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HP 205AG Audio Generator / VTVM

June 3, 2013

HP 205 AG

This is an interesting unit being capable of measuring the frequency response of an audio system over a range of 20Hz to 20kHz. Back in those days, 20kHz was considered enough? My ears give out at 6kHz and I can hear music very well so yes, 20kHz was enough. Maybe we have evolved so fast that our hearing has caught up with that of dogs in this peculiar age of man where technology, fast taking on a life of its own, is getting away from us and the average or even above average human cannot keep up, much less appreciate the ramifications of technological run-away. OK where was I? Oh yes, this ancient audio frequency test unit.

It contains a Wien bridge oscillator, stabilized with a lamp exactly like the original oscillator that Dave and Bill produced to start the amazing company we now know as Agilent. It has three decade frequency ranges, 20 Hz to 200Hz, 200 Hz to 2kHz and 2 kHz to 20 kHz. A push-pull power amplifier (using triode connected 6L6s) is included that allows for an output power up to 5W into loads of 50, 200, 600 and 5000 ohms. Load matching is accomplished by a switched impedance matching transformer. I find the output power useful for in my messing around, sometimes a power signal is handy without having to resort to hooking up an amp. The distortion is given as less than 1% at frequencies above 30Hz. Again, somewhere down the line when transistor amps having large amounts of negative feedback were introduced, our hearing suddenly evolved and we could hear distortion levels of 0.01% or less! Amazing really, how marketing types drive human evolution….. Or not.

There is also an accurate output attenuator, referenced to 0 dBm (1 mW into 600 ohms = 0.7745 Vrms). It has a range from 0 dB to -110 dB in 1 dB steps. A 600 ohm load that can be switched in is provided, allowing accurate setting of the output in dBm. The output is available as balanced or single ended.

To measure the frequency response of the device under test there a VTVM calibrated in dBm from -5 to 8 dBm and in volts from 0 to 2 Vrms is provided. This meter includes an accurate stepped attenuator that extends the meter range to 48dBm and 200 Vrms in 5 dB steps.

Here is the top of the chassis: The push-pull output transformer is directly below the power transformer and has very fine laminations. The tuning condenser is self-evident. To the right is the top of the impedance matching transformer and below that, the switched output attenuator. If you look closely, you can also see the oscillator stabilization lamp that is located between the left-most electrolytic and the 6J7 tube (that has a top grid connection).

HP 205 AG Chassis T

HP 205 AG Chassis U

It was working as received, I did the usual Deoxit job on the switches and tube pins and lubricated the geared tuning drive. The only other thing required was to re-calibrate the output meter and the input VTVM, pre-set pots are provided for both meters.

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