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Post by gtrjunior on Jul 8, 2016 18:32:14 GMT -5
I've read a ton of posts from people saying that they are having trouble getting the bias correct. In other words they have reached the end of what the bias trim pot will do. I believe this is because they are trying to bias tubes that are not "rated" the same as what they had in the amp.
The tubes that came in my amp from Scott have a number "40"( I believe, I'll recheck). I'm assuming that is his rating system. Anybody know if that would be considered early, average or late break-up?
So my question is, when you guys buy replacement tubes from somewhere besides Scott, how do you determine what the proper tubes that will bias correctly without needing to change the resistor....id rather not do that.
And I'm not a big fan of the Chinese tubes that are in the amp, so I won't buy them from Splawn...which I know would be easier, but....
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Post by briman on Jul 8, 2016 20:51:37 GMT -5
I just buy matched pairs or quads and then bias them accordingly.
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Post by briman on Jul 8, 2016 20:56:01 GMT -5
As long as they are matched, you can then bias hotter or colder. Don't know if this answers your question. ..
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Post by gtrjunior on Jul 9, 2016 8:30:26 GMT -5
Not sure if I asked the question properly! I think what I meant is that if a replacement tube is not within the bias range, I've read that some people have had an issue bringing the bias into the proper range. And would then need to replace a resistor in order to do so. I didn't want to replace components so I wanted to make sure I buy the correct value tube. I'm still not sure if I'm explaining it right...
I just emailed Scott, hopefully he understands what I'm trying to get across!!!
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Post by splawndude on Jul 10, 2016 20:24:12 GMT -5
I would let Scott answer and I'm not an amp tech but here is how I think you do it.
Get the plate voltage of the amp in question (either ask the manufacturer or read it for yourself off of a multimeter). Apply the amount of plate dissipation you want (hot or colder) and then find your range you want to shoot for. When they say 'plate' they are referring to the plate that is inside the tube. Electrons collect on the plate when voltage is applied and then dissipate off of there as they burn off. The higher that percentage is, the hotter the amps bias is. EL34s are 25 watts each. Splawns are in the high end of plate voltage. Mine is roughly 475 volts.
25 watt tube / 475 plate voltage = .052 amps
so 52 mA x .60 plate dissipation = .031 or 31 mA
mA = milli Amps
Power tubes can be considered 'matched' if they are between 3 - 5 milliAmps a part from one another. So using my math above you would contact someone like Scott at Splawn, or Laura at The Valvequeen or Doug at Doug's Tubes etc etc and tell them you have a Splawn Quickrod with a plate voltage of 'XYZ' and so you would like a matched set of tubes in the 30 - 35 mA range (or whatever).
Does that help?
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Post by gtrjunior on Jul 11, 2016 5:35:47 GMT -5
Yes.... Thats exactly what I need to know. Once I get the probe I'll be able to have the info I need to get the right tubes.
I just didn't want to order tubes that I couldn't bias correctly.
Thanks man!!!
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