Bias Probe Help

Most meters have a hard time measuring small resistances. I wouldn't be concerned with the 1.5 ohm reading.

Need more info on the 28mA vs 20mA readings. Sometimes the screen is figured into the reading and sometimes it is not. The answer could be as simple as that.

Also the voltage and resistances may not have been read to the same extent. Such as 4.657 volts vs rounded off to 5 volts. The math can throw the answer off quite a bit when the numbers are not accurate.

 

What does your meter read when you short the normal probes on a low resistance scale?. I don't know what your bias probe looks like, or how it sits in the grand scheme of things. Mine sit between the valve and chassis and breakout one of the legs which is then connected to an ammeter.

You could try measuring the bias using your probe, then use direct measurements and/or transformer shunt measurement if applicable to compare the results.

Confirm your ammeter is accurate, and confirm the voltage you're using too. If a schematic says 328V, but that's with a wall voltage of V and your wall voltage is V+/-12, things'll get messy. To get to the voltage check points on mine, I have to pull the chassis. It isn't a job I need to do often, and I take the opportunity to have a look around for signs of scorching or blebbing capacitors.

 

What amp? How was the tech measuring tube current? What was the AC voltage at the tech's shop compared to the AC voltage at your place? Is the battery in your meter fresh?

 

Too many variable to diagnose remotely.

I wouldn't worry about it, it is what it is. Have your amp biased properly by your tech. When you take it home, use your Prob-O-Matic(R) widgit and write down what it is reading. The exact number doesn't matter, your amp is already biased, you're just getting a reference.

When you retube, make sure you get the same number, even if it is totally imaginary. It's just a reference.

 

FYI in most locals the wall voltage varies. Some places it varies a lot. Utah doesn't have many people so it may vary less. You may find the voltage is higher at night or on the weekend. During the day, especially when everyone has an air conditioner *on*, the voltage will be less.
So, it is always a good idea to measure the wall voltage whenever you measure the voltages in the amp.

 

A quick search result for bias probe revealed several that measure plate current only. Different methods can be used to determine bias current of a tube.

When measuring cathode current with 1ohm resistors on the cathode, all of the current from the plate and the screen is measured. The screen current is usually subtracted from the total to have a *more accurate* plate current reading. The tube datasheet will give a nominal screen current listing. Sometimes the datasheet will list it as grid #2. This mA figure is subtracted.

If you are *getting* this you will surmise measuring the bias is not a super accurate measurement. The accuracy relies on many variables that are usually not controlled. Combine that with the knowledge that every tube is different... well... close enough is good enough.

It seems to me your probe functions. Follow Paul G's advice and you will be in the ballpark.

If you are on speaking terms with the tech, ask how he finds the bias current.

For more info on figuring bias, look at the several methods mentioned on the *Robinette bias calculator* site.

 

You would add the approximately 8mA.

Generally you would want to use plate current though. It is best to talk apples to apples.

When I saw the difference in your 20mA and your tech's 28mA, I figured he may not have subtracted the screen current. Many factors come into play though. Wall voltage, meter/probe accuracy, math, etc. Could be one or... a lot of reasons.

If you can, adjust the amp to stage volume and adjust the bias by ear. If you can find a sweet spot, use your bias probe to find the numbers at that bias setting. Many of us find that to be around 60% plate dissipation. The high gain amps may or may not be higher than that. Each amp, tube and ear is different so definitely YMMV.

The main point is to have the tubes run in a safe zone so they do not red plate. Some want tubes to last longer so they choose low percentages. The vintage Fender products mostly ran on the cold side. The point is, don't let the numbers drive your choice of bias setting.

 

Those are max numbers. You want typical zero signal operating conditions because that is where you are when you set the bias. A case can be made that screen current is 0.133 times the plate current in a typical guitar amp. So if you measure 34mA across a one ohm resistor, the plate current is 34 divided by 1.133 which equals a plate current of 30mA.

 

Huh?

 

That doesn't prove anything except that both meters can't accurately measure very small resistances. See post #2.

The resistors in those probes are almost certainly 1r 1%. You just gotta have faith.

 

Where are you guys getting 8mA?

 

Ten Over I was writing about the difference between metering the current at the cathode vs metering current at the plate. Current at the plate is just that. Current at the cathode is plate current and screen current. Screen current would have to be subtracted to arrive at plate current. On the robinette bias calc site 5.5% is subtracted from the cathode current to arrive at the plate current. The 8mA is from the tube chart. The actual may only be ~2mA. the point was to explain the difference in two methods of checking the bias.

 

What tube chart? Can you post it?

 

@Ten Over .
Askwhy posted some of the numbers from a tube chart in post #10. No actual chart (yet).

 

No 8mA in that post.