# Bias Probes



## dcole (Oct 8, 2008)

Hello,

I am looking at building my own bias probes for working on amps. Currently I am measuring the transformers resistance and using this to calculate back the current flow in the tube after I measure the voltage drop across the transformer once the amp is turned on. In researching what others have done for DIY bias probes, I see that everyone (except RG Keen) put a 1 ohm resistor in the cathode lead. Why? Why wouldn't it be put in the plate lead so that the direct current can be measured instead of the combined screen/plate current that makes up the cathode current?

Thanks,

David Cole


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## dtsaudio (Apr 15, 2009)

Safety is the primary reason. Since screen current is very low (5ma or less typically) it can largely be ignored. Then you are dealing with the low voltage at the cathode instead of the high voltage at the plate. This can be particularly important if you are going to put external measurement points on your amp.


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## nonreverb (Sep 19, 2006)

Agreed, safer and far more cost effective as engineering it from the B+ would require more attention to isolation and possibly more robust components.
Design would be critical as arcing is always a potential danger...pardon the pun.



dtsaudio said:


> Safety is the primary reason. Since screen current is very low (5ma or less typically) it can largely be ignored. Then you are dealing with the low voltage at the cathode instead of the high voltage at the plate. This can be particularly important if you are going to put external measurement points on your amp.


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## GTmaker (Apr 24, 2006)

I have one of these
http://www.amp-head.com/product_info.php?cPath=21&products_id=31

for 65 bucks is it realy worth the trouble in building your own?
just thinking out loud...

G.


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## vadsy (Dec 2, 2010)

I was just considering this myself, especially since they just went on sale. http://www.tubedepot.com/bt-bias-scout.html Seems simple enough, instructions are clear, the work is little and with shipping being 6 bucks in could be another way to go if you already have a meter.


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## dcole (Oct 8, 2008)

Thanks guys. I could see safety being a problem, but then there is always danger when working on tube amps. I wanted to roll my own because I really only require it when working on amps with 4 power tubes as there is no other way to find out if the tubes are sharing the current.


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## Paul Running (Apr 12, 2020)

dcole said:


> Hello,
> 
> I am looking at building my own bias probes for working on amps. Currently I am measuring the transformers resistance and using this to calculate back the current flow in the tube after I measure the voltage drop across the transformer once the amp is turned on. In researching what others have done for DIY bias probes, I see that everyone (except RG Keen) put a 1 ohm resistor in the cathode lead. Why? Why wouldn't it be put in the plate lead so that the direct current can be measured instead of the combined screen/plate current that makes up the cathode current?
> 
> ...


Build it yourself...the more sweat you put into something, returns in something that money can't buy. Here's two of several that I have built from discarded tubes and sockets. One has banana plugs on it to interface with an ammeter of your choice and the other is dedicated to a 100mA meter.


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## Latole (Aug 11, 2020)

GTmaker said:


> I have one of these
> http://www.amp-head.com/product_info.php?cPath=21&products_id=31
> 
> for 65 bucks is it realy worth the trouble in building your own?
> ...


Link do not work.
I have those bias probes, one for 6L6/EL34 tubes other for EL84 tubes.
IMO they are not available anymore.
One of those:


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## Latole (Aug 11, 2020)

dcole said:


> Thanks guys. I could see safety being a problem, but then there is always danger when working on tube amps. I wanted to roll my own because I really only require it when working on amps with 4 power tubes as there is no other way to find out if the tubes are sharing the current.


Solder 1 ohms 1/2 or 1 watts resistor between each tubes cathode ( pin 6 for 6L6, EL34) and ground.
Read milivolts = milliamp on each 1 ohms resistor. You'll know how each tube draw.
Resistors can stay there for life.

You can also add test jacks to easy check bias from outside the amp.
Don't do that on a vintage $$ amp


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