The #47 tube (audio driver) in my Philco 70 gives off a blue light on one side. I tested it for gas on my Hickok 534 and it was negative. I put in a NOS 47, and it also had a tiny tinch of blue on one side (it also tested negative for gas). I recapped everything except the tiny value molded caps. The resistors tested within tolerance. Is the blue glow to be expected, or should I look for a problem that doesn't involve caps or resistors???
As always, thanks in advance for your help!
Jeff S.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Not a rarity at all on the higher beam currents experienced on power tubes. Typically coinciding with a manufactured rectangular divot
opening in the plates side superstructure. Typically in about the same blue color spectrum as you would see on a propane torch or blue
natural gas flame.
73’s de Edd
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

:The #47 tube (audio driver) in my Philco 70 gives off a blue light on one side. I tested it for gas on my Hickok 534 and it was negative. I put in a NOS 47, and it also had a tiny tinch of blue on one side (it also tested negative for gas). I recapped everything except the tiny value molded caps. The resistors tested within tolerance. Is the blue glow to be expected, or should I look for a problem that doesn't involve caps or resistors???
:As always, thanks in advance for your help!
:Jeff S.

:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Mmmm...Philco 70...(pause....Droooool...wipe off drool...continue !)
:Not a rarity at all on the higher beam currents experienced on power tubes. Typically coinciding with a manufactured rectangular divot
:opening in the plates side superstructure. Typically in about the same blue color spectrum as you would see on a propane torch or blue
:natural gas flame.
:73’s de Edd
:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:
:
:
::The #47 tube (audio driver) in my Philco 70 gives off a blue light on one side. I tested it for gas on my Hickok 534 and it was negative. I put in a NOS 47, and it also had a tiny tinch of blue on one side (it also tested negative for gas). I recapped everything except the tiny value molded caps. The resistors tested within tolerance. Is the blue glow to be expected, or should I look for a problem that doesn't involve caps or resistors???
::As always, thanks in advance for your help!
::Jeff S.

Philco 70 oooya. Like edd says it normal. you will see this in 45s 46s,your 47 and the great 2a3. these and others fluoresce with this "propane flame" blue. In vacuum tubes there are still billions of atoms of “gas” inside the vacuum not locked up buy the getter. As electrons /ions boil off the cathode and bounce around they are going to strike these. in simple terms the electrons in the gas and stuff jump up and as they return to normal they give off a little visable light. As long as the florescence appears on the glass or around support structures or mica it’s just a wonderful effect that solid state guys will never see. If it appears between the grid(s), cathode or anode you either have a mercury vapor rectifier, gas regulator tube, or a problem. Turn down the lights crank up the volume and you may notice in a big power amp tube, like your 47 the blue glow will pulse with the back beat of the music. Way cool.

I have seen the glow on the beam deflector grids of 7591s and on the plates of some perfectly working tubes. You definitely don't want to see the glow in "gas" form, though. That is, you don't want to see the space between the grids or around the plate glowing. It is beautiful, though, isn't it? I love to watch the glow jump and change shape on my 2A3. You can touch the glass and make the glow move, too.

Also, a gassy tube usually glows purple.

Thomas

Gentlemen,

Thanks for the information. It is as you all described. I turned up the volume and turned down the lights; there is some pulsating in the glow that's neat to watch. I'm considering building a remote socket for the 47 so I can set it beside my P-70 and watch it do its thing.

Jeff S.

Careful, there. Wires that are too long can cause feedback. In order to build a separate socket, you'll have to put all of the wires into a shielding webbing. You may also want to shield the grid wire separately within all of the other wires. As for the socket, since it is for the output tube, it doesn't need to be shielded too much. You may put it inside of a metal or a wooden box of small size (perhaps no more than 4 inches L x W, and a similar dimension for the height) and to the design which you prefer. Placing veneer inlays into the box may make for a really attractive box. Some wood working stores sell some fabulous inlay borders. You can always make your own, of course. Make sure that the box has a heavy lead weight in the bottom of it so that it can't tip over and break the expensive #47 tube.

Thomas

You may wonder, then, why it is so important to shield the wires and not the socket. Well, for one, the socket won't be that close to the radio. For another, the wires expose much more radiation than the box. If the plate wire radiates too closely to the 1st audio amplifier, or if putting on all that extra wire causes it to radiate a lot, for whatever reason, you'll get feedback. Any one of the primary stages can oscillate with the output tube (including the 1st RF stage). If you still have shielding issues with the socket, and you have built a wooden box for it, you may partially line the inside with aluminum. Attach this to one of the socket mounting screws, and attach a terminal to this screw. Tighten well (except if a porcelain socket is used...be gentle here). Then solder one of the wire shields to this terminal. The wire shields should be connected to the radio chassis on the other end.

Thomas

Incense and pepermints