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My favorite receiver used for 40 m AM broadcasts has a cracked valve, but the valve type normally printed on the glass has rubbed away. Can anyone tell me what the numbers for this valve are ?

The radio is mid - late 40's, made in Australia and has no brand or model on it.

The valve is the one audio output valve in the radio. The valve has "Philips Miniwatt Australia" printed on the black base. You can just see the crack in the 3rd picture at the lower left of the valve glass.

Clues are that it has 7 pins and they all seem connected when you look inside so 2 for the heater and 5 left over probably makes it a Pentode. CORRECTON, -> there are 7 pins but only 6 pins are connected, so i'm not sure if it is a pentode now. - It's a Tetrode.

The heater voltage is : 6 VAC. The cathode resistor is : 240 ohms. The G1 resistor is : 543 k ohms

That should narrow it down a bit.

See the pictures of the radio, chassis and valve.

You can see how clean the glass on the front dial is, that took a while to get it like that, i hardly ever clean the valves like that any more.

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    $\begingroup$ Try fogging up the glass with your breath. I've read many tube types like this. Don't clean the glass; that can render this method useless, as cleaning can remove what's left of it. (Don't ask me how I know that. ;-) $\endgroup$
    – Mike Waters ♦
    Feb 24, 2020 at 2:11
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    $\begingroup$ there is usually a tube chart glued inside the cabinet or under the chassis that names each tube in its socket position in the radio. If yours does not have an intact tube chart, perhaps another radio of identical manufacture does. Try searching the web for the schematic of this particular radio model! $\endgroup$
    – niels nielsen
    Feb 24, 2020 at 2:39
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    $\begingroup$ @MikeWaters i tried that and looked with a magnifying glass but the number is really gone. $\endgroup$
    – Andrew
    Feb 24, 2020 at 4:16
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    $\begingroup$ @MikeWaters The valve has 7 pins and one gap where the 8th pin is missing. Looking at the base from the bottom with the gap at the top. pin 1 is the first pin on the right at the top and the elements going in a clockwise direction starting at pin 1 are as follows : pin 1 heater, pin 2 one of the grids, pin 3 not connected, pin 4 heater, pin 5 anode, pin 6 one of the grids, pin 7 indirectly heated cathode. They keyway is between pins 2 and 3. $\endgroup$
    – Andrew
    Feb 25, 2020 at 10:44
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    $\begingroup$ @MikeWaters the heater voltage is 6 VAC, and there is a picture of the base already posted in this question, also see my last comment that has the pin functions, ie: 1- heater 2-grid 3-n/c 4-heater 5-anode 6-grid 7- cathode pin 8 is not present. $\endgroup$
    – Andrew
    Feb 28, 2020 at 9:17

1 Answer 1

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Are you sure that you are correct in the comments about your pin 2 and pin 7? (The conventional numbering for these tubes is as in your drawing, so conventionally pins 8 and 5).

If you look very carefully or use an ohm meter you might find that your pin2 is actually the cathode and your pin8 is G1, as in your drawing of the 6L6.

This would be more normal for an octal beam pentode.

If the drawing is correct for your valve then it's likely a 6v6 (lower power but same pinouts as the 6L6).

It's a 10 watt max audio output tube used in lots of radios from that era. Grid 3 is internally connected to the cathode, so doesn't have a pin.

Max rating currents of 45 mA anode + 5mA G2 would give max 12v across the 240 ohm cathode resistor, approximately right for self biased class A1 operation (6v6 has -12.5v G1 cutoff voltage). Should be probably sitting around 5 or 6v across the cathode resistor at no signal.

I'd give a 6v6 a go if the pinouts are correct, check the quiescent anode and G2 currents (by measuring cathode voltage, I'd expect somewhere around 1/2 max or less) and that the peaks never exceed the max ratings. If there's any distortion find out why.

Capacitors, particularly electrolytics, can fail or change value in sets of this age so worth checking they are good where possible.

If your pinout description in the comments is actually correct then it's a valve I'm not familiar with, sorry.

Good luck.

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  • $\begingroup$ Hi Peter, this appears to be a good answer. Simply edit out your "ooops" etc. ;-) $\endgroup$
    – Mike Waters ♦
    Mar 1, 2020 at 19:18
  • $\begingroup$ The 6L6 heater draws about twice as much current as a 6V6, so it may make the transformer overheat if the original tube was a 6V6. $\endgroup$
    – Mike Waters ♦
    Mar 1, 2020 at 19:39
  • $\begingroup$ Peter thanks for the answer, the drawing is correct, it took a few goes to get it right, the comments are wrong. I tried a 6V6 and that worked so i'm pretty sure the valve is a 6V6. $\endgroup$
    – Andrew
    Mar 2, 2020 at 20:41
  • $\begingroup$ @Andrew I sure would love to know what the voltages on those pins are! :-) $\endgroup$
    – Mike Waters ♦
    Mar 8, 2020 at 18:15

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