Pro One Bias Probe for Octal Base Power Tubes.

This is a very useful device to set the correct bias of the output tubes. Saves a lot of time having both milliamps and DC plate volts displayed at the same time. I really like the fact that it does not need batteries. Great product!

Bought this to bias my ENGL E840-50 amp head after installing new Apex matched JJ 5881 quad. Never done a tube bias before but have some background in electronics and felt confident about doing it.The ENGL is a stereo 50 watt per side (left/right) designed for either 8 ohm or a 16 ohm speaker loads (separate outputs for 8 or 16 ohm). I used a Marshall 2X12 stereo extension cab with two inputs for 16 ohm speakers, connected to the 16 ohm outputs on the ENGL amp. Apex recommends 31ma plate current to the 5881 tubes.When I looked at the plate current to the 5881’s I was seeing ~14.7 ma. This was with the bias adjust on the ENGL E840-50 at the lowest point in the adjustment range and a plate voltage of 455VDC. Maxing the bias adjustment range displayed a max plate current of ~ 21 ma.I assumed that the discrepancy in the measured plate current is due to the fact I am using a 16 ohm load as opposed to an 8 ohm load; that it was halving the measured plate current per the formula: tube wattage/plate voltage * .7 = plate current. As such, I adjusted the plate current for 15.5 ma, with is ½ of the Apex recommended plate current of 31 ma and 4 ma less than the result of 23 watts / 455VDC * .7 = 35 ma.Testing the amp – sounds GREAT, plenty of volume with gain stage enabled, tiny bit of presence chime. Signal/Noise is excellent; the amp is dead quiet even off standby. I am hearing some low-end tube crackle but that is my pre-amp, not the ENGL.Regarding the discrepancy -- all my cabs are 16 ohm except a combo amp and don’t feel like pulling an 8 ohm speaker out of a combo amp or borrowing an 8 ohm cab to bias my ENGL head with an 8 ohm load, especially since my ENGL head is designed to handle 16 ohm speakers, which is what I have available.If the plate current discrepancy displayed by the Pro One when using a 16 ohm load on an amp designed for 16 ohm speakers is a valid concern and my assumption about dividing the plate voltage by half is correct, I suggest that the Eurotubes update their documentation to reflect this.I emailed Eurotubes asking for clarification. The response was that I was not matching the impedance of the amp to the impedance of the cab, that I was not “following the rules” and I was “on my own”. I did provide them links to the ENGL E840-50 amp documentation showing that the amp is designed for either 8 or 16 ohm speaker loads and I was using a 16 ohm speaker load. I did not receive any further response from Eurotubes after this point.I feel that the Eurotubes Pro One is a useful, well-designed product. However, I am taking one star off an otherwise 5-star rating for the response I received from Eurotubes regarding my question.

Eurotubes Pro One Bias Probe for Octal Base Power Tubes.TUBE AMPS CONTAIN POTENTIALLY LETHAL VOLTAGES. IF NOT FAMILIAR WITH TUBE AMP SERVICE IT IS BEST TO TAKE IN THE AMP AND HAVE THE BIAS SET FOR YOU.I purchased both the Eurotubes Pro One bias probe for octal tubes as well as the Eurotubes bias probe.These are excellent products. In five minutes time I was able to measure and set the bias for a pair of 6L6GB/5881 tubes. To use the Pro One, you simply remove one power tube from your amp (turn off power first, of course), insert the Pro One socket into that tube socket, insert the tube into the Pro One ceramic socket, power on your amp and read the display. You will find both the plate voltage and the bias displayed. It's easy enough for a musician with little experience to measure the bias and plate voltage. You need both, because you have to do a simple calculation to arrive at the correct bias for your tube. All explained in the sheet that comes with it, however (briefly) you divide the tube's maximum dissipation (in watts) by the plate voltage, multiply by .7, and that is your target for the bias. In my case, the plate voltage is 450v (as displayed) and the tube dissipation is 23 watts (from the specs), and so [23 / 450] * .7 = 36ma. Just that simple. Tube power dissipation charts are available all over the web, or you can refer to a tube manual. 6L6GC is common, and the dissipation is about 30 watts for that tube. Regarding the actual setting of the bias: I have a bias pot on my amp so I simply set it. Every amp will have a different procedure for setting the bias, but the Pro One makes the measurement part of the task simple.I put the probe back in action and snapped a couple of pictures. and they are attached.The basic probe (this is much cheaper) I also tested. In this case, you insert the probe and tube as stated above, insert the test leads into your DVM, set the DVM for milliamp reading (all meters are different, but assume up to 60 ma) and then turn on the amp. You will need to know the plate voltage and in this case you will need to measure it or locate/find the information elsewhere. The measurement is done on your meter and setting target is calculated and set as described above.Notes:1. Warm the amp up for a better setting.2. For new tubes any tech would recommend that you recheck the bias after a few hours of run time. It's easy to do.3. The calculated target is approximate. You can run your tubes hotter (up to the spec max) or colder depending upon your preference for the tone and tube-life trade off.If there's an interest I can set up the simple bias probe and snap a couple of pictures for you.(I have no affiliation whatsoever with Eurotubes, however we both share a love of tube amps)

Pleased it works like it should, reasonably well built

Worked great. Was able to set the bias on my new output tubes and the 1964 amp sounds wonderful!

After 1.5 years of working great this thing failed. It stopped reading the plate current. Should have just thrown it out, but decided to test it on another amp. Bad idea, it smoked a resistor below the power tube on a JTM45. Any signs of malfunctioning, just can it, or you will be paying a visit to the amp technician.

The Eurotube bias meter is unbelievable easy to use and actually kind of cool. Worked perfectly - my amp sounds amazing!

This is such a great product, quick and easy you have the milli Amps and the plate voltage, no probing or resistors needed. And tweaking bias and playing was never this good.