OT - Amp Biasing

Adam Shame · October 17, 2003, 12:05:07 AM

Can anyone point me in the direction of how to bias my Carvin amp? I have checked out some FAQs, but they confuse the hell out of me. What I really need is a simple step-by-step walkthrough.

And before anyone asks, I am really handy with an iron and multi-meter. The reason why I need help is because none of the places around me work on Carvin amps (not even the Carvin factory store!!) and the one place that does, want's to charge me an arm and a leg ($60!!) I figure that I can do it myself, and won't fry myself.

Oh, and the amp is a Carvin VTR-2800. It has 4 EL-34 output tubes, and I have opened it up before and can find a pot that probably has something to do with biasing. I have the schematic in PDF format (if needed). Any help would be appreciated!

Mike Nichting · October 17, 2003, 12:36:52 AM

Thats tricky business if you haven't done it before. I personally don't think $60 is a lot to have your amp setup. Don't get me wrong I don't have $60 for that type of thing but I would wait untill I did to have it done.
I bias probe would help tremendously in this case since you have 4 tubes. You want the 2 highest mA tubes in the middle and the lower on the outside. Your swing for EL34's is between 40 and 60 mA.
You don't want to under bias or over bias.
If you KNOW that that pot is for bias not balance adjust you can do it by ear but watch the tubes carefully. You don't want them to glow orange or you will kill them.
I really suggest that you have someone do it for you with a new set of tubes installed. Keep in mind you should have the amp biased every time you install new tubes.

Check this site if you are dead set on doing it your self: www.fenderforum.com there is a section for non fender gear and the people are as nice there as they are here. VintageJon is a good friend of mine and is a virtual GURU when it comes to amps.

Best of luck.

mike N.

ErikMiller · October 17, 2003, 05:55:43 AM

I use the cathode-resistor method:

http://scobro.150m.com/Rivera/LVBias.htm

It's pretty step-by-step, and he tells you what pins on the tubes to measure, so you don't necessarily need a schematic.

Remember to keep one hand in your back pocket whenever you have that amp powered up and out of the case. I'm serious. Clip the black probe to ground using an alligator clip and probe only with the red one.

keninverse · October 17, 2003, 10:02:14 AM

THIS IS SOME SERIOUS BUSINESS!!!!!!

If you haven't ever done this before I would highly recommend watching an amp tech prior practice. I always tell people that want to start servicing their amp this very same thing. Biasing an amp for $60 isn't that expensive and I'm willing to bet that if you pay your tech a couple extra bucks he will be willing to sit down and let you watch what he's doing.

As far as methods, someone already cathode resistor method, which is what I use as well as transformer shunt. But to tell you the truth...I bias my amps according to the sound I like and then go back and make the measurements. This way I can dial up the sound everytime I replace tubes.

Mike Nichting · October 17, 2003, 11:25:59 AM

Yeah, like I said before you should have someone do it. I agree with what
keninverse said. Think it over seriously before you do it, we would like to keep you around for a while longer :-) One slip up could be your last.

Mike N.

Peter Snowberg · October 17, 2003, 12:44:27 PM

I agree with Mike and keninverse here. Slip the tech a couple extra bucks to watch. Even at $60 this will be cheap education.

Can you provide a link to the schematic online? If it's less than 5 megs, you could e-mail me at petersnowberg at yahoo dot com and I'll see what I can tell you.

-Peter

Mike Burgundy · October 17, 2003, 01:42:51 PM

for EL34's is between 40 and 60 mA.
BEWARE!
This depends on plate voltage and the tubes used! (edit)
If these are running on a plate-voltage of even 350V, 60mA will have the tubes running too hot. They will idle okay, but when in use, they will very probably glow a nice orangey-white where they shouldn't!
I suspect the tubes in this amp run closer to 450V or higher, which suggests an upper limit of 38mA. That's below 40mA! (duh

This is based on Randall Aiken's recommendation of idling a tube in class AB at NO MORE than 70% of it's max dissipation.
(so MAX idle current equals 0.7*(Pa/Va) with Pa being max plate diss. and Va being plate voltage)
In my experience, most situations with new-production tubes will have them running too hot at full-tilt when you bias close to this point, even with the cathode-resistor method and ignoring the (small) grid current contribution.
Point being: without knowing plate voltage and some calculation, just choosing a certain current can be rather hazardous to your tubes - and if they overheat and fail who knows what they might take with them. Fortunately the math is very easy.

(actually: 60mA/450V would take the tube beyond it's maximum plate dissipation of 25W at idle)

http://www.aikenamps.com/

Adam Shame · October 18, 2003, 12:40:52 AM

I appreciate all the help/advice. The two big reasons why I wanted to do it myself is because I am a poor college graduate AND the turnaround time on the amp is like 2.5 weeks. I really like the suggestion of giving him a few bucks for him to show me how it's done. If you teach a man to fish, right?

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OT - Amp Biasing