Alastair Reynolds wrote:You've gotta love old outside broadcast gear, built like a tank, basic, but everything you need to get the sound out..
Hi Alastair. (and all!)
(~8
Was reading your post above, on this thread, and thought it worth asking..
Do you happen to have any info on the AWA "PORTABLE O.B. AMP TYPE G56830"?
I have the original old service manual booklet for it here (but not the device itself, unfortunately!)
Looks like a cool earlyish model (mid 50's?) unit with three mic inputs.
Input transformers are AWA Type 1XZ562380.
Output transformer is AWA Type 1TU57705.
Tubes are: CK512AX sub-miniature (x5), 1T4 (x3), 1S4 (x2).
It's designed to run off batteries:
(2x) Eveready Type 467 "Minimax" in series for HT, and (1x) Eveready Type 742 for LT.
B+ is 135v, B- is -11v.
A- is -1.5v, A+ is ground.
(i think?)The booklet has an amendment at the start explaining that valves 1-5 are changed from type CK512AX to type XFW30 for model
1G56830, along with the addition of a dedicated output connection for telephone line feed.
Interesting to see these input stage tubes are the sub-miniature type. Not something I've seen in any of this kind of gear before, myself. (actually these tubes are 3x input stage, 1x mixer, and 1x phase inverter)
Just as interesting is the addition of a built-in compression circuit "using four GEX55/1 germanium crystal rectifiers (or equivalent) fed via blocking capacitors and potential dividers from the power amplifier output; these provide DC voltage which varies the grid bias on the push-pull driver stage, thus preventing the power output stage from seriously overloading."