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cathoderay57 · 21st Jun 2021, 11:17 am

Hi folks. I have started work on a Barker 88. These sets seem topical at present! From my initial checks I found when metering the audio output transformer that one half of the centre-tapped primary has a DC resistance of 286 Ohms (about right) but the other half measures 3.1k. So basically it is knackered! Ed Dinning has kindly offered a rewind. It is marked with a primary impedance of 10k which would be right for 6F6 but a bit high for 6V6 (8k). The set came fitted with a pair of 6V6GTs and a shared cathode resistor of 300 Ohms (definitely original) which seems to be a median value as a compromise for several different valve pairs, because:

A.M. Ball’s valve databook specifies 320 Ohms for 6F6 and 250 Ohms for 6V6, whereas Trader Sheet #747 specifies 220 Ohms for 6F6 and 180 Ohms for 6V6. Not sure why lower values were specified – these would result in higher anode current owing to less bias and therefore higher output, maybe to compensate for the fact that the anode voltages are only 225v. The Balls values are quoted for anode voltages of 315v (6F6) and 285v (6V6) so the operating points would be different from that used in the Barker. Anyhow, it’s not worth tying myself in knots over it, because in the grand scheme of things it probably makes precious little difference in a domestic radio!

Once I get the transformer rewound I'll fit a matched pair of 6F6 and leave the cathode resistor as-is (it measures 296 Ohms). Thankfully the mains transformer checks out OK. Any comments welcomed.

Cheers, Jerry

21st Jun 2021, 11:17 am	#1
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Barker 88 Output Stage Hi folks. I have started work on a Barker 88. These sets seem topical at present! From my initial checks I found when metering the audio output transformer that one half of the centre-tapped primary has a DC resistance of 286 Ohms (about right) but the other half measures 3.1k. So basically it is knackered! Ed Dinning has kindly offered a rewind. It is marked with a primary impedance of 10k which would be right for 6F6 but a bit high for 6V6 (8k). The set came fitted with a pair of 6V6GTs and a shared cathode resistor of 300 Ohms (definitely original) which seems to be a median value as a compromise for several different valve pairs, because: A.M. Ball’s valve databook specifies 320 Ohms for 6F6 and 250 Ohms for 6V6, whereas Trader Sheet #747 specifies 220 Ohms for 6F6 and 180 Ohms for 6V6. Not sure why lower values were specified – these would result in higher anode current owing to less bias and therefore higher output, maybe to compensate for the fact that the anode voltages are only 225v. The Balls values are quoted for anode voltages of 315v (6F6) and 285v (6V6) so the operating points would be different from that used in the Barker. Anyhow, it’s not worth tying myself in knots over it, because in the grand scheme of things it probably makes precious little difference in a domestic radio! Once I get the transformer rewound I'll fit a matched pair of 6F6 and leave the cathode resistor as-is (it measures 296 Ohms). Thankfully the mains transformer checks out OK. Any comments welcomed. Cheers, Jerry Attached Thumbnails