|
Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc. |
|
Thread Tools |
3rd Jan 2019, 10:19 am | #1 |
Pentode
Join Date: Nov 2010
Location: Tenby, Pembrokeshire, Wales, UK.
Posts: 139
|
Vortexion CBL6 output pentode EL84
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 |
3rd Jan 2019, 10:46 am | #2 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
|
Re: Vortexion CBL6 output pentode EL84
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 |
3rd Jan 2019, 12:13 pm | #3 |
Pentode
Join Date: Nov 2010
Location: Tenby, Pembrokeshire, Wales, UK.
Posts: 139
|
Re: Vortexion CBL6 output pentode EL84
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 |
3rd Jan 2019, 7:01 pm | #4 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
|
Re: Vortexion CBL6 output pentode EL84
It looks like you have enough gas for a nice blue glow, but not quite enough to whiten the getter.
|
3rd Jan 2019, 8:33 pm | #5 |
Pentode
Join Date: Nov 2010
Location: Tenby, Pembrokeshire, Wales, UK.
Posts: 139
|
Re: Vortexion CBL6 output pentode EL84
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 |
3rd Jan 2019, 8:39 pm | #6 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,996
|
Re: Vortexion CBL6 output pentode EL84
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.
|