Odd tube thing happened...

M

Mazzownya

New member
Hi Everyone,

I've just swapped out the original tubes on my VH4. They were EL34BHTs and I've put in Ruby 6L6GCs. I figured I've biased it ok. I flicked the switch over to 6L6. I've biased 43.4mA per tube. My plate voltage is 483V. After about 15 minutes of playing (at low volume) after changing the tubes, they suddenly went a fiery red really quickly, getting hotter and hotter like they were about to burn out (I was sitting right in front of the amp so I noticed straight away) I shut off the amp immediately. I waited a minute or two and turned it back on and everything was fine. I continued to play for another 40 minutes or so with no problems.

Should I be worried?
 
Mazzownya":13v57cgn said:
but I was using 6L6GC, not 6L6.
6L6GC at 70% is 43.4ma. Do I have it wrong?
Click to expand...

What is the plate voltage - 43mA certainly sounds to high even for 6L6GC's and the change in the tubes you describe certainly sounds like red plating!

Assuming your plate voltage is ~500 Volts then 42ma would be 70% ((0.7 * (30/500)) = 42mA) for those tubes so I would suggest dropping them to 42mA or lower, otherwise you are going to red plate the tubes and burn them out.
 
Plate voltage is 483v. 70% was 43.4ma on a couple of calculators I tried so I set it as that. I played it for a couple hours yesterday and everything was fine. It was just weird that it happened once. I will drop the bias current down to 60% (37.3ma) to be safe I guess
 
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