Tube DIY Asylum

A few important power supply notes, etc :

97.115.105.224

This Post Has Been Edited by the Author

In Reply to: RE: A push-pull triode EL84 schematic by Thomas Martens posted by LowRedMoon on May 24, 2009 at 10:16:49

Thanks indeed Adam, for the nice review!
The amplifier design was originally built using the much more robust (big!) Hammond 372 JX power tranformer which is rated at 300V-0-300V @ 250 mA. The reason why I initially specified the 372JX power trans, was so the builder could, if desired, slightly re-configure the amplifier for ultralinear operation, which requires considerably more current from the power supply than triode-mode. With the original 372JX transformer, and the amplifier operating in the triode-mode as shown, the addition of a 25-Watt 270-Ohm series dropping resistance between the rectifier cathode and the input filter capacitors was needed to drop the main supply voltage to the specified +325 volts.
The optimized lower-current triode configuration shown, however, has been pretty much universally prefered by listeners, and therefore the selection of a smaller power transformer than the 372JX for a dedicated 'triode-mode-only' amplifier makes good sense.
Adam has found the smaller Hammond 370EX power transformer (now shown) to be an ideal match for this amplifier, and I fully agree. however...
Just one SUGGESTION: the power supply schematic posted now shows no series dropping resitor used between the 5AR4 rectifier cathode and the first filter capacitors. Be sure to measure the MAIN SUPPLY VOLTAGE of your unit at the point shown on the diagram. It should be very near the +325 volt reading as shown. Plus or minus 5-volts is fine, but if it is 10 volts or so higher, then I STRONGLY SUGGEST you install a 25-Watt main supply series dropping resistor between the 5AR4 cathode and the first filter caps: here are the values you might need:
If the actual main supply voltage is found to measure approximately:
335V then choose a resistor of 90-Ohms
345V then choose a resitor of 180-Ohms
355V then choose a resistor of 270-Ohms
Keeping the main suppy voltage near the specified +325 Volts(as measured at the audio output transformer's center taps)will ensure best operation, and the longest tube life.
Also: the .5 amp HV fuse shown is a bit large. I would suggest instead a 1/4 amp fuse be used.
Cheers,

-Thomas

Edits: 05/24/09

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