Dodgy 300B

[Clive]

A pal sent me a 300B which is making strange sounds. There's a whistle, probably about 6kHz, it's not loud, it can be heard about 2 feet from my 100db speakers. Clearly the mesh plate valve is failing but this characteristic is not something I've come across before. Any idea what's happening?

[Mike H]

Never heard of this so did a Google search, a few matches, so a few others have done it too, but no definitive answers. One was a WE of 800 - 1,000 hours (Tube Asylum, but no answers to OP).

Presume this is in two different amps (?) if so it's not another component problem, so my best guess is it's now bin fodder.

[Clive]

Never heard of this so did a Google search, a few matches, so a few others have done it too, but no definitive answers. One was a WE of 800 - 1,000 hours (Tube Asylum, but no answers to OP).

Presume this is in two different amps (?) if so it's not another component problem, so my best guess is it's now bin fodder.

Yep, 2 different amps. I certainly won't trust it!

[Mike H]

I have had 300B's doing crackling and banging, this is because of gas, even got one that does a gentle 'chuffing' noise, accompanied by a convincing pulsing blue gas flame impression, and very pretty it is too, first time I've heard of whistling though, and not sure what would do that.

Feasibly possible it's an audible manifestation of RF oscillation, this was suggested on some of the topics, in one case where a guy couldn't see it on his 'scope, but could see his sig gen signal. Possible though only 'cause he wasn't locked onto the whistle but the sig gen signal, as one poster suggested.

Whatever it does seem by all accounts to be an age related thing.

[Paul Barker]

I have had 300B's doing crackling and banging, this is because of gas, even got one that does a gentle 'chuffing' noise, accompanied by a convincing pulsing blue gas flame impression, and very pretty it is too, first time I've heard of whistling though, and not sure what would do that.

That describes the sound soft BR4 VR tubes inspire from an amplifier:-

you may recall me explaining how variable the balloon shaped British base VR tubes are.

I have it in black and White now. This one

at maximum current can settle at 135v or it can settle at 115v or anywhere between.

I have found these particularly week in regulation. Shame because they look so cool. But it seems the technology was young when they were made, better tolerances emerged with later Octal versions.

I may try to use them somewhere because of their cool factor.

The most realistic data sheet which matches my experience
an unrealistic data sheet

Whatever it does seem by all accounts to be an age related thing.

I am learning a lot about age related things at an exponential pace

[Mike H]

Does suggest it's gas discharge then. Didn't think that could do a whistle though. Although I suppose it would if the oscillation was fast enough.

It does make sense, insofar that in my experience (and others!) 300B's will mostly tend to fail in 1 of 2 ways, 1. gas / air leak, 2. broken filament.