Bias probe leads for octal / tube testing

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Tubetec

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Nov 18, 2015
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I see many tube amp suppliers offer bias adapters , mostly they simply insert a 1 ohm resistor in the cathode to allow current to be measured , one called the bias scout also provides a connection to measure anode voltage ,
A simple switch box could be made , four octal adapters  , two pole rotary  could then switch in or out the various tubes volts and current to a multimeter , it could make a handy simple rig to check/match tubes ,

I did of course see some of you have the Utracer,  but to be honest its a bit to involved and its only one tube at a time , in circuit testing in real amps is more what I want .  I have my eye on an orange divo ov4 ,the software is fairly basic , and theres a whole load of waffle about accreditation to use the full capabillities  , its a dead project now ,products off the shelves ,but I was half thinking of buying it purely to construct a test jig , the makers of the board module ,tubesync now offer an internal version designed to allow the roadie to monitor and control the tube amp bias from beside the stage , and keep track of faulty  tubes etc .
The software is very boring , one page allows you to set some variables relating to the  currents at idle  and under dynamic conditions for each tube , thats it , Tubesync must have a proper software console to get  real time readings back from the tubes
but the screen shot below is all the public see's ,
So that orange box of tricks isnt likely to be much use to me for any realtime measurements via a computer , the usual passive plug in for a multimeter that measures cathode current is a bit limited , adding anode voltage ,its own negative variable grid supply and the possibillity of a balanced input from a signal generator would be cool , maybe REW could be used to view the sweet spots in the harmonics at various biasings, signal levels and frequencies .

I see most bias probes arent designed to be run under actual dynamic conditions ,you simply set the tube current to the correct value ,then disconnect it before testing it out for sound .
If anyone does use bias probes or has other ideas for measuring tubes in circuit any ideas welcome .
 

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Fuzz Face

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Nov 22, 2018
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Anaheim
Most of the tube amp guys that I know use the Weber BiasRite

https://www.tedweber.com/br-4

They measure idle current and are not meant to be used under dynamic conditions as you mentioned
 

Tubetec

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Nov 18, 2015
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3,184
The webber probe is nice , but still doesnt allow dynamic testing like I want , multimeters are so cheap nowadays ,you could afford to dedicate one per tube  for this purpose , Of course you can always probe around to find voltages and work out bias points ,but the bias probe would allow hands free ,one hand to switch the tube 1,2,3,4 ,then another hand to note down a bias value for each . adding the variable grid bias,test tone input  and load resistor/watt meter /REW distortion analyser  on the output of your amp you could probably graph up each tubes performance very nicely . A simple grid volts vs current graph could be penciled in for  each tube on a sheet of paper pre prepared with the graph axis's and scale . Making the whole thing semi automated with a computer to do the data collection and display would be very nice , I have emailed tubesync about their product ,but that only  displays the preset cathode current and doesnt give any realtime data to the end user .

End of day maybe making it computer automated is probably not nessesary unless you wanted to test hundreds of valves , simple multimeter usb interface could be used to data log the anode currents over a burn in period .Regular sig gen could provide test signals to the grids ,20 volts from my digital arb or 50 volts from the Hp 201cd , theres a temptation to over complicate with technology ,but a simple universal bench test jig for 2 or 4 output tubes that is capable of dynamic testing , you could have a simple work sheet prepared ,then run through a preset series of tests , note down readings , match up the pairs or quads ,job done .

I went into my local music store lately , they wanted nearly 200 euros for a quad of EL34's ,mesa/boogie ,that was all there prepared to offer the customer , because the  rep/sales has them totally paranoid about their proprietary testing and matching process .....load of fkn blsht
If I could buy in bulk tubes ,burn  in and test,  then box,match with credentials and charge half the boogie price ,selling a quad would be equivalent to a around a days work busting down electric motors ,plus it would include the heating bill for my new workshed , 100% valve powered of course  ;D


 

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