Hey everyone, hope you are all well and doing fine.
Quick question....Do class "A" amps have to be biased when installing new tubes? I just purchased my first the other day and I have no idea about that question. Thank you for your time and regards,
Mass.
Well here's some more info about "class A":
http://www.aikenamps.com/VoxAC30classA_2.html
Most amps like the AC30 or other cathode biases amps don't need the worry when changing tubes, but it wouldn't hurt to have the amp checked to make sure you're running within spec.
Just for the record, I don't know whether the Crate amp in question is or isn't true CLass A.
I did, however, notice that their website says "Class A" in quotes.
Maybe that's in reference to the popular misconception that an AC 30 is Class A.
Somehow folks equate "cathode biased" to "class A", but that's not always true.
Go to the Mesa Boogie website. There is a very good article (kind of long mind you) about Class A amps by Randall Smith.
brianf
Oh Man!!! I never knew Fender made amps too!!!
:ahemOriginally Posted by brianf
I think you're much better off reading the article by the other Randall linked above.
Several guitars in different colors
Things to make them fuzzy
Things to make them louder
orange picks
I've not had to bias when installing new EL84's in my Vox amp. Not sure if the Crates are set up the same way. I didn't have to do it when I had a Peavey Delta Blues, either (4xEL84).
"I'm gonna find myself a girl
that can show me what laughter means
And we'll fill in the missing colors
In each other's paint-by-number dreams..."
Ditto.:ahem
I think you're much better off reading the article by the other Randall linked above.
Randall Aiken = :yay
Randall Smith = :nay
So anyway... If the amp/sound you were going for is Vox AC30 and that is sometimes called a "Class A" amp, then they should have said, "Cathode biased amp with EL84 power tubes" and you'd get the same amp with that characteristic sound.
Class A or not shouldn't really matter. And the answer to the original question is still no, you don't NEED to rebias when changing tubes.
Wilko, why is it that Fender amps need to be biased whenever you swap power tubes? And Voxes do not?
"I'm gonna find myself a girl
that can show me what laughter means
And we'll fill in the missing colors
In each other's paint-by-number dreams..."
Most Fender amps are fixed bias. Some are cathode biased, mostly early tweeds, but the majority of 'em are fixed.
What's the story on this??? Fill me in.
brianf
Oh Man!!! I never knew Fender made amps too!!!
Mass - If your amp is cathode biased (regardless if it's Class A or not...let's leave that alone for now), for all practical purposes you probably don't need to rebias each time you stick in a new set of tubes. The cathode bias circuit will take care of that for you.
Meanwhile, you can adjust the characteristics of how the tubes operate under cathode bias by adjusting the value of the bias resister by removing it and replacing it with a new value, but you probably won't need to do that. The amp manufacturer should have the bias resister value dialed in close enough to accomodate most tube manufacturers. Just make sure you use matched tubes so that one side of the signal isn't running hotter/colder than the other side of the signal. Shoving any old tube in there, while it will achieve its bias level relative to the amp, won't guarantee that the tube you stuck in there will bias to the same point as the other tube you stick in there. You may end up with one tube sounding great and the other tube sounding weak and not even know it. That's one reason to have them matched...so, at least they'll perform together in concert with one another.
Last edited by PhatTele; 05-17-2005 at 09:37 AM.
IMO, Randall Smith is a hypemeister. A successful one though. Randall Aiken is an amp builder. Not as successful, but he builds better amps, and knows what he's talking about.
Here's another great Aiken paper on Class A
Last edited by fezz parka; 05-17-2005 at 11:38 AM.
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