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Old 21st Jun 2021, 10:17 am   #1
cathoderay57
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Default 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. Ballís 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 Ė 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, itís 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
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Old 21st Jun 2021, 10:50 am   #2
Edward Huggins
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Default Re: Barker 88 Output Stage

I never really understood why 6F6s in PP were originally specified over 6V6s. Maybe so as not to produce too much power for the 8" speaker?
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Old 21st Jun 2021, 10:55 am   #3
paulsherwin
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Default Re: Barker 88 Output Stage

I think Barker were keen users of government surplus stock in their designs - they may have used 6F6s simply because they'd managed to get hold of a batch cheaply. When they used them all up they changed to 6V6s.
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Old 21st Jun 2021, 11:08 am   #4
Herald1360
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Default Re: Barker 88 Output Stage

I'd be inclined to keep the 6V6s. If nothing else, the lower heater current is attractive for less transformer stress!
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Old 21st Jun 2021, 12:36 pm   #5
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Default Re: Barker 88 Output Stage

As you say, it makes precious little difference in a domestic radio. I'd also be inclined to keep the 6V6's. Mine has 6V6's and it is LOUD!
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Old 21st Jun 2021, 4:00 pm   #6
cathoderay57
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Default Re: Barker 88 Output Stage

Quote:
Originally Posted by Herald1360 View Post
I'd be inclined to keep the 6V6s. If nothing else, the lower heater current is attractive for less transformer stress!
You're right. The mains transformer is marked 6.3v 3A and the total budget inc. 2 x 0.3A pilot lamps is 3.5A for 6F6 pair and exactly 3.0A for 6V6. I will leave the 6V6s in and also probably replace the pilot lamps with LEDs or else use 0.15A filament lamps. Cheers, Jerry
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