unclej Posted April 26, 2005 Report Share Posted April 26, 2005 i recently came across geo-fex's plans for a 1 to 4 way bias probe and it sure seems like a time saver. have any of you tube mavens made one, used one, used one like it, have any opinions about bias probes in general? Quote Link to comment Share on other sites More sharing options...
lovekraft Posted April 28, 2005 Report Share Posted April 28, 2005 They appear to work great - however, so does wiring a small .1% power resistor between each power tube cathode and ground (or cathode bias resistor, if it's cathode biased) and reading the voltage drop across it to calculate the total current. Use a 1 ohm resistor and the voltage will read directly as amperage (25mV = 25 mA). It might not be a good idea to throw this mod into a minty '63 blackface, so you may want to build the bias meter to use on those. Quote Link to comment Share on other sites More sharing options...
unclej Posted April 28, 2005 Author Report Share Posted April 28, 2005 thanks lk..think i'll give the probe a try..don't have near enough toys yet. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted April 28, 2005 Report Share Posted April 28, 2005 The 1 ohm resistor trick works great. Quote Link to comment Share on other sites More sharing options...
unclej Posted April 28, 2005 Author Report Share Posted April 28, 2005 hey paul..thanks..i'll try the resistor method also..sounds easy enough..so let me double check.. i wire the resistor between cathode and ground on the power tubes..set my meter to mV..pos probe to hot side, neg to ground and take reading. and that reading will change as i adjust the bias pot..right? Quote Link to comment Share on other sites More sharing options...
lovekraft Posted April 28, 2005 Report Share Posted April 28, 2005 That's it! Keep in mind that it'll also read the screen grid current, so you can actually run the bias up close to max plate current and still have a small "safety margin". Just make sure your resistors are accurate - this is definitely not the place for a 10% carbon comp. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted April 29, 2005 Report Share Posted April 29, 2005 IMO, I think it's also a good idea to use 1 watt resistors in this application. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.