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Kevin Nybo · April 10th, 2003, 06:33 PM

I just went and took a few photos of how I bias my main amp. You don't need to go inside the amp to find any of these numbers. You can zoom in on any of the photos for a better look. As you can see I forgot to rotate the last 2 and they appear sideways.

http://www.villagephotos.com/pubbrow...elected=219008

Value 1-This probe's instructions said to measure the plate-to-cathode voltage. On this amp it's always the same as plate-to-ground measurement. I use an old set of meter leads and connect one to pin#3 and one to pin#8 of one of the power tubes(see pic). I then plug the tube in and power up(see pic). Using this rectifier and the 6L6 Svet shown, the plate voltage is 341. This is one of the numbers you will need to figure bias.

Value 2-After that reading is taken you start using the bias probe. Mine's a duel model I purchased on ebay for $57/new. SRS continually lists these on there. Knowing what I know now I would have built one myself and saved the money but didn't know how they were designed when I ordered it.

You can switch between the 2 tubes and see their mA(mV) reading. These are the numbers you hear everyone talking about usually, "Biased at 39", etc. FWIW-these are the only matched set I've seen that exactly match. Usually the numbers have a difference of around 3-4mA.

Value 3-The other number you will need is the max-plate-disipation of the power tube. For instance, the 6L6 Svetlana in the photo has a mpd of 30watts, while most 6L6's are at 20.5W, 6V6's at 13W, etc. You can find these values all over the web.

I have seen so many formulas I don't know which one to go by. Given the ones in these directions we get-

30watts(Svet 6L6) X 500= 15,000

15,000 -divided by- 341(plate voltage)= 43.9mA

Acording to these instructions, 43mA will put you in the "low side of the midpoint" range.

A common furmula is "max-plate-dissipation -divided by- plate voltage =X(variable)". Then take 70% of X variable. For instance-

30W(mpd) -divided by- 341(plate voltage)=.0879 or 88mA. 70% of 88 =61.6.

61.6 would be the highest mA setting I would want, in theory.

For those who noticed-
I realize these Svetlanas are a little too much for my low plate voltage. They just happened to be in there when I took the pictures. I've been waiting for another set of 6L6's to arrive this week for testing and left them in. I also have several other sets of 6L6 and 5881's.

April 10th, 2003, 06:33 PM	#6 (permalink)
Kevin Nybo TDPRI Member Join Date: Mar 2003 Location: Mini-Soda Posts: 77	I just went and took a few photos of how I bias my main amp. You don't need to go inside the amp to find any of these numbers. You can zoom in on any of the photos for a better look. As you can see I forgot to rotate the last 2 and they appear sideways. http://www.villagephotos.com/pubbrow...elected=219008 Value 1-This probe's instructions said to measure the plate-to-cathode voltage. On this amp it's always the same as plate-to-ground measurement. I use an old set of meter leads and connect one to pin#3 and one to pin#8 of one of the power tubes(see pic). I then plug the tube in and power up(see pic). Using this rectifier and the 6L6 Svet shown, the plate voltage is 341. This is one of the numbers you will need to figure bias. Value 2-After that reading is taken you start using the bias probe. Mine's a duel model I purchased on ebay for $57/new. SRS continually lists these on there. Knowing what I know now I would have built one myself and saved the money but didn't know how they were designed when I ordered it. You can switch between the 2 tubes and see their mA(mV) reading. These are the numbers you hear everyone talking about usually, "Biased at 39", etc. FWIW-these are the only matched set I've seen that exactly match. Usually the numbers have a difference of around 3-4mA. Value 3-The other number you will need is the max-plate-disipation of the power tube. For instance, the 6L6 Svetlana in the photo has a mpd of 30watts, while most 6L6's are at 20.5W, 6V6's at 13W, etc. You can find these values all over the web. I have seen so many formulas I don't know which one to go by. Given the ones in these directions we get- 30watts(Svet 6L6) X 500= 15,000 15,000 -divided by- 341(plate voltage)= 43.9mA Acording to these instructions, 43mA will put you in the "low side of the midpoint" range. A common furmula is "max-plate-dissipation -divided by- plate voltage =X(variable)". Then take 70% of X variable. For instance- 30W(mpd) -divided by- 341(plate voltage)=.0879 or 88mA. 70% of 88 =61.6. 61.6 would be the highest mA setting I would want, in theory. For those who noticed- I realize these Svetlanas are a little too much for my low plate voltage. They just happened to be in there when I took the pictures. I've been waiting for another set of 6L6's to arrive this week for testing and left them in. I also have several other sets of 6L6 and 5881's. __________________ See my Toys!