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Old Tube Transmitter Questions

Feb 12th 2015, 19:00

WM3X

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I am planning to build a transmitter/power supply combination that appeared in QST, August, 1936, p.23, and later in the 1937 Handbook, p. 183. Period parts are hard to come by, so I have to make do with what's available, and my knowledge of tube circuits is a bit limited when it comes to understanding 1930's schematics. So here are a couple of questions that have come up.

Question 1: The power supply shows a transformer secondary winding of 2.5 v and a grounded center tap powering the filaments of the the oscillator and doubler stages. The final amplifier tube has the same filament circuitry with another secondary winding of 7.5 v, no center tap. The filament transformer I have lacks the ct on the 2.5 v winding. Can I just go ahead and use it, ignoring the missing ct?

Question 2: On the schematic it looks like the keying circuit puts +45 v directly to ground. The voltage is from a battery. The text says you don't need the battery. What I need is an explanation of what is connected to what here. Are crossed lines connections?

Question 3: The filter capacitors were dual unit can types, 8 mfd wired in series to give 4 mfd and 475 v. I plan to use new electrolytics, 2 at 10 mfd in series to give me 5 mfd at 600v. They look pretty small. Should I be worried about that?

Question 4: Similar to question 3, the original bypass and coupling "condensers" were pretty large in the photo. I am using new silver mica capacitors of the same capacitance, 500 v or greater, that look pretty small. Again, should I be worried about that?
Feb 12th 2015, 22:32

WM3X

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Well, here's a big oops! for question 3. Must have been a senior moment. The original filter caps in series gave 4 mfd at 950v, and my two 10 mfd caps will result in 5 mfd at 1200v. They still look pretty small.
Dec 29th 2015, 11:49

W4BOH

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
How are you doing on this?
I can help if you are not finished.
Wilson
W4BOH

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