V @ 42A]? I don't see any thing about that in the Eimac spec
I've wondered about that for the amp I am building. I have a variac on
the filament transformer because I wanted to set the voltage and my
filament transformer is over-sized for the requirements of the tube.
Your design sounds good because I have wondered how to prevent
forgetting to reset the variac after shut down. I never thought of a
constant voltage transformer on the filament circuit. That's a good
On the other hand, if I revise the filament circuit to 240 VAC I have a
properly sized filament transformer for the tube so I don't have to
worry about inrush current.
On 6/3/2011 6:44 AM, Jim Thomson wrote:
> Date: Thu, 2 Jun 2011 15:11:39 -0600
> From: "Jim Garland"<email@example.com>
> Subject: [Amps] Should a blower run after amp shutdown?
> Content-Type: text/plain; charset="us-ascii"
> 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
> 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
> 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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