Vortexion CBL6 output pentode EL84

[BillDWVA]

Hi, I discovered that one of the output pentodes had gone ‘soft’ on a CBL6 that I am working on. The valve had cracked across the base, between the pins.

The soft valve was replaced with a used ‘Mazda’ branded EL84 from my spares pile (of uncertain origins) and as the HT starts up this blue glow appears, accompanied by a ‘squeak’ and a ‘pop’ from the loudspeaker. However, operating like this it does not amplify.

What is happening here? Searching the web it seems that this valve may have an imperfect vacuum? Do I need to find another EL84 or is there something else that may cause this?

All the smoothing and reservoir capacitors have been replaced in this set and the HT voltages measure 10% higher than shown on the circuit, possibly owing to lower leakage.

Thanks,
Bill

[snowman_al]

Most likely a 'gassy' valve.
Is the gettering still shiny at the top of the valve and does the filament light up before the glow comes on?
Before you replace it, have you checked / replaced C10 and made sure there is no voltage on pin 2 (g1) of the EL84?
Alan

[BillDWVA]

Thank you for your reply.

The top of the valve is a very dark grey. Yes, the heater comes on first but the glow only starts when the HT rises.

C10 has been replaced but the original (a ‘mustard’) was actually OK.

I think I should try this valve in the other channel’s amplifier, as a test.

Bill

[G8HQP Dave]

It looks like you have enough gas for a nice blue glow, but not quite enough to whiten the getter.

[BillDWVA]

Thanks chaps. Looks like it has got gas in it; I pulled a ‘known good’ valve from another set and it works OK.

Here they are; ‘glowing’ one between the original and a good one.

Bill

[G6Tanuki]

Yes on the middle one you can clearly see how the getter has 'retreated' back up the cylindrical part of the envelope leaving behind a no-longer-shiny deposit.