BIAS Probe

[MrBump]

Hey Guys.

Anyone have any experience using a BIAS probe?  I'm thinking that it might be an easy way to DIY my Orange, but I've never one anything like that before (and I'm a wee bit worried about the current involved...)

Ta.

Mark.

[ailean]

I've been thinking about getting a multimeter so I can bias my Orange myself. The pre-built probes look very expensive for what they are, that said if you don't have much electrical experience then they are probably the safest bet.

The other thing of course is that you still need to get you hands dirty changing the resistors as I don't think Orange use variable resistors.

I think if I do decided to get any gear it will be a multimeter.

[hamfist]

A multimeter will not quite be enough on it's own. A Multimeter will allow you to directly measure the plate voltage, but not the plate current.
  There are a few ways to measure plate current. First, one can install a 1 ohm resistor in the cathode lead from each power tube. To measure the current flowing through the resistor, you measure the voltage drop across the resistor. If a 1 ohm resistor is used, the plate current in milliamps is equal to the voltage across the resistor in millivolts (50mV across the 1 ohm resistor indicates 50mA plate current).  
   Alternatively, one can buy a tube socket adaptor, to fit between the socket and the tube. These will have leads coming out of them that will plug iunto your multimeter, which you can either directly measure the plate current, or use a similar arrangement to the 1 ohm resistor in the cathode lead, as mentioned before.
   I've just spent ages researching all this, and just put in an order for an SRS Dual Bias tool - http://www.srs-webstore.com/pages/biastool.html . With my multimeter, it will be all I need to bias new tubes.

Some great biasing info here - http://www.aikenamps.com/Biasing.html

  So Rich, don't rush into buying anything as I'll have the kit, and would be happy to come around and bias any new tubes for you.  Of course, if you just fancy getting into tinkering around with amps yourself I would understand completely !!

[martinw]

Be careful out there!  :!:
Using a bias probe is one thing..worst case you damage your amp.
Measuring anode (or plate) voltage using a DMM is another...worst case you die. You can also damage the amp if your meter probe slips off the pin and shorts to the next one.

BTW you need both. Bias probe readings (which measure cathode current from which you can approximate anode current) but you also need to measure the anode voltage to use the equation Power = V x I. This gives you quiescent Anode dissipation (not audio power).

[ailean]

...worst case you die.

Yeah I know, that's why I haven't got one yet. But I do want to learn to do this myself. It's been a while since I played with circuits and power formulas but they don't scare me. 400volts and current to kill, does scare me however, so I'll take my time over this :)

[hamfist]

Be careful out there!  :!:
Using a bias probe is one thing..worst case you damage your amp.
Measuring anode (or plate) voltage using a DMM is another...worst case you die. You can also damage the amp if your meter probe slips off the pin and shorts to the next one.

  Agreed, you have take the time to educate yourself about how to go about things safely, and also take your time and don't rush when working on an amp.
  However, I also think that we should not be too frightened of the high voltages in an amp . Just be naturally cautious, as one is when driving or crossing the road, for example - you know it can kill, but follow all the basic safety rules and you'll be fine.
  I do think it's something that virtually everyone on this forum could probably achieve safely and accurately, with a little bit of time and effort.

BTW you need both. Bias probe readings (which measure cathode current from which you can approximate anode current) but you also need to measure the anode voltage to use the equation Power = V x I. This gives you quiescent Anode dissipation (not audio power).

Absolutely. It's amazing how much interweb (and dodgy amp tech) knowledge out there, talks only about plate current, when biasing.
  One can only work out the power dissipation of each tube if you know the plate voltage as well (which can vary enormously between amps).

   eg. you might hear someone recommending biasing EL34's at 46mA. Well the actual power dissipation would vary enormously between someone with an amp B+ (plate voltage) of 380V, and someone with a B+ of 490V.

Amp 1 - power diss = 380V x 0.046A = 17.48W (70% of max diss power for an EL34 of 25W)

Amp 2 - power diss = 490V x 0.046A = 22.54W (90% of max diss power for an EL34 of 25W).

  It's generally considered that about 70% of the max quoted power dissipation is about as hot as you'd want to go before you start eating tubes quicker than you'd want, so Amp2 would b running waaaay too hot with a plate current of 46 mA - which equates to a power dissipation of 22.5W, which is 90% of the max quoted diss power for an EL34 !!

 Can anyone tell I'm really getting into this stuff !! -  I'll hopefully have even actually biased my first tubes in a week or two, when my probe arrives !!

[MrBump]

Cheers for the replies, all.

I've read enough to understand the principal, I think.  What I was asking was more along the lines of the tools that people would use to accomplish the task.

Digital multimeter, obviously.  I've seen bias probes online, that you can plug your valve into and gives you an easy point to measure the (I think) cathode resistance... But I've also seen devices online that do the calculations for you.  However, these cost a couple of hundred dollars.

Just wondered how people achieve this.

Mark.

[martinw]

  However, I also think that we should not be too frightened of the high voltages in an amp . Just be naturally cautious, as one is when driving or crossing the road, for example - you know it can kill, but follow all the basic safety rules and you'll be fine.
  I do think it's something that virtually everyone on this forum could probably achieve safely and accurately, with a little bit of time and effort.

I don't actually agree with this. In an industrial setting, working on live electrical systems ususally requires additional training, even for experienced engineers. Please don't downplay the risks.

[hamfist]

Cheers for the replies, all.

I've read enough to understand the principal, I think.  What I was asking was more along the lines of the tools that people would use to accomplish the task.

Digital multimeter, obviously.  I've seen bias probes online, that you can plug your valve into and gives you an easy point to measure the (I think) cathode resistance... But I've also seen devices online that do the calculations for you.  However, these cost a couple of hundred dollars.

Just wondered how people achieve this.

Mark.

I think it all comes down to how much you want to spend and how easy and foolproof you want it to be. The compubias is probably the most utterly easy to use (and one of the most expensive), as it even does all the calculations for you.
  However, if you feel handy with a multimeter, and feel confident about learning how to measure the B+ safely in an amp, then a cheaper one would be fine, as the mathematics is fairly simple and, as you say, there are a number of online bias calculation tools.
   Also be aware that you'll be wanting to bias within the safe limits of both tubes (in a 2 tube setup), so you will need to check the plate current in both tubes. So getting a single probe makes the whole process a bit more awkward, as you have to shut the amp down between doing each tube, and then move the probe onto the other tube, tweak bias again, shut down , move the probe again etc.  The easier solution is to get a double probe so you can just switch between measuring each tube by flicking a switch. the SRS dual probe I linked to above does this (at a very reasonable cost). The Weber Bias tool can do this too, and is apparently very well thought of.
  It's all just a matter of balancing cost with how easy you want the whole procedure to be.

[hamfist]

  However, I also think that we should not be too frightened of the high voltages in an amp . Just be naturally cautious, as one is when driving or crossing the road, for example - you know it can kill, but follow all the basic safety rules and you'll be fine.
  I do think it's something that virtually everyone on this forum could probably achieve safely and accurately, with a little bit of time and effort.

I don't actually agree with this. In an industrial setting, working on live electrical systems ususally requires additional training, even for experienced engineers. Please don't downplay the risks.

 I absolutely agree, that the risks are there. But I also think that in the context of a tube guitar amp, a careful hobbyist, with a working knowledge of how electricity works and a basic knowledge of circuits can learn how to do things safely. There is a huge amount of great info out there, to learn how to do it safely and properly.
  There are thousands, maybe tens of thousands, of "careful hobbyists" out there around the globe doing all sorts of stuff within their amps. These people seem mostly self-taught, along with support from more experienced folks. It can be done, going slowly one step at a time, following good safety advice, with the right tools.
  So I think we're going to have to agree to disagree slightly.

What I will agree on is that no-one should ever put anything inside an amplifier whether it's on or off, unless they have found out the risks involved, and also how to minimise those risks by doing things safely.

[jpfamps]

The more sophisticated bias probes can measure plate voltage and current. I've built one that does this, but would not advise anyone who isn't familiar with high voltage electronics to try this.

Regardless, one point that isn't mentioned herein is that the bias adjust pot (if there is one fitted to the amp!) is often inside the chassis and this means you ideally have to have the amp running with the chassis out, thus exposing the high voltages inside. I suppose you could measure the bias, turn the amp off, remove the chassis, adjust the pot, put the chassis back in, turn the amp on, measure the bias, turn the amp off, remove ........... etc

In an ideal world all manufacturers would provide externally accessible  bias test points and adjust pots, however I expect that this is too much to ask....

[PhilKing]

Both of the Ceriatones that I just received have built in probe points and an accessable pot (you do have to remove the back from the cabinet, but if you are changing valves you would have already done this).  I was very impressed with this feature.  In the picture you can just see the trim pot behind the left hand tube red probe insert point.  The common -ve probe is behind the triode/pentode switch between the KT66's.

[hamfist]

Both of the Ceriatones that I just received have built in probe points and an accessable pot (you do have to remove the back from the cabinet, but if you are changing valves you would have already done this).  I was very impressed with this feature.  In the picture you can just see the trim pot behind the left hand tube red probe insert point.  The common -ve probe is behind the triode/pentode switch between the KT66's.

  That's a great feature, Phil.  I didn't think to ask to have that installed on my recent Ceriatone 1987. How much extra was that feature ?  I'll bet something ridiculous like $10 !

[PhilKing]

That's a great feature, Phil.  I didn't think to ask to have that installed on my recent Ceriatone 1987. How much extra was that feature ?  I'll bet something ridiculous like $10 !

It's standard on the Overtone Special, and my JTM45/18 is a custom amp with cathode bias on the power tubes (unlike a standard Marshall).  I think that Nik added it with the cathode bias.  However none of his mods are expensive so I am sure it isn't much.

[martinw]

Bias points aren't much use on a cathode biased amp.