Today I've been bringing up a 1956 vintage Healing 312 17" TV
This is very much a 1st generation Oz TV and was Healing's first.

The cabinet is all steel and so this thing is HEAVY!

Original RCA 17HP4B 70 degree CRT is strong, which makes the restoration worthwhile.

After a clear dim bulb check, the set gave me a raster, despite being full of those pitch-sealed Ducons "High Seal 70" caps - I thought they were all gone by 1956..

No video. Scoped the IF path and found 50 volts of signal at the detector, nothing at the other side.

The detector is (was!) a 6AL5 double diode. It was bad.

In more years than I care to remember I've never before encountered a bad 6AL5!

Since I didn't have a replacement (why would I?) I fitted a pair of 1N4148s. Nice contrasty picture!

But that wasn't the end of it. After fixing a swathe of minor problems caused by leaky caps and resistors that had gone high, I moved on to the sound,

Audio amp OK, nothing from the detector. Scope showed plenty of signal at the primary of the ratio detector transformer. But no audio coming out.

A power-on diode test read OC on the pins of the ratio detector diodes, which were also a 6AL5.

No! This could not be!

But it was. A 2nd dead 6AL5!

A 2nd pair of 1N4148s brought up perfect sound, I didn't even need to align it.

A 6AL5 has to be the simplest and least stressed valve ever to grace a 50s TV chassis. They just did not fail in service. What are the chances of getting two dead ones in the same chassis? With one of the two being a much newer one, so it wasn't a batch issue?

Interesting coincidence.

Assuming it's not obvious (such as open circuit heaters), suggest putting them on a valve tester and see if there's any similarity in failure mode.

Hi Ian, they have a tiny cathode to plate clearance.
Maybe the cathodes have 'grown whiskers' and the things are shorted?
If so a 'zap' with HT volts across may clear the short.
Back 100 years ago I had a valve with a short and the HT treatment fixed it.
Applied HT (through a limit resistor), flash inside valve, short gone!

Fred.

A thing common to the different 6AL5s is the bases. Maybe they have whiskers, or some other common fault.

One of my radios has a 6AL5, in its own metal shield. It's my first ever valve radio.

Surprising for your ones to blow up though. They normally last forever.

There has been talk here recently about the pesky 6BV7 valve.

Could you replace the pentode section with a 6BQ5/EL84 and use a 6AL5 as a detector? Or use general purpose germanium diodes; or 1N4148’s?

I reckon I would have over 100 loose 6AL5’s in my stash + heaps of NOS NIB, including CV versions.

Not sure if the 6BQ5 would have the gain of a 6BV7. A 6M5 would be pretty close though. Or you could use a 6BM8 or 6GW8 with rewiring the socket.

I was able to determine, before I pulled each 6AL5, that they were effectively O/C according to the meter's diode test. The video detector had 50 volts of 35MHz on one side and zip on the other!

Don't think either had an O/C heater because they were both slightly warm when pulled.

The one from the audio looks like an original and it has a depleted getter flash, so it might be it shipped some gas over the last 70 years.

But do I care? Not really! The 1N4148s will last forever.

On the unrelated issue of the 6BV7 replacement, I looked into the best triode-pentode replacement candidate and found the 6GV8 and a 1N4148 needs the least socket changes. Because they are not an audiophool valve like the 6GW8 they are cheap and plentiful.

Use the audio amp circuit from the Kriesler 11/99.

Just looked in my collection, there's I guess about 40x 6AL5 and equivalents (EB91, 6D2, etc).

I did a 6BV7 replacement using a 6GV8 in my AWA 212C I'll draw it up and send it to Brad. It works extremely well.

The extra audio gain allowed me to move the sound IF B+ from the 260 volt rail to the 150v rail. This has several advantages:

1. Allows some FM limiting action to happen so that the fine tuning does not alter the volume.
2. Improves the AM rejection (measured displaying PM5544 test card) by 6dB.
3. Lowers the 150V rail slightly so that the black level tracks contrast pot settings better (after my DC coupled video mod).
4. The 6AU6 no longer runs stinking hot - my set had a dead one in that socket.

The pentode in a 6GV8 is a bit like the old 42 / 2A5 / 6F6 output bottle, it needs 560 ohms in the cathode. Very different to the similar-looking 6GW8. But with this and 3.3k in the triode's cathode, no bypassing is needed to get more than adequate combined audio gain. Makes it very simple to implement.

6GV8s are plentiful and don't carry an Audiophool price premium.