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Old 13th January 2021, 08:07 PM   #11
brl0301 is offline brl0301
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Join Date: Dec 2019
Originally Posted by vernonb View Post
Thanks for the info brl0301

Coincidentally! my tubes are also Gold lions
Does your bias settle eventually?

Mine doesn’t seem to stop drifting till I crank the pot the other way
And with the other channel doing the same but in the other direction…its been confusing as to which tube is tube/channel is causing it,

What are your operating points for your 300b if you don’t mind me asking?
My operating point was ~405v at 68ma and a 3k opt….

So Ive found that the iffy tube will run up pretty high over the normal operating point and after maybe an hour or so will eventually drift back down to the where it should be.

The reason the other channel moves opposite the one you are adjusting is because as one tube has more current flowing through it, it draws power away from the other tube. Same in the reverse, if you drop the current flowing through one, the current in the other will slightly increase. This is pretty normal and expected behavior from the tubes.

I would start by letting your tubes get nice and warm and then adjust both tubes to your 68ma target. If one of the tubes is having stability issues, it will drift significantly from where you set it while the other tube may move a much smaller amount in the opposite direction. Whichever tube is having big swings is the one with issues, or at least that has been my experience.

If both of your tubes are moving a ton without you adjusting bias then you may have some other issues going on.

I currently am running my tubes at around 400v b+ and 75ma. My bad tube will run up to around 100ma before it settles down. If i try to adjust the bias before it settles down, it continues to drift in whichever direction I had adjusted it last.
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Old 14th January 2021, 07:21 AM   #12
vernonb is offline vernonb  New Zealand
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Join Date: Oct 2015
Location: New Zealand
Thanks brl0301
I think what is going on with my bias is slightly different,

It never settles, one tube will always “runaway”, kind of making the amp unusable (I think)
I’ve never been game to let it get much beyond 100ma when I’m watching
Maybe it would stop at some point but its still moving fairly smartly (say 30sec to get from 68ma to 100ma, I guess)

I will do some more tests over the weekend, I would like to be able just test one side of the amp at a time,

but I’m not sure running it with only one output tube is a good idea…

Maybe I just need to focus a bit harder on watching for changes between sides

Good to know your operating points are not too dissimilar to mine…Maybe they are just to hot for Golden lions?

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Old 14th January 2021, 03:01 PM   #13
brl0301 is offline brl0301
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Join Date: Dec 2019
Im pretty sure you can run woth just one output tube but id wait for someone more knowledgeable to confirm.

What you should try is adjusting the runaway tube halfway to where you want it ( say its at 100ma and you're shooting for 70ma, id adjust it to 85ma and let it drift down to 70ma) and see if thatll work. Thats how i eventually got my bias pots set close to where i needed them. After mine runs away on warmup, the bad tube will usually settle back down where i want it as long as i dont mess with the bias pot again
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