09-14-2005, 10:35 PM
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Join Date: Apr 2005
Location: Louisville, KY
The Maco 300 feeds 12.6 Volts to the heaters in the 8950 tubes. If you remove the center one of the five final tubes, you can wire pairs of tube sockets in series. This permits the use of (slightly) more common tubes with 6.3-Volt heaters, like 6LF6, 6KD6, 6LR6, 6LB6 and similar.
The heater is wired to pin 1 and pin 12 on the tube. These are the pins either side of the "gap" in the circle. On each socket, one of these has the 12.6 Volt wire and a ceramic disc bypass cap. The other one goes to ground.
To wire the heaters of the two driver tubes in series, the hot (12.6 Volt) wire is removed from the corner tube. The ground wire is removed from the socket of the other driver tube. A jumper wire now goes from the pin you just took loose from ground, and goes to the (now unhooked) pin on the other driver tube's socket. Now you can use 6.3-Volt tubes for the two drivers.
Repeat the same procedure for the front pair, and the middle pair of final tubes, and get rid of that fifth final tube in the center of the other four. It blocks air flow from the fan and doesn't add more than 15% or 20% to the peak output.
This trick requires an even number of tubes to work.
Just make sure that both driver tubes are the same type. And that the four finals match each other. The type used for the driver sockets does NOT need to be the same as the four final tubes. You can change over ONLY the drivers, and leave four (or five, if you wish) 8950 tubes in the final stage.
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