Voltage doubler? Unknown PT ratings.

[Phil_S]

Phil, I must be missing something here! How did you arrive at 425vac in your last post? If you have 200vac times 1.4 you wind up with 280volts.

No, it was me missing my morning coffee? Shoot, I posted that in the evening. That excuse doesn't work.
Seriously, it's actually a typo. I ran that calculation on 300V. 300 * sqrt(2) = 424 and change. Thanks for catching it. Sorry for any misleading info.

[flood]

R for the 6.3v won't tell you much. Five tubes is a decent number. I would assume the heater winding has at least 2A. The exception would be if there was a 6.3v rectifier, in which case, its rating is about 1A more. Jus tdon't look to put in a high mA Octal tube like KT66 or EL34. I doubt it will be able to support the current draw or produce adequate B+ supply to wake up that sort of tube. You should be fine with 6V6 or EL84.

Put your meter on the primary and find out how many ohms it is and use the chart. Be aware, that unless you've got a Fluke, your margin for error for low ohms is fairly high. You can test your meter on a low R resistor and see what it reports on the display.

thanks phil, i'll give it a shot. luckily i have a small fluke, and it is pretty accurate - it even tests capacitors rather well. will give it a shot on a low R value nonetheless.

i don't remember whether the radio had a 6.3V rectifier - i do have most of the tubes i pulled from it lying around somewhere. in any case, if it doesn't add up, i'll save it for a 6V6 design instead.

[Phil_S]

Really, there is every reason to use a solid state rectifier. Four diodes will cost less than $0.50, including the solder and electricity required to melt the solder. SS rectifier will save you on the filament requirement and give you the highest B+ supply.

Build a FW bridge (remember to cap off the HT center tap if there is one), put a cap in the circuit, and test that PT for B+ under a particular load. When the load drops rectified voltage 5%, you've reached the prudent limit (assuming the PT doesn't get too hot). Using the load in ohms and the measured voltage, you can figure the mA limit. Then you know what's appropriate to build.

[skyboltone]

Really, there is every reason to use a solid state rectifier. Four diodes will cost less than $0.50, including the solder and electricity required to melt the solder. SS rectifier will save you on the filament requirement and give you the highest B+ supply.

Build a FW bridge (remember to cap off the HT center tap if there is one), put a cap in the circuit, and test that PT for B+ under a particular load. When the load drops rectified voltage 5%, you've reached the prudent limit (assuming the PT doesn't get too hot). Using the load in ohms and the measured voltage, you can figure the mA limit. Then you know what's appropriate to build.

I agree up to a point Phil, but the higher the DC resistance of the secondary winding the more sag you'll get. The famous AO-35 Hammond Organ Reverb Amp transformer has 260 ohms across the secondary in a 300-0-300 secondary winding. The Tonslut Express tranny (I'm guessing) has about 40 ohms. With the Hammond, you run a pair of EL-84s biased to 11 volts with two gain stages and a PI and you'll run about 345VDC on the supply. It sags that much. That's why DR Z, Matchless, Goodsell, and others love that thing so much. You can experiment with changing the output winding resistance in Duncans and see what happens. It's not overloaded, it was just designed to be soft on regulation. The lower the secondary impedance the tighter the regulation and the less sagyou can allow.

[skyboltone]

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