Thoughts on Dc'ing Filament Supplies.

[ka3zlr]

Good Day Everyone,

 I've always wondered about this here and there, and have listened to differing opinions down through the years..

Would anyone like to comment....

[K1DEU]

Jack; In most cases when the first speech amp tube socket is accessible I would not run DC on its filament.

I would however re-wire it to balanced filament (at the tube end) with equal .01 RF bypasses on each side of the filament to ground.

In all transmitters I use new braid RG-58A/U from mike in jack to first audio grid. Also make sure you do not have ripple on any Push to Talk parallel line. Next, in All tube transmitters, I use balanced filament wiring to the first speech amp tube usually a AX7 or AU6.
 This makes a night and day difference in High impedance mike hum pickup. Do the standard stuff. Twist two equal small insulated conductors together to run from New floating (at tube) filament pins (neither grounded to chassis) to the filament transformer winding where one side is grounded to chassis (fine). No current is required to light one tube so use tiny wire. Make sure RF, filament bypass caps at first speech amp filaments are .01 from each side to ground. Now we cannot have a ground loop bringing 60 Cycle hum directly up through our grid leak resistor to the control grid ! This great technique came from Peter W1VZR many years ago.
middle of page http://hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/universal_speech_amplifier.htm

I have used 6 VDC to a VFO tube filament that wasn't accessible. To make sure the new 6AU6 DC filament supply was Clean and ripple free I used a 7506 I.C. 3 pin regulator on the DC full wave filament supply and .1 RF bypass disc cap near the filament. These regulators easily track/regulate at 120 cycles and can take ripple down an additional 30 or more Db. But, normally this DC should not be necessary, except maybe after being 4X to 10 meters.     73  John

[ka3zlr]

Thank You John,

 This makes for a good Sunday reading and Study..I am much Obliged OM..

[WBear2GCR]

The use of DC on the fils is common today in audio gear that is for low level mic or phono signals.

It probably doesn't really matter for ham applications in most instances.

I think it is ok fine.

For thoriated fils there are issues with using DC on the fils... but indirectly heated, no problem.

That's my 2 cents worth...

                   _-_-bear

[k4kyv]

For thoriated fils there are issues with using DC on the fils... but indirectly heated, no problem.

Actually, some of the very earliest broadcast receivers used tubes with thoriated tungsten filaments, and ran them off DC.

Those were the early sets with the 201-A family of tubes.  The '01-A has a 0.25 amp thoriated tungsten filament that runs white hot, almost like a light bulb.  It was designed to run off a 6-volt storage battery.  The older UV-201's (without the -A suffix) used a pure tungsten filament. They required about one amp per tube.  The filament was changed to thoriated tungsten in order to save drain on the battery.  Receivers with the older version of the tubes might draw 5 amps from the battery just to run the filaments.

I run the filaments in my mic preamp with regulated DC, as well as my VFO tube filaments.  I use an old 12-volt DC 2m CB power supply with a dropping resistor to get the voltage to exactly 6.3.  I always had a little residual hum I couldn't get rid of with a.c. on the 12AX7 filaments, and the DC cured it.  I run the transmitter VFO tube off the same power supply for the regulation.  The VFO uses a Collins type PTO (T-368 master oscillator,  highly modified).  Even minute changes in a.c. mains voltage would change the filament voltage enough cause the VFO to drift to the point that it was unusable on 40m CW.  Since I put it on regulated DC about 15 years ago, it is now rock stable. 

I  had the same problem with the 75A-4.  I now run the whole receiver off a Sola constant voltage transformer.  This is a newer model, designed to run a PC in an office environment, rated for just about enough power to run the receiver but not much more.  It is very quiet, almost silent, whereas the vast majority of the older Sola CVT's I ever tried vibrated so badly that they sounded like chainsaw running.

[ka3zlr]

Very Good,

 Thank You Don, Bear for your comments, this is an interesting subject and Not spoken of enough..it does have Advantages.

[W2JBL]

    i have used DC on the filaments of low level audio stages here for years, as well as balanced AC. either one makes your world a lot quieter. in VFO/PTO
applications i like the 7506 and 7812 regulator IC's with rectified filament voltage to drive them, as mentioned in another post. it eliminates that 60 cycle FM on a lot of oscillators. in my R390/R390A's i yank the ballast tube, half wave rectify the 24V AC, and stuff it through a 7812 to the PTO and BFO. this makes for one very stable radio.

[k4kyv]

Wasn't there an article in Electric Radio a few years ago for building a plug-in replacement module for the near-unobtainium ballast tube that did just that - rectified filament a.c. and regulated it with one of those 3-legged ICs' to light up the BFO and PTO tubes?