I need help with fundamental tube bias information

by mrconclusion Thu Oct 18, 2012 1:55 am

mrconclusion: Posts : 15
Join date : 2012-10-18

Post n°1

I need help with fundamental tube bias information

by mrconclusion Thu Oct 18, 2012 1:55 am

Hello! This is my first post but I've been reading the forum for a while. I have one of Bob Latino's VTA ST70's and am about to build another. The problem is, I don't really understand the jump from calculating desired bias in amps, to measuring it in volts. I want to play with various tubes without always asking "where should I set the bias for this one?" so it would be nice to know what I'm doing!

I understand measuring plate voltage, dividing it by 70% of the tube's rated dissipation, and getting the answer in amps... but how is this translated into volts measured at the test points? I've found info online (mostly re: guitar amps) that talks about transformer resistance or cathode resistor value... am I barking up the wrong tree?

I bet this is a very stupid newbie question and I apologize if it's been covered before. If so, point me at a useful old thread and I'll be happy! (The existing threads I keep finding concern bias problems or bias for one particular tube.) Fortunately I have no amplifier problems - just a lack of knowledge. Thanks you in advance for your assistance!

by **Bob Latino** Thu Oct 18, 2012 11:17 am

Bob Latino

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: I need help with fundamental tube bias information

by Bob Latino Thu Oct 18, 2012 11:17 am

Volts = amps X resistance therefore amps = volts divided by resistance. In all the VTA amps we use a 10 ohm bias resistor therefore the amps that the tube will be using will always be 1/10 that of the measured voltage. If you measure the bias voltage across that 10 ohm resistor and you measure .400 volts or 400 millivolts then that tube is using 1/10 of that value in amps - so - the tube is using .040 amps or 40 milliamps.

The reason you use VOLTS to measure bias is that it is easier to do with a meter. To measure amps with a meter the meter has to be in series with the circuit and the meter itself with it's own resistance becomes a part of the circuit. It is hard to get an accurate reading this way.

Bob

Last edited by Bob Latino on Thu Oct 18, 2012 2:49 pm; edited 1 time in total

by mrconclusion Thu Oct 18, 2012 12:10 pm

mrconclusion

mrconclusion: Posts : 15
Join date : 2012-10-18

Post n°3

Re: I need help with fundamental tube bias information

by mrconclusion Thu Oct 18, 2012 12:10 pm

Thanks you very much Bob, that's an excellent explanation. This explains why I couldn't get the math to make sense when I was looking for a cathode resistor. I feel silly because the term 'bias resistor' should have clued me in. Using a 10 ohm bias resistor sure does make the math easy!