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Post by wraparound (Steve) on Jan 17, 2017 13:58:49 GMT -7
Would it be accurate to say the reading in milliamps using a bias probe should be slightly higher than readings using a Transformer shunt method?
My understanding is that cathode current is comprised of both plate and screen current. Where as it is the plate current alone that is measured in the shunt method. The bias readings given in the info section are given using the Transformer Shunt method which I am not prepared or equipped to do. I use one of the single tube bias probes and think the readings there may include the screen current as well. So if the Galaxie suggested bias is 27ma using the TS method, would it be more like 29 - 30 using the cathode resistor method the bias probe is based on? I don't have a Fluke meter BTW just a pretty good Craftsman.
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Post by wraparound (Steve) on Feb 4, 2017 13:43:08 GMT -7
Anybody out there in the ole experts section?
Thoughts?
A good guess?
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Post by benttop (Steve) on Feb 5, 2017 8:39:24 GMT -7
Well I'm no expert by any stretch, but one thing that I've learned about biasing is that there is no absolute correct value. There is a value we all agree as a starting point, but from there it really depends a great deal on what you are hearing, and what level of reliability you're willing to accept or give up. A tube biased especially hot might sound fabulous for a short period, while it is attempting to fail prematurely. A cold biased tube may last forever and sound awful the entire time. You want to be in a place somewhere between those extremes, and still have good tone.
I suggest you try a few different values, and make a paper record of your B+ and cathode current for each. And keep notes on your perception of the tone. I predict you'll notice the differences, as I can hear them and my hearing is awful.
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Post by wraparound (Steve) on Mar 22, 2017 10:18:34 GMT -7
Thanks for the response Steve. I guess I was just curious as to a basic milliamperes difference between the shunt method and the cathode reading of the bias probe. I've usually hovered around the suggested value up or down a bit by ear, and currently find the Galaxie sounds more balanced and sweet right around the 27 or 28ma mark than above 30 where many have put it. But I am using a Craftsman meter and a bias probe. If it was sent to the shop on Broadway and they used the shunt method and set it up with the stock tube compliment, I wonder if the milliamperes reading on the bias probe would be higher as a base value.
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Post by benttop (Steve) on Mar 23, 2017 11:24:18 GMT -7
Why don't you set it with the shunt method, then slip in the bias probe and see what it says?
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Post by wraparound (Steve) on Mar 23, 2017 14:22:24 GMT -7
Why don't you set it with the shunt method, then slip in the bias probe and see what it says? I don't have the knowledge and the correct tools. It would be something I would feel the need to be taught firsthand.
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Post by zpilot on Mar 23, 2017 18:53:29 GMT -7
Just about anything can be learned off of YouTube nowadays. I wish that information was available when I was learning how to work on tube amps. This guy does about the best job I've seen of instructing:
This is a pretty good tutorial on biasing. Most other videos I've seen don't cover cathode bias. You still need to have a basic knowledge of things like ohms law, using a multimeter, and of course safety.
He gives a good foundation of theory so you not only know how and what to do, but also why.
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Post by wraparound (Steve) on Mar 24, 2017 21:30:56 GMT -7
Thanks guys appreciate the input.
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Post by Lefty on Mar 25, 2017 4:16:41 GMT -7
My thoughts on many years of biasing are... If you have an amp that has an adjustable bias, get a bias meter. If you have an amp that is cathode or a fixed bias, buy tubes within the preset or manufacturers recommended range.
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