More bias Q

Hi again.
After having some nasty experiences (but lucky I was, so with no bad damage) while biasing my amp, I decided to get something like the Biasrite, which seems to turn the whole process into something really safe and idiot proof (I'm one of those). The only thing is that I don't know how much the shipping from USA will be... anyone there having bought one already? tried to ask them, but no answer.

here is THE QUESTION:
I read around that for calculating the bias-idle dissipation, you can use these tables in which you put some data and give you the ideal value.
my doubt is whether in the field
"Output Tube Power Rating (in watts)"
I should put the rating of my 6L6s, which is 30 watts, or use as reference the output transformer rating, which seems to be 40 watts (divided for 2 tubes, makes 20 watts each).
what do you think?
thanks for help!

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Riz, I don't bias my amps myself, precisely because I don't like nasty experiences. I prefer to pay some tech to zap himself while the amp's running. That said, here's my opinion.

Weber will probably use UPS or a similar courier which could be expensive. Also, read Lee's "sticky" about import tax when bringing an item in from outside the EU. Because the BiasRite is ca $150, you will be paying a significant tax on the item AND the shipping cost. Personally, I think the BR is an expensive piece of kit for what it is, especially if only used occasionally.

On eBayUS, there is a company called, I think SCS who make single and dual octal bias adjusters. Caveat; I have heard of one or two people not totally happy with the product. The principle is exactly the same as the BR. The plug-through socket has a 1ohm resistor so that the current draw of the valve can be read as mA using a standard digital multimeter (set to 0-200mV and the voltage is converted to mA).

Whichever you use, the method is not completely ideal. The idea is that you take the published maximum current for the valve (TDSL, Duncanamps website) at the B+ for your amplifier and adjust the bias until you have a current draw of about 70% of this value. The valves are then considered to be correctly biased. This is simple, but not as elegant as using an oscilloscope to set at the point of crossover distortion.

What you could do is use the 70% method to get you in the right area, then make adjustments by ear until the amp sounds as you would like it. Don't forget to have a load on the output while you are checking things.

ok, so I'll take the published rating as reference.
it's hard to get through all these methods... one of those, like I wrote, had this excel-format table, in which you could put values and the result would be the mV value for correct biasing. the thing that made me think was thatthe table asked to dial in the rated power of the valve, instead of the rated output of the transformer, which sometimes differ, like in my case.

anyway, I had a look at the ebay link, thanks a lot.
at this point I'm not sure wether it would be best to buy this probe, or the Weber (they sell the probes only, for something like 30$ for a single). for 15$ more they provide probes with an options which allows a measurement of idle current AND plate voltage, making this a more accurate tool, according to my very small knowledge. (check https://weberspeakerscom.secure.powweb.com/biasrite/br_page.htm) if only a good tech was available, I'd have had a lot of troubles saved, and you too I think.

Thanks for your help, as always!

I think I'd get the Weber probe rather than some unknown eBay merchant. At least Weber is a company with a good reputation. I'd use my own DMM to save money rather than buy the whole BiasRite. However, it's worth paying the extra to have the facility to measure B+. You'd have everything you need then. I was forgetting; you said you can'tfind a good tech in your area :(

sorry, I've been SOOO busy these days.
yeah there is no tech around here, or at least, no one you would trust.
this is why I'm trying to learn at least how to do the basic service to my amps. the fender is quite simple so I'm nearly confortable to deal with it (even if I can do some mistakes caused by lack of experience). the ampeg is much more complicated, so I'll have to take that one to some good tech somewhere, when I've got money AND when I'm shure the guy is a good one and not too expensive. you know, driving 100km or more just to have your amp biased is not such a joyful experience...
I'll go for the weber, one of these days, and then let you know how it works.

one thing I'm not sure of yet: you wrote I have to check the max power rating of the tube... the fact that the transformer is not rated for the same power but less does not matter for calculations then?
thanks!

You will need the B+ voltage for the amp, riz. From this, you adjust the current draw of the valves until you achieve 70% of the quoted maximum power for the valves you are inserting. This is the "70%" method of biasing. I'm only going on what I've read - I do not bias amps myself and leave this to others to do for me. I can check it out further if you'd like or perhaps Weber will have it in their FAQs.

Post new comment

The content of this field is kept private and will not be shown publicly.
CAPTCHA
This question is for testing whether you are a human visitor and to prevent automated spam submissions. Do not include any spaces in your answer.
Image CAPTCHA
Copy the characters (respecting upper/lower case) from the image.

Contact | Advertise | Contents | Privacy Policy | Forum

This site is published by Hitsquad Pty Ltd. Copyright © 1999 - 2017 , All Rights Reserved.