GR 1201-B Unit Regulated PSU

I have one of these neat power supplies each for my 1217-B Pulse Generator and 1210-C RC Oscillator so I thought I would do a quick post on it. It supplies a floating precision 300V at 70mA and 6.3V at 4A.

A search on the net did not bring up the schematic for this unit so I have traced it out:

(P.S. After doing this I did find a link to the manual for this unit! Go here)

Apart from the primary control loop from the output to the series pass tube via the error comparator (12AT7), amplifier (6AN8 P) and cathode follower (6AN8 T), there are two other signal paths: C531 and R533 feed input fluctuations forward to the grid of the series pass tube in cancelation phase via the amplifier (6AN8 P) and R540 serves to make the open loop gain infinite thereby allowing very low output impedance. The technique of regenerative feedback to allow infinite open loop gain coupled with degenerative feedback to realise extremely solid stability and freedom from drift is also used in the TS-375 A/U VTVM DC amplifier which is the subject of my previous post.

I haven’t characterised the impedance vs frequency however, the use of a grounded grid pentode amplifier with a cathode follower to drive the grid of the series pass tube clearly indicates that it was designed to have excellent performance at high frequencies. My own regulator designs usually include feed-forward (DC coupled) but I have never attempted to make the open loop gain infinite.

Here is the inside with the PCB flipped up:

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