I always wondered why AWV went down that path rather than making an uprated version of the 6AB8 triode-pentode that was around at the time.
Having to ground the common cathode in the 6AB8 was something that was easy to organise with back bias.
But then along came the 6BM8 from Philips which is what the 6BV7 should have been.
Funny thing is, when AWA designed the P1 portable they used a 6BM8 and AWV started making them, along with other Philips designs like the 6CM5 and the frame-grid 6EH7 and 6EJ7 that made a 2 stage vision IF practical.

Part 4 Uploaded.

Thanks Brad. Interesting story.

I assume the reference to 6AQ5 was meant to be 6AV6.

I've had no trouble with 6BV7, as long as they are treated properly, which they rarely are by default. The common problem is they are allowed to overheat, causing all kinds of internal issues.

However the use of a 6DX8 is perfectly fine, as would most other output valves. Especially in my case where I have over 100 of them and 6BM8s.

The issue with the crackly mica on high voltage is pretty well known, I've had the issue before and I'm sure Marc could tell you lots of tales about them.

However the main thing is you've been able to sort out all the issues and now have a perfectly working system. Well done Fred!

Any mica capacitor that is NOT across a coil is now suspect. Particularly if it has more than 100 volts across it!

In TVs they can cause all manner of long-term intermittent faults.

Hi Rob, yes, 6AV6 !
Sorry everyone I don't know where 6AQ5 came from!
Funny how you can write something that's wrong and be blind to it.

The build of 6BV7 I have are definitely not the best.
They were quite microphonic so the grid structures must not have been that rigid.
Also the envelopes are covered internally with what looks like black soot.
For all the world like they had been glowing for ages.

Anyway its a nice set now and when I die its going back to the original owner.

fred.

Two things come up here, I often think 6AV6 tubes are psychic. Any sort of issue on the chassis tends to send these horrid little swine's microphonic & they ring like a bell.

The Americans tend to have a lot of issues with Silver-Mica caps. Silver grows whiskers like a mans face and these migrate and cause it to short. My normal description of its fail manifestation is as either a "Snap, Crackle or Pop". Usually the ones that die, have high voltage DC across them; As opposed to ones like IF coils with HV on them not across them.

With non obvious faults and to an extent writing articles. The mind can actually get into a rut like a cracked record. Over and over again, it will miss one of its mistakes. The answer when you have one of those senior type moments, is to walk away and go do something different, like mow the lawn, or wash the vehicle; to clear the mind.