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Audio and Video Talk
The Vintage Audio Section
Lafayette Superheterodyne Radio - 1935
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<blockquote data-quote="El Sid" data-source="post: 1005820" data-attributes="member: 18550"><p>As it turns out, I do have an EL84 PP OPT lying around, which is destined for a guitar amp. The Type 47 wants a 7k load according to the data sheet, so not too far off, and should work. A second hand PP OPT may well be a practical solution - even if it's not optimal it's better than f@k@l, and I doubt my mate will want to cough up the couple of R '000 for a custom-made one.</p><p></p><p>There's another subtlety around the speaker field coil though, which took me a little while to figure out. All current to ground is actually returned through the field coil to the 21H choke and then through the choke to the centre tap of the 360-0-360 power transformer. So the voltage across the coil is actually negative with respect to ground, and this negative voltage is used to bias the grids of the 47s (and some other valves). According to the data sheet the 47s want about -15V on the grid. The 47s use about 37 mA cathode current each, the 55/57/58 about 10 mA, a total of 125 mA. So a 120ohm (5W) resistor should do in place of the field coil.</p></blockquote><p></p>
[QUOTE="El Sid, post: 1005820, member: 18550"] As it turns out, I do have an EL84 PP OPT lying around, which is destined for a guitar amp. The Type 47 wants a 7k load according to the data sheet, so not too far off, and should work. A second hand PP OPT may well be a practical solution - even if it's not optimal it's better than f@k@l, and I doubt my mate will want to cough up the couple of R '000 for a custom-made one. There's another subtlety around the speaker field coil though, which took me a little while to figure out. All current to ground is actually returned through the field coil to the 21H choke and then through the choke to the centre tap of the 360-0-360 power transformer. So the voltage across the coil is actually negative with respect to ground, and this negative voltage is used to bias the grids of the 47s (and some other valves). According to the data sheet the 47s want about -15V on the grid. The 47s use about 37 mA cathode current each, the 55/57/58 about 10 mA, a total of 125 mA. So a 120ohm (5W) resistor should do in place of the field coil. [/QUOTE]
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Audio and Video Talk
The Vintage Audio Section
Lafayette Superheterodyne Radio - 1935
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