BIAS PROBE VS Voltage drop across 1k1Ohm resistor

OD1

Hey guys,

So I built a bias probe after not being able to find any to buy anywhere in SA. The reading is in milliamps which I set mine to 31.5. My amp has a bias resistor connected to pin 8 of the powertubes (whats that, grid I think) but whatever, it measures the voltage drop across a 1k1ohm resistor, which is in milivolts.

My question is why if I set the milivolts to 31.5 the biasprobe has a reading of almost 40, I think it was 38.8 miliamps, isn't that too far apart?

When I set the miliamps to 31.5 my milivolt reading is 27.5 or thereabout.

Why would this be the case?

OD1

To anybody that comes across this post and has a similar issue, I think I found the problem.

So the 1Ohm resistor is used to use the equation V=I x R which is handy when you bias your amp -given you know the resistance, multiplying current by a known resistance value gives you voltage across that resistance.

This then all means that if you get a reading of 31.5 milivolts you assume that it will also equal 31.5 miliamps.

The problem is apparently the resistors are rarely exactly 1 Ohm and if you do the math you realize that a small error on the resistor has a dramatic effect on your bias at the end of the day.

My resistor is less than 1 ohm that is why I'm seeing higher bias current compared to the voltage drop across the resistor method. So check the resistor value or better yet, just build yourself a bias probe, it's pretty simple to do and tube sockets etc. can be found at tonetubes. There are also ample videos on youtube but you basically connect a multimeter in series with pin 3, you dont use pin 1 or pin 6 and the rest of the pins are connected.

In series is literally positive lead goes to tubesocket pin 3, negative lead goes to tube base pin 3. Hope this helps

modulator

Component values are one issue, multimeter drift another, and then, drift as the amp warms up, and so forth. Frayed multimeter cables, unseen, can drive you nuts. And fatigue, they do.
I bias to approximate spec, seeing as we have line voltage variations to muddle matters. I have no desire to adjust bias for "best tone", just not practical for my setup. So I tend to end up with conservative settings close to what the specifications say. Which, according to the internet, might not be the "best" bias.
What I would like to achieve, is to feed each amplifier the voltage it needs to have the heaters at 6.3 V, and then adjust bias to suit.
I should build a bias probe. On the other hand, why can they not build the amplifiers so one do not need to disassemble it to get to the probe points and bias pots?
I really consider going to Cathode Biased amplifiers exclusively, but I have a few lekker fixed bias amplifiers... and I still need to check one, but have to date (years...) not had the energy to take out the chassis to do it. One should, my Marshall was grossly out of spec, and I played it for years before I checked it, must have damaged the valves. Still going.
I re-valved my Laney Cub 10 recently, the bias was OK afterwards, but then, the factory bias specification is rather conservative, effectively making the amplifier "plug and play".

OD1

modulator Haha you mention power variations.....multimeter drift...component values.....I'm happy that amps function on a somewhat consistent basis tbh, just so many things that influence the sound that comes out the speaker....and if you want all of the chain to operate optimally I almost feel you will never be able to sleep again. Ignorance is bliss I suppose....as these things didn't bother me, now......kinda does 🙂

My new thing is dirty power, apparently I have this......I'm looking at a power conditioner....hopefully a schematic so I can try and build one myself and learn something in the process.

modulator

Ignorance is indeed bliss. You sleep much better.