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Author Topic: Preferred tube filament wiring method?  (Read 462 times)
KC9KEP
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« on: February 09, 2009, 04:20:00 AM »

Hello Elmers!

I've seen several schemes for tube filament wiring in vintage radio receivers:

Twisted pair to each tube, one wire to each filament connection and the other filament lead grounded, and use of shielded cable to run the filament lead.

Do you suppose than any one method has a decided advantage over another?

(Of course, connecting one lead to chassis or using a low value resistor or pot from each filament lead to ground seems to be a common method of reducing hum as well.)

Thank you all and 73!

--Tom Nickel / KC9KEP
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WA3SKN
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« Reply #1 on: February 09, 2009, 05:33:32 AM »

The best method was shielded twisted pair to each tube.  This was done to minimize any pick-up.
73s.

-Mike.
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W8JI
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« Reply #2 on: February 09, 2009, 06:04:46 AM »

Tom,

In most cases you will see no difference at all, that is why most receivers and transmitters never bothered with doing anything fancy.

A few systems years ago used floating twisted filament wires and a "hum balancing" pot to zero out any hum, but that largely wound up being a waste of time.

The only case where filament induced hum can be an issue, besides the obvious defective tubes with H/K shorts, is in tubes with high voltage filaments or in very low signal level high gain amplifiers or very sensitive oscillators that have the cathode floating at a fairly high impedance at 60 Hz. This is VERY uncommon, so it usually turns up that any special wiring is a total waste of time.

73 Tom
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AC5UP
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« Reply #3 on: February 09, 2009, 08:33:02 AM »

I have been piddling with boat anker restorations off an on since the 60's and have noticed a few tendencies under the chassis of commercial and consumer gear... Whether any of this makes a night & day difference in the performance of the circuitry is in the ear of the beholder, but here 'tis:

AC line voltage will generally be routed along one end of the chassis as far from everything else as possible, especially low-level audio. Filament leads are twisted pair routed at right angles to the socket and snug to the chassis. General goal is to have as little filament wiring directly under the passive components as possible. Not unusual to see a .001 ufd ceramic bypass cap or similar from one side of the filament to chassis ground at each socket.

Audio leads for the volume, bass, treble, tape monitor switch, etc. are kept short, twisted and/or shielded to minimize stray coupling. In RF sections, especially oscillators and pre-amps, lead lengths are kept ungodly short to the point where you wonder how in the hell the factory dudes crammed all that in without smoking parts when it was soldered. Then you wonder how you're going to replace R152 without tearing up everything around it. Eventually you realize you'll have to replace three adjacent parts and start surfing the junk box for a 68pf NPO ceramic cap 'cuz that's the trick part in the clump... Until you notice the piston trimmer cap and how delicate the leads are, so you cut R152 off the socket and install the new one "ugly".

Other tricks I've noticed are resistors arranged with their tolerance bands all oriented in the same direction N-S and E-W so they're easier to read at a glance. If there are three caps in a row on a terminal strip the center cap is 90 degrees out from the other two for minimal coupling and usually arranged so the part value is visible. If the marking on a cap is buried it's because the outside foil polarity made it go that way.

With all that said, I see things in modern gear like every wire bundled into one harness and RF leads bent at sharp right angles that would have been a bad idea in tube gear, but doesn't seem to matter today...

Chances are it does matter, but if the compromise is between production efficiency and a tenth of a dB better noise floor you know the bean counters will win that argument every time. Wink
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KC9KEP
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« Reply #4 on: February 10, 2009, 08:32:47 AM »

OK, everyone ..

Thank you for your responses!

73

--Tom Nickel KC9KEP
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