Members bryank Posted February 6, 2006 Members Share Posted February 6, 2006 what is the tone differnce if one were to bias (el34) the tubes at say 35 mA VS. 42 mA? and what the heck is biasing a tube 'cold" or hot" mean? the higher the bias number , the hotter the tube? Link to comment Share on other sites More sharing options...
Members MetalHed Posted February 6, 2006 Members Share Posted February 6, 2006 Originally posted by bryank what is the tone differnce if one were to bias (el34) the tubes at say 35 mA VS. 42 mA?and what the heck is biasing a tube 'cold" or hot" mean? the higher the bias number , the hotter the tube? The first thing that you need to understand is that the number of mA that you want running through the tubes is dependent on the plate voltage. Running your amp at 35mA sounds reasonable if the plate voltage is around 450 volts. If the plate voltage is much higher or lower than that, you might be running either too much or too little current. So, if you mention the mA without mentioning the plate voltage, you are only giving us half of the picture, so to speak. Link to comment Share on other sites More sharing options...
Members guitarman967 Posted February 6, 2006 Members Share Posted February 6, 2006 The results will differ depending on the amp. If you have an amp that doesn't have alot of preamp gain then a bias setting of 40mA or above may work out well in an EL-34 based amp. I personally wouldn't go any higher tha 35mA in a modern high gainer for that particular tube. Keep in mind that These settings are all relative to the amp's Plate voltage. Each tube type will have a max dissapation that you will not want to come close to exceeding. So, 40mA may be O.K. for a El-34 tube but not for a 6L6. The terms hot and cold refer to how negative the bias setting is. If you wanted a colder setting you would adjust the bias to be more negative thus reducing the electron Flow. Hotter would be the opposite. As a general rule of thumb 70% of max dissapation is a good guideline to follow. Link to comment Share on other sites More sharing options...
Members bryank Posted February 6, 2006 Author Members Share Posted February 6, 2006 sorry, this is a Carvin legacy amp. the Plate voltage is 470. is 42 mA to high for el34s? Thats what i have them set to now. I dont know if should lower the bias to 37 or 39, but if i did so......what would happen to the sound of the amp? and what gives you more headroom.....a higher or lower bias setting? Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 6, 2006 Members Share Posted February 6, 2006 Originally posted by bryank sorry, this is a Carvin legacy amp. the Plate voltage is 470.is 42 mA to high for el34s? Thats what i have them set to now. I dont know if should lower the bias to 37 or 39, but if i did so......what would happen to the sound of the amp? The rule of thumb for the maximum safe bais current is 70% of max power dissipation. Most EL34's are rated at 25W each. So I=(P/V)*.7 or in your case, (25/470)*.7=37mA. You can go higher but it will shorten the life of your tubes. Lower is perfectly safe. and what gives you more headroom.....a higher or lower bias setting? Neither. You get the maximum perfomance from the amp when it is biased to the point where crossover distortion is minimized. This requires an oscilloscope and experienced eye to measure. Just use the equation above as a starting point and adjust it down from there by ear to find whatever sounds best. Link to comment Share on other sites More sharing options...
Members bryank Posted February 6, 2006 Author Members Share Posted February 6, 2006 the lower the bias, the more crossover distortion you get, right? and why would someone want to bias thier tubes hotter than normal (safe zone)? whats the benifit of doing that? Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 6, 2006 Members Share Posted February 6, 2006 Originally posted by bryank the lower the bias, the more crossover distortion you get, right? Lower bias current, yes. and why would someone want to bias thier tubes hotter than normal (safe zone)? whats the benifit of doing that? Some people think it sounds better. Purely a matter of preference AFAIK, I can think of no other good reason to bias it hotter than that. The two amps I've exerimented with I actually like set a tad colder than the "rule" (JCM 800 2204 and homemade SLO based amp) to get a tight crunchy tone, hotter gets a little to bold and loose for my taste. Link to comment Share on other sites More sharing options...
Members bryank Posted February 6, 2006 Author Members Share Posted February 6, 2006 by "bold and loose"...do you mean it gets to be on the bassy side? i noticed my amp to get a little flabby and bassy when i biased it at this setting (42 mA). im thinking of just setting it to a cooler 37 mA. Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 6, 2006 Members Share Posted February 6, 2006 Originally posted by bryank by "bold and loose"...do you mean it gets to be on the bassy side?i noticed my amp to get a little flabby and bassy when i biased it at this setting (42 mA). im thinking of just setting it to a cooler 37 mA. Ya, sounds like you're probably hearing the same thing I'm talking about. Link to comment Share on other sites More sharing options...
Members bryank Posted February 7, 2006 Author Members Share Posted February 7, 2006 heres a link of a photo of my volt meter........where do i set this at to get a reading of the bias?http://www.upload2.net/download2/9gYxaeQHJpMDCcB/4614504_o.jpg.html Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 7, 2006 Members Share Posted February 7, 2006 How did you bias your amp to 42mA if you don't even know how to set your meter??? Link to comment Share on other sites More sharing options...
Members guitarman967 Posted February 7, 2006 Members Share Posted February 7, 2006 Set the meter to DC voltage. Preferably auto ranging. I didn't look at your meter BTW. Unless you have an external bias I wouldn't mess with it. Link to comment Share on other sites More sharing options...
Members guitarman967 Posted February 7, 2006 Members Share Posted February 7, 2006 Originally posted by tommythelurker How did you bias your amp to 42mA if you don't even know how to set your meter??? Link to comment Share on other sites More sharing options...
Members bkkornaker Posted February 7, 2006 Members Share Posted February 7, 2006 i used the setting that closley matched what i thought it should be............ originaly when i put in my tubes......i just turned the dial untill i got a reading that showed numbers in the range of 20-50.....then just assumed that must be the right setting on the voltmeter cause it was getting numbers in that range for el34 tubes. is that so wrong? yes.......... thats why im double checking with you guys so, when i change my bias.....i use the right setting this time! Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 7, 2006 Members Share Posted February 7, 2006 Are you trying to confuse me by changing your name? It depends on how you're actually measuring the current. If your using the 1 ohm cathode resistor method (common with bias probes like the Bias Rite) then it should be set to DCV 200m. If you're reading the current directly you should be setting it to DCA 200m but this requires putting your meter in series which usually means desoldering stuff so you probaly don't want to do that. You can also read the voltage accross the OT primary, that would be DCV 20 or 200, but you need to measure the DC resistance of the OT also and use I=V/R to get the current. Link to comment Share on other sites More sharing options...
Members bkkornaker Posted February 7, 2006 Members Share Posted February 7, 2006 im using the bias probe method, and when i took a reading...i set it at the DCV 200 seeting on the voltmeter, and nothing showed............ so i set it at the DCA 200m setting, and got a reading of 42. so i fugured that must be the right setting on the volt meter..........but im assuming its not cause everyone says to set it at DCV. im lost now Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 7, 2006 Members Share Posted February 7, 2006 Originally posted by bkkornaker im using the bias probe method, and when i took a reading...i set it at the DCV 200 seeting on the voltmeter, and nothing showed............so i set it at the DCA 200m setting, and got a reading of 42. so i fugured that must be the right setting on the volt meter..........but im assuming its not cause everyone says to set it at DCV.im lost now DCA will get you kind of close, but not as accurate as you can be. What's happening when you set it to DCA is that the meter bypasses the resistor and current starts flowing through the meter instead of the resistor. The problem is some of the current will still flow through the resistor so the current you measure will be less than the actual current from the tube. Exactly how far off you are depends on the internal resistance of the meter but you could potentially be off by as much as 50% or more if it's a lousy meter. So what you should be doing is setting the meter to DCV (the 200m range would be the correct range). In this mode, no current will go through the meter, all the current goes through the resistor. What you actually measure is the voltage across the resistor. When you measure voltage you have to convert it to current using I=V/R, but the makers of the bias probe make this easy for you by using a 1 ohm resitor (R=1). So the mV reading on you meter is the same as the mA that you wanted to measure in the first place. Link to comment Share on other sites More sharing options...
Members bkkornaker Posted February 7, 2006 Members Share Posted February 7, 2006 oHHHHH! OK. so, even when its set to DCA, its the same as DCV. or at least close enough. I was getting worried about not getting a reading on DCV...............but if DCA is close enogh.....works for me! Link to comment Share on other sites More sharing options...
Members bkkornaker Posted February 7, 2006 Members Share Posted February 7, 2006 wait....i just read you post agin.....and if it could be off by 50% or more.......thats not good. Damn! Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 7, 2006 Members Share Posted February 7, 2006 Originally posted by bkkornaker wait....i just read you post agin.....and if it could be off by 50% or more.......thats not good. Damn! No, not good at all. Definitely do NOT use DCA. Kinda close is not close enough. Probably wasn't the best choice of words, what I meant was it could be close but you don't really know. Ideally the internal resistance when measuring DCA should be very small, but you're comparing this to a 1 ohm resister which itself is very small. Most circuits use resistors in the thousands of ohms range in which case the resistance of the meter isn't significant but this is a special case so you have to be careful. Bottom line, use DCV with the bias probe. Link to comment Share on other sites More sharing options...
Members bkkornaker Posted February 7, 2006 Members Share Posted February 7, 2006 OK, my amp is open, and the bias probe is in....i have the voltmeter set to DCV 200, and i get a reading......................of only 20 mA!!?? i adjust the bias pot and notice when i push the pot full left, i get a max reading of only 30 mA, and full right is about 15 mA. when i set the voltmeter to DCV 200m, i get no reading. but when i set ti to DCV 200, i get these readings........ now what? Link to comment Share on other sites More sharing options...
Members bryank Posted February 7, 2006 Author Members Share Posted February 7, 2006 for some reasonm im getting wacky readings on my voltmeter, and i have it set at DCV 200. when i push the bias trim pot far left, i get a reading of 30 mA, and far right i get a reading of 15mA. one thing i did notice, when i adjust the bias trim pot far to one side, the tubes were glowin hot, and red in the plates! and the meter was only reading like 30 mA! along with alot of white noise comming from the speakers. but when i adjusted the trim pot to the other side, the tubes were barley glowing at all. So i have the bias trim pot in the middle, and the plates are not glowing red, and it seems to have a nice blue glow in the plates...........even though the meter only reads 20 mA................. Link to comment Share on other sites More sharing options...
Members tommythelurker Posted February 7, 2006 Members Share Posted February 7, 2006 That doesn't sound right at all. If your plates are glowing you're putting more than 30 mA through the tubes. You should get a reading with the meter set to DCV 200m (not 200). Is there more than one place to plug the probe into the meter? Post a picture showing the whole meter or the make and model number of the meter. Link to comment Share on other sites More sharing options...
Members bryank Posted February 7, 2006 Author Members Share Posted February 7, 2006 thats what im sayin! its crazy! the inputs for the DMM are 3. one is a common (black), and the other 2 (red) are for 10A input, and another for V mA regular resistance/voltage etc readings. I was told not to use the 10A input. I tired using the setting of DCV 200m, but the reading on the DMM just shows the number 1 and doesnt change at all when i make bias adjustments. so i used the DCV200 setting. heres a link from ebay to see the pics of the DMM i bought.http://cgi.ebay.com/Digital-Multi-Meter-DT-830B-multimeter-VOLT-OHM_W0QQitemZ7588619860QQcategoryZ25412QQrdZ1QQcmdZViewItem Link to comment Share on other sites More sharing options...
Members bryank Posted February 7, 2006 Author Members Share Posted February 7, 2006 i also read that some bias probes mat OR may not have a built in 1 ohm resistor. how can i check to se if mine does or not? maybe thats why i get strange readings on th dmm? Link to comment Share on other sites More sharing options...
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