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Post by Richard on Mar 4, 2024 10:58:49 GMT 12
  I'm just restoring this unit, which is similar to the standard Radio console, but has a Garrard record deck in the top. On removing the chassis and all the tubes, I notice that there is a 6C6 UX6 base, when all the rest are Octal tubes as per the schematic. I've checked the different schematic versions of the model 75 and its definitely this version  However instead of a 6J7 as an Audio preamp, it had a 6C6(Radio Corp must have been short of 6J7?). Its been "repaired" sometime, though someone has put a 6K7 where the 6B8 should be, so it would never have worked, they had assumed that the model was the other version of the schematic which had the 6B8 as the detector and first audio amp. |
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Post by Richard on Mar 4, 2024 11:01:00 GMT 12
 The serial number and model 75A.  The chassis with the UX6 socket , where normally a 6J7 octal Tube would be. So far, the Field coil is open circuit. no doubt there will be other issues! |
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Steve
Society Members
vintageradio.co.nz
Posts: 734
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Post by Steve on Mar 4, 2024 13:08:36 GMT 12
Interesting. 1942 model, so right at the end of the 75's production... possibly shortages due to stockpiling for the model 90 which was not too far from production by that stage, or just using old stock? The country was running short of all kinds of things at that stage, and the radio control notice was about to come into effect, banning production and forcing factories to give their spare stock to the government for 'war use'...
It's a very interesting set for several reasons then - not just the rare upright console cabinet.
I have a couple of spare 75 chassis' in the shed, I'll have to look closer at the valve lineups!
Cheers, Steve
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Post by Richard on Mar 4, 2024 13:55:12 GMT 12
 It also has factory installed switch and terminals on the rear to change from "Internal speaker" to "External Speaker". The Gram/Radio changeover switch, as well as changing the input signal, also changes the negative Feedback to the plate of the 6C6/6J7 audio preamp, by placing a 75K resistor in parallel with the 270K Feedback resistor. |
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However instead of a 6J7 as an Audio preamp, it had a 6C6(Radio Corp must have been short of 6J7?). Its been "repaired" sometime, though someone has put a 6K7 where the 6B8 should be, so it would never have worked, they had assumed that the model was the other version of the schematic which had the 6B8 as the detector and first audio amp.
