Tube Bias Meter Questions

  • Thread starter Thread starter MetalThrasher
  • Start date Start date
mcstinger said:
Huh, if that 40 - 70 milivolts are going to zap you, than be careful with 9V batteries. Those are capable of tearing you apart :)

But seriously. On that banana plug is a voltage, which is directly measured by the connected multimeter on 200 mV range. Under normal circumstances, this is a completely safe voltage. Potentially a bit more more dangerous is that recommended bias scout probe.

It is of course completely wise not to touch any potentially dangerous parts around you. It is also wise not to give any advice in a field in which one is not familiar and mistakes in it are life-threatening.
Click to expand...
From the website regarding the probe - "This probe takes a direct DC plate current measurement by breaking the plate connection (pin #3)"

 
And the following is from Robert Hull in the Tube Amp Builders group. Seems to agree with my sentiments. The method works, but it's worth knowing what you're dealing with:

2a - plate current (break) method using passive bias adapters
tools required:
current meter, 0 - 200mA DC
bias adapter with serial plate circuit access (ex: early EuroTubes bias probes)
speaker / dummy load
Notes:
This biasing method uses specially designed adapters that break the plate circuit. EuroTubes offers this type of biasing probe. The banana plugs are connected to a current meter. With this adapter, the highest voltages in the amp are accessible at the banana plugs, therefore there is a chance of shock when not installed in the current meter. This is a very accurate method of testing
ease – 4
Math needed - none
accuracy - 5
safety - 2
expense - 3
Pros - this is one of the most accurate measurement methods
Cons - rather unsafe because the measurement requires extending the most dangerous voltages of the amp out to the meter.
example:
EuroTubes

You can see the full post here: https://www.facebook.com/groups/tubeampbuildersdiy/posts/1320939078437095/
 
I apologize for stupidity, if possible. I have edited my previous post. I did not notice that the described probe is a current probe. This completely changes the situation.
:doh:
 
mcstinger said:
I apologize for stupidity, if possible. I have edited my previous post. I did not notice that the described probe is a current probe. This completely changes the situation.
:doh:
Click to expand...
Hey man, no worries at all. This actually came up recently when I made some of my own probes for testing some electrical components. The banana plugs didn't sit fully flush in my meter and some of the metal was exposed. Totally different scenario since this was just low voltage stuff for effects pedals that I was breadboarding, but it was one of those moments where you're like, "Hmm... This could possibly be unsafe in certain situations." And it just so happened that the cheaper EuroTubes meter could possibly be one of those situations.
 
This is all great info! Keep them coming! If I have time this weekend, I may attempt to do this on my 2555X Silve Jubilee. Dumb question, what's the reasoning for putting one hand in your pocket while doing this?
 
MetalThrasher said:
This is all great info! Keep them coming! If I have time this weekend, I may attempt to do this on my 2555X Silve Jubilee. Dumb question, what's the reasoning for putting one hand in your pocket while doing this?
Click to expand...
This is a precaution in case you do receive a shock there's less of a chance it won't go directly across your heart and stop it. If one hand is on the amp and the other on anything else, the path of least resistance to ground would more likely flow from one hand to the other, which goes directly across your heart. With one and in your pocket the more more likely path would be from your hand to your foot. You'd still receive a hell of a shock, but less of a chance to go directly across your heart and stop it.

Just to be clear though, hand in your pocket is not a guarantee that you'd be unscathed if you did get a heavy jolt. There's always the chance that it can kill you. But you want to take as many precautions as you can no matter how small they are.
 
Last edited:
Code001 said:
And the following is from Robert Hull in the Tube Amp Builders group. Seems to agree with my sentiments. The method works, but it's worth knowing what you're dealing with:

2a - plate current (break) method using passive bias adapters
tools required:
current meter, 0 - 200mA DC
bias adapter with serial plate circuit access (ex: early EuroTubes bias probes)
speaker / dummy load
Notes:
This biasing method uses specially designed adapters that break the plate circuit. EuroTubes offers this type of biasing probe. The banana plugs are connected to a current meter. With this adapter, the highest voltages in the amp are accessible at the banana plugs, therefore there is a chance of shock when not installed in the current meter. This is a very accurate method of testing
ease – 4
Math needed - none
accuracy - 5
safety - 2
expense - 3
Pros - this is one of the most accurate measurement methods
Cons - rather unsafe because the measurement requires extending the most dangerous voltages of the amp out to the meter.
example:
EuroTubes

You can see the full post here: https://www.facebook.com/groups/tubeampbuildersdiy/posts/1320939078437095/
Click to expand...
Robert is a family friend and of the utmost skill, knowledge, and integrity.
 
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