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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

need power tranformer ideas


2007-08-26 11:02 pm
Hi All;

I am planning a guitar amp along unconventional lines. The plan is to use a pair of 6CA7's in push-pull class-A. The plate voltes will be around 270VDC and they will pull about 85 ma each using fixed-bias.

I could use either a bridge or center-tapped supply but most commercially available trannies don't offer secondaries that produce that kind of voltage at that draw.

The amp itself will be pretty simple. Two 6SL7 preamps (one for gain the other a long-tailed pair PI). Right now I am thinking about 200uf after the rectifier and to the plates, a 1k dropping resistor to the screen supply (with 1k screen resistors) and a 40uf cap and two more small dropping resistors to the PI and preamp.

If you have any experience or a recommendation on an off-the-shelf power transformer, especially available in the US, I'd love to hear about it.

Thanks, Skip
For those of us with 240V AC power and who work in the US regularly there are a heap of 120V to 240V stepup trannies available at various power ratings. Full Wave Bridge rectify the 240V side for around 290 to 300V Dc under load. Separate Filament transformer (toroid would be available) and you are all set.
Just another option.
Boris hit the nail on the head. Voltage double a Triad N-77U, as was done in "El Cheapo". Please observe that the trafo is rated for over 800 mA. RMS, but the PSU is good for 200 mA. The rail comes in a bit over 300 V. So, use a modest cap. after the choke. Then, follow with a gyrator and a final "tank" capacitor. The resulting PSU will be dead quiet and on the money, voltage wise.

Allied Electronics sells the Triad N-77U for $25.82 each. Allied also sells the Triad C-24X 1 H. choke for $7.74, which will permit a few mA. beyond 200 to be safely drawn.

Folks living in "240" V. zones can use the Triad N-77U, courtesy of dual primaries. :)
Try a Hammond 272JX (300V 250mA) SS-rectified and with choke input; you should hit 270V on the head. Unless, of course, the guitar experts here do not advocate the sound of such a supply :scratch2: I suppose it depends on what kind of tone you're looking for? Class A is, as you say, already unconventional and should have some interesting breakup characteristics. Being from a Hi-fi perspective, though, breakup is something I try to minimize in design, so I won't be much help :smash: