[Amps] monitoring tube seal temps

Jeff Blaine AC0C keepwalking188 at yahoo.com
Mon Jul 6 21:44:01 PDT 2009


I saw one of the Acom 2000 on the bench of a buddy once.  That thing has 
some sensors in the tube exhaust.  It scales up the fan speed based on the 
temp so the fan only runs at a speed as needed.  Now that's a nice 

I implemented a similar design using the single chip pwm using the Microchip 
TC649.  It's an 8-pin unit that bases the attached dc fan speed based on 
thermistors.  I used 5k thermistors, one above each tube, in parallel.  From 
the thermistors I ran through a pi-net to kill off any rf (probably 
unnecessary), and there is no modulation with rf exposure.  only the 
thermistors are exposed - the wire connecting the thermistors is shielded. 
Another nice feature is that the chip will sense a locked up fan and provide 
a error control signal.  So no separate airflow sensor is required.  In 
addition, there is a separate threshold input that can be used so that the 
fan will be turned off after cool down.  So the fan runs at a lower speed 
when the amp is on, speeds up based on the heat produced, and remains high 
even if the amp is turned off (and hot) - slowing down as the amp cools - 
and then when it reaches the threshold, realizing the power is off, the fan 
will cut out completely.  Lot of functionality and minimal additional glue 
parts needed to make it work.  The IC sells for about $3.  And DC fans with 
up to 300 cfm capacity are available - Pabst makes some very nice ones - 
reasonabley quiet at full speed - and silent at low speeds... 
Implementations for AC fans are provided in the spec sheet as well, if I 
remember correctly.


From: "Radio WC6W" <wc6w_amps at yahoo.com>
Sent: Monday, July 06, 2009 5:45 AM
To: <n1miw at cox.net>; <amps at contesting.com>
Subject: Re: [Amps] monitoring tube seal temps

> Hi Larry,
>  There are a couple manufacturers (names escape from my moldy old mind 
> this afternoon) making complete sensors, with lenses and signal 
> processing, in a transistor sized case covering up to 300 degrees C.  I 
> believe that they cost about $30 ea. in small quantity.
>  A homebrew version might be cobbled together by cutting the top off an 
> old germanium transistor, covering it with a piece of opaque trash can 
> plastic, and using a small mylar conical concentrator in lieu of a lens. 
> A temperature sensor IC, or a second transistor of the same type, could 
> provide the ambient reference.  A small PIC or other uP, with an integral 
> A/D converter & a PWM output, could process the somewhat nonlinear (4th 
> power function!) raw signals to drive a meter.  Calibrate it against a 
> mercury thermometer in the kitchen oven and viola!
>  The latter approach could share the processing circuitry with multiple 
> sensors, for multiple tubes, or multiple location sensing.
> 73 & Good afternoon,
>  Marv WC6W
> http://wc6w.50webs.com/
> --- On Sun, 7/5/09, Larry <n1miw at cox.net> wrote:
>> From: Larry <n1miw at cox.net>
>> Subject: [Amps] monitoring tube seal temps
>> To: Amps at contesting.com
>> Date: Sunday, July 5, 2009, 6:26 AM
>>  I've been following the RTTY vs.
>> tube seal temp thread with interest. It
>> was mentioned that you can use the crayon to monitor the
>> seals, but those
>> are one-time use. What about a circuit such as the one used
>> in the hand-held
>> temperature guns (IR, I believe?). Are there available
>> components out there
>> where we could make our own small-scale circuit to monitor
>> the tube temps?
>> Personally, I've only seen electronic temp sensors which
>> work up to around
>> 150 deg C. What do you think?  Larry N1MIW
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