Eurotubes bias probe - accurate and safe?

[spayne99]

I probably should've asked this before I ordered it, but I've read a few mixed forum posts on this bias probe. Including one where someone insinuated that the probe is not safe to use.

Anyone know much about this? I'd be using it with the Eurotubes 'cheap' multimeter, to bias my PRRI.

Anything to be worried about here? I figure as long as I follow the instructions and don't do anything stupid, it'll be fine. No problem. I'm also curious about how accurate it is, vs. having a tech do the bias?

[robt57]

1st I've heard Euro is doing one, I know Weber used to [and may still]

[StephaninMelb]

Your Back Pocket – The Most Important Biasing Tool

When working inside a live amp Put your Second Hand in your Back Pocket. When working with one hand if you get a jolt it is going to sting. If you have your second hand touching the amp chassis or other ground point the current path will flow through your heart. Chances are you are doomed if you take a high voltage jolt with both hands completing a circuit.

You will be working around 300-500V circuits!


Other Safety Tips

• Use a nylon potentiometer screwdriver
• Alternatively, use a metal tip insulated screwdriver
• If you drop your tool while the amp is on - don't reach for it
• If you drop your tool turn the amp off and discharge the caps before attempting to remove tool
• Don't leave your amp unattended with the enclosure open if children are around
• Don't connect/disconnect test leads with the amp on
• Don't touch any of the circuitry outside the potentiometer with the amp running
• Rubber soled shoes/rubber mats can provide additional safety
• Do not touch any other plugged in electrical item or pipes while you are working

If you are not experienced it is best to have a tech work on setting your bias. Alternatively seek detailed specific instructions for each of your amp models from experienced people here on TDPRI.

[BobbyZ]

Nice thing about the bias probes is thay fit between the tube and the socket so if you can get at the bias pot you don't need to pull the amp apart. (if this is like the others I've seen)
Pretty safe way of doing it.

I don't use one because I'm too cheap to buy one and too lazy to build one.

[tazzboy]

Never used it Now that Weber has re issue the Bias Rite Meter which is what I have.

[bdgregory]

I have 2 of Eurotube's probes that I use with a cheapo multimeter - they work great (accuracy tested using a good quality meter and internal test points). I can't imagine why they would be unsafe - except for the fact that on many amps the bias pot is inside the amp - in that case use Stephan's guidance.

[gwjensen]

I've had no problem with it. Works good.

[robt57]

So do you need multiple devices to do a multi power tube amp ? Or does the adjustment carry across a pair of tubes or all 4 in that case ?

[spayne99]

Originally Posted by robt57

So do you need multiple devices to do a multi power tube amp ? Or does the adjustment carry across a pair of tubes or all 4 in that case ?

Theoretically you buy matched tubes, and adjust the bias measuring one tube only. Hopefully it works that way.

[BobbyZ]

robt57 most amps just have one bias pot for all four tubes. So if the tubes are matched and the amp is in good shape one probe should do the trick. But it would be a good idea to put the probe on each tube just to make sure there all running the same. Or close.
Even with a two tube amp you'd want to cheak each tube.

[robt57]

Thanks for the answers there... ;)

[riff]

Safety is paramount. I wouldn't worry about accuracy so much. I know what range I'm shooting for so I start within that range and leave the setting where it sounds best within it. My Fenders usually sound best well below 70% plate dissipation, usually 55-60%. If I do it that way it only needs to be reasonably accurate.

[spayne99]

The probe and multimeter were delivered yesterday. I tried it out on the stock EHX tube (v5 socket) in my Princeton Reverb Reissue and after about 5 minutes of warm up time I'm only getting a reading of 9.7 ma. Eurotubes said I should bias to 23-25 ma. Could the factory bias really be that cool, 9.7?

This weekend I'll give it a try with the new Tungsols.

[TxTeleMan]

It's not the bias probe that's unsafe, it's the 400 or so volts that can get you. The one hand rule is important, and before you touch something, make sure you know what it is.

[AnthonyTurco]

Just some info on the cheaper bias probe from eurotubes, it got me thinking since I have one, I thought that with out a doubt if I switched my probe while the amp was off or in standby to dcv high voltage setting 1000, that the probe should read plate voltage, and it did, gave me 425 for jj 6v6 in a fender deluxe reverb reissue. They don't tell you that it will also read plate voltage, then I put the amp in standby again, and switched my meter to dc amps 200 is fine, and I set my bias accordingly. With all of this I let the amp sit in the on and standby on for about 10 minutes for each reading, this way I knew it was settled out. You must make sure you do not switch your meter while the amp is on, the power can be on but you must switch it to standby mode first before you switch, if you do not you will could blow your meter or cause awful popping sounds to come out of the amp. So again make sure your in standby mode FIRST then switch to and from your DC Voltage or to Dc amps to do the baising. DC Voltage for Plate Voltage, DC Amps for Bias setting. Just to make sure you understand I am talking about the cheap 25 dollar probe from Eurotubes. You don't need the more expensive one to get your plate voltage, and the great thing is your not in the amp poking around! Hope this helps! If doing this is wrong which I feel it isn't I would love some feed back. To get my bias where I like it I use formula and ears, I take

Plate Voltage x milliamp reading for example 22 milliamps is 0.022 then I divide all of this by the tube spec wattage, 14 for jjs

that will tell you what percentage dissipation you are at, i tend to try to stay just under 70% as I like a little bit more clean headroom.

Plate Voltage x Milliamp / tube wattage = % of maximum plate dissipation for that tube.

thanks

anthony

[Clint80]

Originally Posted by AnthonyTurco

Just some info on the cheaper bias probe from eurotubes, it got me thinking since I have one, I thought that with out a doubt if I switched my probe while the amp was off or in standby to dcv high voltage setting 1000, that the probe should read plate voltage, and it did, gave me 425 for jj 6v6 in a fender deluxe reverb reissue. They don't tell you that it will also read plate voltage, then I put the amp in standby again, and switched my meter to dc amps 200 is fine, and I set my bias accordingly. With all of this I let the amp sit in the on and standby on for about 10 minutes for each reading, this way I knew it was settled out. You must make sure you do not switch your meter while the amp is on, the power can be on but you must switch it to standby mode first before you switch, if you do not you will could blow your meter or cause awful popping sounds to come out of the amp. So again make sure your in standby mode FIRST then switch to and from your DC Voltage or to Dc amps to do the baising. DC Voltage for Plate Voltage, DC Amps for Bias setting. Just to make sure you understand I am talking about the cheap 25 dollar probe from Eurotubes. You don't need the more expensive one to get your plate voltage, and the great thing is your not in the amp poking around! Hope this helps! If doing this is wrong which I feel it isn't I would love some feed back. To get my bias where I like it I use formula and ears, I take

Plate Voltage x milliamp reading for example 22 milliamps is 0.022 then I divide all of this by the tube spec wattage, 14 for jjs

that will tell you what percentage dissipation you are at, i tend to try to stay just under 70% as I like a little bit more clean headroom.

Plate Voltage x Milliamp / tube wattage = % of maximum plate dissipation for that tube.

thanks

anthony

Thanks for the heads up on PV. I just biased my PRRI with the Eurotubes Bias Probe and cheap multimeter and it was easy.

[telex76]

Originally Posted by spayne99

Theoretically you buy matched tubes, and adjust the bias measuring one tube only. Hopefully it works that way.

Yes, but one of the reasons you check is to be sure the tubes are matched, so I always check both tubes.