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Old 22nd May 2013, 08:46 AM   #11
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Join Date: May 2010
Location: Hawkes Bay
2x 6BL8's,
12v wall wart
240v/12v transformer
5 watt 100v line transformer.

See schematic on Page 140 of 100 buck amp challenge.
See picture on Page 133 ditto.

Have fun, don't touch the wires.

Cheers
JimG
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Old 22nd May 2013, 10:50 AM   #12
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For the schematic - you just take out the rectifier valve, the 5Y3GT on the diagram (which needs a 5 volt heater winding anyway) and replace it with a pair of 1N4007 diodes (or whatever the modern equivalent is) with the white-striped end toward the reservoir capacitor, the other end to the transformer windings. That's it; no other mod required. They cost about a cent each, if you buy twenty.

Before going to the sheet metal shop, check that chassis are not available somewhere ready bent (as they always used to be) – surely not everything goes into plastic boxes? A quick google brought up mainly ridiculously expensive units in the USA (where are you, by the way? helps for suggestions.) but that wasn't conclusive. If you do go to a sheet metal place, check whether they've got a fly press (or modern equivalent), and can do the round holes for you. Just involves planning and drawing out in advance, and then you drill the holes for mounting screwa, tag board or tag strip, fuse holders… those latter are frequently difficult to do well. I used to have Qmax chassis punches for all standard holes (Octal, b9a, b7g) and I think my 19mm for male switchcraft XLRs would probably do b9a) but I haven't been involved with this for some time, and I get the impression that prices have gone silly. If you're using aluminium chassis, you can use a hole saw, or buy a conical drill bit (nasty, but it works) - you'll need a decent drill, anyway, for pot, switch and jack holes.

Fifties and early sixties everything electronic had valves – but there just wasn't that much domestic electronics. And the most prevalent, the television, cut weight and cost (and any slight hope of safety) by not using mains transformers. I've salvaged ex medical gear, devices from university experiments, office intercoms and valve pape recorders, but by now most of that backlog has gone. And not all of them were suitable for power amplification, anyway; not only because they didn't always have enough current, but if they were designed for preamp style work they didn't have enough voltage. You really need at least 400 volts, and you'll find circuits on this forum exceeding 1000 volts (but the UHT winding of an old cathode ray tube oscilloscope is useless, because of its negligible current rating).
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Old 22nd May 2013, 01:36 PM   #13
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Quote:
Originally Posted by chrispenycate View Post
For the schematic - you just take out the rectifier valve, the 5Y3GT on the diagram (which needs a 5 volt heater winding anyway) and replace it with a pair of 1N4007 diodes (or whatever the modern equivalent is) with the white-striped end toward the reservoir capacitor, the other end to the transformer windings. That's it; no other mod required.
I would suggest you should place a wirewound resistor between the diodes and the reservoir capacitor, this simulates the 'highish' impedance of the valve rectifier and will keep the HT rail at a similar level.

Otherwise the amp runs at a higher voltage, and doesn't have the PSU 'sag' which is common with valve rectifiers.
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Old 27th May 2013, 03:19 PM   #14
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Awesome, thanks for all the replies - they are prompting me to do some research. I will re read until I completely understand what's going on. Thanks again
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