TheRadioBoard

Back to this schematic for another question:



I have the high voltage part of the power supply mounted in a chassis and wired.

On this schematic it shows one side of the filament voltage to ground. Many of the books I read suggests that when using AC to light filaments that it should be ungrounded and you should use a twisted pair of wires between the transformer and filament pins on the tube sockets to reduce the possibility of 60 Hz induction into the other wiring.

Good idea in this case, or is there a reason one side of the filament voltage needs to be grounded?

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Secret of Electronics - keep the smoke _in_ the wires.

Hi Mike ,

It looks like they are trying to keep RF from geting on the fillament wires .

Mark

Ok. that makes sense. Grounded it is.

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Secret of Electronics - keep the smoke _in_ the wires.

Another silly question:

The filament winding on my transformer puts out ~7.1 volts with no load. Under a load equal to what the 2 tubes (6ag7 and 6bg6) will put on it (~1.5 amp) the voltage drops to ~ 6.7 volts. Is this close enough to 6.3 that it won't shorten the lives of the tubes, or do I need put resistors in series with the filaments?

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Secret of Electronics - keep the smoke _in_ the wires.

Its not enough to worry about , when the transmiter is keyed it will drop .

Mark

Just be aware that a lot of digital meters are not known for their accuracy when measuring AC voltages that are lower than about 10 volts or so.
Curt

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Curt, N7AH
CW forever
Connoisseur of the cold 807

That's probably within spec for those tubes, a good tube manual will give the upper and lower limits for filament voltage. If you ever have to drop the filament voltage a few tenths of a volt, it is easier to do it by using a longer filament lead between the transformer and the filament pin on the socket. Keeping the wire to the minimum required gauge will introduce some drop.

Pete

I measured it with an analog meter set on the 10 volt scale, so it should be close.

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Secret of Electronics - keep the smoke _in_ the wires.

It passed a smoke test, and with the dummy antenna on it I can hear it when I key it up on a receiver a few feet away.

Now I have to string an antenna for it.

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Secret of Electronics - keep the smoke _in_ the wires.

Lookin' good, Microbes. Well done.

6.7 Volts is a bit high for a 6.3 Volt rated tube. One of the reasons for this is that the nominal 117 Volt AC line voltage has been gradually increasing over the years. Some areas of the country now regularly run 125 Volts (or more!). Another reason is that some transformers put out a slightly higher voltage with no (or small) loads so that they produce their nominal voltage at full load.

I'm not sure what the exact relationship is with tube heater voltage versus life, but it's pretty dramatic for light bulbs (Somewhere around a -21st power relationship!!!).

Thus, I'd think about dropping that heater voltage a bit. One way would be to use a series dropping resistor. That may also help in that it will produce a slow turn-on effect for the tubes. Another way would be to use a bucking winding in series with the primary.

Dave

I've always heard +/- 10% as a general rule of thumb. That would allow up to 6.9 volts.

But, that said, if you're in the 125vac range then you probably would want to buck it down for other reasons. When I had a spell of high line voltage I saw many xfmrs that didn't like it...and lost a couple too! The power co finally did some work and got me back down to 120.

Nice work, Mike. Whats in the small square cans? Is that where you hid your chokes?

A long forgotten trick Uncle Pete reminded me of. Hang a NE-2 bulb off the plate cap. Leave the other end loose. Makes a great tuning indicator (I don't see any meters?) It should glow a nice orange, if you see it go pink or purple its an indication of parasitics.

-Bill

I ran the numbers and it called for a .23 Ω resistor to reduce the voltage to 6.3. I didn't think I had a suitable resistor but I dug thru my junk and found this 5 watter:



I stuck it in the circuit ahead of the filaments and .22 Ω turned out to be exactly right, the voltage drop across the filaments is 6.3 volts right on the nose (give or take a milli volt) .

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Secret of Electronics - keep the smoke _in_ the wires.

2 of them. The other two are mounted under the chassis at right angles to each other. I took the warning about feedback seriously.



See the two round black things on the top front right? They will take meter probes, and I have a small pocket multi-meter that will read up to 150 ma. that jacks right into it. There is a toggle below that lets it hit 2 test points. Tuned up on a dummy load with the coils in resonance at 80 meters I get 100 ma... So I think I'm looking at about 38-40 watts.



I might do this, sounds like a good idea.

I was thinking of using Curts idea of a can to shield the bottom of the 6BG6, but some of the local guys that looked at it were worried about ventilation, so I scrounged through my stuff and came up with a shield that reaches just above the grids, and leaves lots of air space.

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Secret of Electronics - keep the smoke _in_ the wires.

I just put a NE-2 on the plate cap like you said, and with the dummy antenna (a 40 watt light bulb soldered onto a coax) on the radio I can make it go "purple" if I over shoot on the final variable cap too much, but getting a nice orange on the neon is really easy to do. I put a RF watt meter between the radio and the dummy antenna and when it's properly tuned on key-up it shows something like 30 or 35 watts on the RF meter and the light bulb glows a nice semi bright red color. I think it will work when I get a proper antenna put up.

800K video:

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Secret of Electronics - keep the smoke _in_ the wires.