Hewlett-Packard HP-200AB audio generator

Hewlett-Packard HP-200AB audio generator

Having repaired several HP instruments, I quickly grabbed this HP-200AB at a swap meet. When these pieces were new, they cost too much to experiment with. Now they can be obtained for relatively low cost and are fun to use and experiment with.

HP-200AB audio generator

The Hewlett-Packard HP-200AB audio generator will generate sine waves from 20 Hz to 40 KHz. Like the other HP audio generators, the circuit uses the famous light-bulb-stabilized Wien bridge oscillator patented by William Hewlett in the late 1930's. This HP-200AB is an early one with serial #632. Price in the1953 catalog was $120.

A PDF manual is available from the HP site linked on the home page. As usual, I started with safety checks.

The coupling caps in the amp section had been replaced earlier in the life of the set. I cleaned the power switch, the amplitude pot, and the range switch with deoxit contact cleaner. After a slow power up, the set performed as expected when connected to my oscilloscope. No further repairs were needed.

What's different in the HP-200AB
The 200AB has a very different circuit from the HP-200CD oscillators that I repaired earlier. While the 200AB has the classic Wien bridge oscillator, it has a separate push-pull audio amplifier with an output transformer with 600 ohm output and a separate winding that provides inverse feedback for its excellent relatively flat response from 20 Hz to 40 KHz. The amplitude control (think "volume control") separates the oscillator circuit from the amp circuit. Note the shield separating the two sections.

According to the 200AB manual, "The output transformer contains a tertiary winding for overall negative feedback in the amplifier section. The cathode bias resistor in the first stage (V3A) is not by-passed giving additional negative feedback. This large amount of negative feedback (over 30 db) in the amplifier section results in very little distortion being introduced by the amplifier section of the instrument." (page 8)

HP-200AB chassis showing oscillator section (top) and amp section (bottom)
HP200AB chassis
Amp section ------ 6K6GT -------- 6SN7GT -------- 6K6GT --------- 5Y3GT rectifier
The amp circuit uses a 6SN7 as preamp (V3A) and direct-coupled phase inverter (V3B) . A pair of 6K6 tubes in push-pull are the output tubes (V4 and V5). While the specs for the HP-200AB call for only one watt output into 600 ohms, the circuit parameters allow for nearly 10 watts RMS output given a sufficient input level to the 6SN7. A simple 70 volt audio tranformer can be used to match the 600 ohm output to a speaker impedance such as 8 ohms.

The HP-200AB does double duty as an audio amp.
Because these old HP instruments are now relatively cheap, I like to experiment with them for alternate and fun uses. I decided to try the 200AB as an amp. I hooked up a speaker and 70 volt transformer and fed the 6SN7 with a relatively high level audio signal. It turns out that the 200AB works quite well as a quality monobloc amp!

HP-200AB showing the power supply and audio amp section
HP200AB chassis showing amp section

A Hallicrafters WR-600 receiver was the previous item on the bench.

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