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 Post subject: what is wrong with my tube? it is flickering brightly.
PostPosted: Sep Mon 28, 2009 4:12 am 
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Joined: Apr Thu 23, 2009 12:25 am
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Hi,

I recently restored a Colin B. Kennedy model 20B, which is a console receiver, 7 tubes, TRF type, circa 1929.

It works really well, except something strange is happening with the first audio/detector stage tube. This tube is a type 227 globe tube, which is a triode. Its filament is flickering really brightly, bright-dim-bright-dim-bright-brighter-dim, etc. The filament is much brighter than the other 227 in this radio, and it is flickering.

I have ordered a replacement 227. But, what might be the cause of this? Could this cause pops and static noise in the audio amplifier circuits? (sometimes i do get this). Or, could i just use this tube anyway with the flickering until it dies?

Thanks for any advice on my flickering tube.

Greg


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 Post subject:
PostPosted: Sep Mon 28, 2009 4:41 am 
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Is there another type 27 in the set?? If so, swap them around. If the same tube flickers in the other socket position - I'd suspect a bad tube... possible a filament about to blow. If the flickering transfers to the other tube in its new position - there may be something deeper. Did you recap the radio???

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 Post subject:
PostPosted: Sep Mon 28, 2009 10:48 am 
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227 is an indirectly heated tube (heaters not filament).

The heater may be breaking up & shorting to the cathode, or there is too much voltage between it and the cathode & its flashing over.

I would check the plate voltage, the plate resistor, if it has one and the grounding of the cathode. The heaters may float and be grounded by a "humdinger"? Have a serious look at its integrity and connexions especially if it shares earth with the cathode. I had a similar fault to that in a Lyric 70 and corrosion in the humdinger caused that.

Tube substitution is worth a try if you cannot test it, or try heating it external to the radio from another source, to see what happens?I would check everything around it before risking the second tube.

You can often pick up a heater cathode short with an ohmeter, but this one may only short when its hot? (have not looked at circuit)

Marc


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 Post subject:
PostPosted: Sep Mon 28, 2009 12:10 pm 
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Hi jgj6331 and Marc,

Thank you for your replies!

Yes i have re-capped the radio. A schematic can be found here (except that the audio output stage in mine is a single-ended pentode and the cathode resistors are 1.5K in the RF section), scroll down to the bottom last schematic:
http://www.nostalgiaair.org/PagesByMode ... 039547.pdf

Yes there is a second 227 tube. Per your advice I have swapped them both and found that the bright flickering one continues to flicker in the OTHER socket. Where the normal red/orange hot one continues to glow at its lower temperature as it was before.

None of the other tubes in the radio flicker.

Although the RF tubes, the 224's are all pretty bright compared to any modern tubes i have used before. I'm just wondering if they are normally like that anyway considering i have never worked with globe tubes before. What do you think?

These are not flickering, they just seem bright like an EL34 or something.

Thanks for your help, this is very interesting.....

Greg


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 Post subject:
PostPosted: Sep Mon 28, 2009 12:25 pm 
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I'd bet $$ that the tube in question is bad. Replace it. They're not expensive.

Gene

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 Post subject:
PostPosted: Sep Mon 28, 2009 12:40 pm 
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I'd vote bad tube also.

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 Post subject:
PostPosted: Sep Mon 28, 2009 1:46 pm 
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One last thing you could try, before consigning the 227 to the junk drawer, is to resolder the filament pins. Occasionally a tube shows up with a weak solder connection there, right from the factory, and it has worked loose over the years. An intermitant connection might be causing the flickering.

The filament is connected to the fat pins, 1 and 5. Remove the tube and heat the side of the pin with your soldering iron and see if you can feed a little solder into the end of the pin.

If you think the filaments are glowing too brightly, you could measure the AC voltage at the heater terminals while the set is in operation. It should be about 2.5 volts.

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 Post subject:
PostPosted: Sep Mon 28, 2009 1:57 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Some of the early AC tubes, and the 27 in particular had massive ceramic insulator sleeves between the actual filament wires and the cathode. Some of them take what seems like forever to heat up the cathode, while the filament burns at a white heat. And when they are turned off, the filament goes out almost immediately, but the cathode takes a considerable time to lose its brilliance. This is normal with those tubes. Another one to watch for with these characteristics is the early 24 and 24A tubes.

However, it should not be "flashing", as you describe. It may have an intermittant heater/cathode short. If this is the case, take and connect both filament pins together and discharge a large capacitor, maybe 50uF that is charged up to a couple hundred volts between the shorted filament pins and the cathode. It may blow clear any short, or else it could ruin the tube. 50% chance of either, so it depends on your luck the day you try it.

But the tube sounds useless to me, anyway, so what is there to lose? Also make sure the filament pins have been resoldered and the tube socket is making good connection to the pins, as already stated.
Curt

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 Post subject:
PostPosted: Sep Mon 28, 2009 2:46 pm 
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This is looking very much like a dodgy tube.

These have a fair bit of current draw so the wiring needs to be in good order. 2.5V @1.75 Amp

I would check the sets heater voltage to make sure its not too high.

I note multiple heater windings, check all of them.

If all are too high check which tapping the primary is on?

Marc


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 Post subject:
PostPosted: Sep Mon 28, 2009 2:48 pm 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
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Location: Hueytown, AL
Very rarely the heater itself will partially short to itself. If this isn't too severe such that the tube burns out, it may flicker as you have observed. Most have seem incandescent lamp bulbs do the same, tree lights and so on, often just before they burn out. The majority of tube heaters consist of straight wires folded back on themselves and depend on a coating to keep them from shorting to each other. Some of the early tubes used a spiral or coiled filament which I would think might be less likely to do this and if one did probably would not be as noticeable.


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 Post subject:
PostPosted: Sep Mon 28, 2009 6:10 pm 
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MARCC: I drive a humdinger, and I think I'm married to one, but I've never seen one in a radio. Could you define it??


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 Post subject:
PostPosted: Sep Mon 28, 2009 7:23 pm 
Silent Key

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Location: Hueytown, AL
Betcha DKinYORKpa could describe one!

Haven't heard from him here in some time.


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 Post subject:
PostPosted: Sep Mon 28, 2009 9:15 pm 
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Curt Reed wrote:
discharge a large capacitor, maybe 50uF that is charged up to a couple hundred volts between the shorted filament pins and the cathode.Curt


I do that to rejuvinate old NI-CD batterys. Use a 100mf 350v or so charged up to about 200-225V.

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 Post subject:
PostPosted: Sep Mon 28, 2009 9:38 pm 
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Hi

I've also seen filaments on early globe 27's go between bright and dim. The filament is made from strands of insulated wire going up and down within a cathide. Where the filament bends to change direction there isn't any insulation. Possible a couple lengths of filaments shorts intermittently at this point.

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 Post subject:
PostPosted: Sep Tue 29, 2009 2:17 am 
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Hi everyone,

Thank you for your advice and conversation. This is a very interesting problem.

Per your suggestions i have ordered a 'new' 227 tube.

Also, i pulled mine out and hooked the filament pins up to a bench supply. Watching the current meter on my bench supply i could see that the filament current was varying once it warmed up.

If i held the tube horizontally and slowly rotated it, i could see the current bounce around more or less depending on how much i rotate the tube (to what extent i rotate, full CCW, half CCW, etc).

In fact, using my steady ham radio tuning skills, i was able to rotate the tube just right so that the filament completely open-circuited! No current flow.

I also checked for shorts from the filament to any other pins. No shorts in my 227.

In summary i think the filament is broken inside the ceramic insulator part. It is opening and closing while it is warmed up, like a blinking christmas bulb. It could also be shorting against itself too. If the tube is horizontal and at the right angle the filament will open-circuit all together.

I will check back in with you guys on this thread once the new tube shows up later this week or early next and let you know if that fixes the problem :)

Amazingly enough, the radio still works great for now. I will continue to listen to it. Old jazz standards sound better on a 1929 console radio than my 5 channel tube stereo/home theater system (i have added connections to the 1929 radio for my ipod where the phono input connection is located).

Cheers,
Greg


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 Post subject:
PostPosted: Sep Tue 29, 2009 3:55 am 
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Marcc wrote:
227 is an indirectly heated tube (heaters not filament).......Marc


Sheer semantics.... All heaters are "filaments" but not all filaments are "heaters" ... perhaps you meant not all filaments are cathodes - but, here again, that wasn't implied in my statement and therefore an invalid argument... :wink:

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 Post subject:
PostPosted: Sep Tue 29, 2009 12:12 pm 
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[img][img]http://antiqueradios.com/forums/gallery2.php?g2_view=core.DownloadItem&g2_itemId=94163[/img]

Ok hope image works. Quality is not good as it is cropped out of a full picture of the pan, which has 8 tubes, in line. and the chassis is 12.5Kg (x 2.2 for LB)

The big silver thing that actually sits under the 50 in a Lyric 70 is R5, a potentiometer. The AC filaments are floating and the centre goes to ground (earth). This is part of the Hum supression.

That device is stamped "HUMDINGER". Its made in USA. So for those who have never seen a "real humdinger" ...... now you have.

Obviously the thing is a real humdinger for stopping hum!

Back to the pedantics:--
One tends to refer to the tubes where the filament & cathode are one eg 50, 5Y3. as filament tubes. If you look at the data sheet for 27 (Franks electron tube pages) it will be referred to as a heater type. The cathode and the heater are totally seperate entities.

Marc


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 Post subject:
PostPosted: Sep Tue 29, 2009 1:59 pm 
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OK, it's what I have always called a hum balance pot. Thanks.


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 Post subject:
PostPosted: Sep Wed 30, 2009 2:38 am 
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Just take the glass off, and solder the heater back together :D

While you've got the glass, off, might as well clean it :o

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"What does matter is the size of your heart and the strength of your character."
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In a short time we may speak of three or six crystal sets, the same as we speak now of a three or six tube set.
-- Hugo Gernsback, 1924


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 Post subject:
PostPosted: Sep Wed 30, 2009 12:56 pm 
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Hi everyone,

I have posted an article about restoring this radio on my website:
http://www.mit.edu/~gr20603/Dr.%20Grego ... Radio.html

Thanks again for your help and conversations!!

Greg


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