[Test-Equipment] re: HP8640B audio oscillator thermistor

[Marco Bruno]

At 19.03 17/01/2006, you wrote:
>As I remember the first product was an audio oscillator. Many 
>designs used a lamp bulb
>as an AGC element. The Wein bridge puts out a very good sine with 
>low parts count,
>
>-John

And the first set of five oscillators - model 200A, if I remember 
well - was delivered to Disney Studios to produce "special" effects 
in the cartoon Fantasy (1938 or 1939, right?)

However nobody answered Ken's question on how to replace the bulbs in 
the potted "thermistor" mount. I suggest to measure the cold filament 
resistance of one of the survived lamps (with a low current!) and 
find a miniature lamp with approximately the same resistance; if 
possible find three identical lamps. In Europe they are quite easy to 
find on RS or Farnell catalogs; try DigiKey and so on. As a 
rule-of-thumb cold filaments have roughly one ninth of the hot lamp 
resistance, so a 6V 40mA telephonic bulb has approx. 150Ohm hot and 
17Ohm cold. Then you may try different lamps sets and measure distortion.

Please post your results, they will be useful to others!

Marco IK1ODO