AndyKubicki Posted June 24, 2003 Share Posted June 24, 2003 <?xml:namespace prefix = v /><?xml:namespace prefix = o />Craig has been trying to get me to do a bias mod on my 222D for some time now, and I am FINALLY about to do it...but I can't help wonderering: What is the ultimate goal of setting the bias in a push pull output like the 222D using 7189s? Is it that there be no voltage difference across the plates of each set of tubes? Is it that the grids should have the same potential? The mod involves replacing a common 10 Ohm Cathode resistor with two (one per tube instead of one per two tubes) and measuring the voltage drop across each resistor and balancing the cathode potential. The way I see it, the plate voltage should be equal...but I'm probably wrong. <?xml:namespace prefix = v ns = "urn:schemas-microsoft-com:vml" /><?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /> Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 24, 2003 Share Posted June 24, 2003 Its not the Plate voltage that should be equal ! Its the current draw ! This is the easy way to set it without having to handle 400+ plate voltage. The plate current and cathode current is all in relation to one another . Your really not changing anything here your just making it so you can adjust and measure each tube rather than as a pair like it is now and then use some half a$$ed way to balance the current draw. Really Andy almost every amp with full bias control used this same design and has forever and ever !! Just do it !!! Craig PS, Remember you cut the bias setting on half after you do this so if you were setting it to .44V then you now set it to .22V Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 24, 2003 Author Share Posted June 24, 2003 For some reason, I thought that you don't want to have the plate voltage vary on the paired output tubes so as not to have that get to the output trannies and that the bias was one way of balancing this...told you I was probably wrong! BTW, I wasn't questioning this method as much as I am trying to understand what's going on. BTW II, I like the idea of this mod as it makes use of the existing switch and jacks...what do you think? http://home.netcarrier.com/%7Erstevens/biasmod.html Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 24, 2003 Share Posted June 24, 2003 I think its fine but if I remember right yours is not laid out like that ! Yours has 2 test point back by the tubes right ? So this setup will not work. You need to add 4 test points like I told you or just set it from under the chassis in that case all your doing is replacing 2 resistors and adding 2. No big deal. Once again just do it ! Craig Quote Link to comment Share on other sites More sharing options...
garymd Posted June 24, 2003 Share Posted June 24, 2003 BTW Craig, I'll be calling you for biasing instructions this weekend now that all my components are in place. I'll email you tomorrow so we can set a time. Thanks, Gary Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 24, 2003 Author Share Posted June 24, 2003 No...the test points will be used as grounds for the new resistors and the RCA plugs will be the test points while the switch selects the tube pair to be balanced and biased...that's what makes this mod so desirable... Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 25, 2003 Share Posted June 25, 2003 Andy, It is the same thing as I do just uses the factory existing setup no big deal if your amp has the RCA jacks then by all means use them. I think its more trouble them its worth but go for it. It accomplishes the same thing. Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 25, 2003 Share Posted June 25, 2003 Andy, I didn't think your amp had the RCA jacks and switches not all 299D and 222D's have them. Its only been about 5 months since I seen your amp there have a many amps thru here since then . I can't remember what yours had. Quote Link to comment Share on other sites More sharing options...
Marvel Posted June 25, 2003 Share Posted June 25, 2003 mdeneen is also saying, by way of inference, that each tube will also be slightly different. Add that to the error of the winding, makes being able to adjust each tube by itself is more desireable. Marvel Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 25, 2003 Author Share Posted June 25, 2003 Thanks guys! I did erroneously ask about biasing, but on the 222D there is also a balance pot and this is probably where my question lacked clarity. I had been setting the balance so as to get as close to a 0 reading across two plates with a volt meter, then I would adjust the bias current level itself, the assumption being that the balance control (in theory) is there to divide the bias equally between the two tubes, so apparently turning the balance pot one way or the other will put the plate voltages a bit out of balance...now (after this mod) I will be measuring the voltage drop across each tube's cathode resitor and balancing that. So I suppose my question is, is the objective of balancing and biasing more to get the grids equally negative rather than the plates? Or will this in fact lead to the plates being equal? Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 25, 2003 Share Posted June 25, 2003 No it is to equalize the current draw of the plates on the pair of tubes the negative voltage on pin 2 of your amp will be different for each tube. The problem with dealing with 400+ volts on the plates to do it 1) its dangerous 2) its not accurate 3) you have to go under the chassis to do it The amount of ground that is drawn thru the Cathode is in direct relation to the current drawn thru the plate. How much Negative voltage is required on pin 2 of your amp to make this equal varies from tube to tube. Craig Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 26, 2003 Author Share Posted June 26, 2003 The total current measured across this sensing resistor should be no greater than 46 mA. Thanks for the explanation Ryan. If you recall, you said I should balance by listeneng for the lowest hum level and adjusting the balance control pot, yet when I did that and measured the voltages at the grids, they were further off than when I started. It didn't seem right to me that each one had to be set to its extreme position to eliminate the hum. As to the above statement, I though current flows through a device and that voltage can be measured across a device...am I wrong? It just seems logical to me that this mod would allow a more accurate balancing and biasing process.... Quote Link to comment Share on other sites More sharing options...
Marvel Posted June 26, 2003 Share Posted June 26, 2003 Andy, You are correct. But knowing the voltage across the resistor (10 ohms), you can easily the current flow through the resistor, which will be the current flow through the rest. The current is the voltage divided by the resistor value. As an esily understood example: 10 volts across a 10 ohm resistor would be 1 amp. 10/10=1 (the resistor would be toast pretty quickly here) But you want to know a specific current, .046 amps (46 ma), so current x resistor value = voltage. .046 (amps) x 10 (ohms) = .46 volts You should be aiming for that voltage reading. Help me out guys, did I misplace a decimal point somewhere? Marvel Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 26, 2003 Share Posted June 26, 2003 Just to make this clear ! If you set the voltage drop across the 10 Ohm resistor to .46v which is accurate and then switch your meter to Ma it will be way off. There is no accurate way to measure across the 10 Ohm resistor with the Ma feature of a Multi Meter. You have to remove one end of the 10 Ohm resistor to use Ma and flow all the voltage thru the meter. Been there done it checked and confirmed it. The meter is not going to completely shunt the resistor so the measuement will be inaccurate using MA feature. Craig Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 26, 2003 Share Posted June 26, 2003 Yea okay my Simpson and my $300 Fluke are both out of wack !! No your information is wrong ! To properly use the MA feature of a meter the resistor would have to be desoldered and the meter put into the circuit. Oh and I use 1% vishay dale resistors very accurate ! I suppose Dynaco and Fisher and Pilot and on and on are all crazy when they use the voltage drop to bias there amps ? Scott never put out detailed specs to the amp owners because they intended the unit be seviced by competent tech that would know to convert the MA spec to voltage across the resistor to set the Bias this is the reason no Scott manual comes with Bias instructions. Just service bulletins that were sent to the service departments ! Craig Quote Link to comment Share on other sites More sharing options...
Marvel Posted June 26, 2003 Share Posted June 26, 2003 I would say that my Fluke also measures correctly when I measure the voltage drop across a resistor. Who in their right mind is going to unsolder the part to place your meter inline? Unless you have a switch to open the circuit with test points for your meter in between. I'll do the voltage drop thank you. If you read your manual for your meter, you would know not to try to measure current by placing it across a component. Marvel Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 26, 2003 Author Share Posted June 26, 2003 This is interesting....I didn't realize the ammeter acts as a shunt...if that's the case, what you say makes sense...and this should be easily verified with a voltage source, multimeter and resistor. I'll be working on this mod tonight and will experiment and see what readings I get...anyone else is welcome to post results! Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 26, 2003 Share Posted June 26, 2003 Ryan, No I am not missing the point and neither is marvel !! you are what I suggest is you yourself do a simple test. Go set the bias on your 222D with your method then desolder the 10 Ohm resistor and check with the meter in Ma the way it is designed to be done and you will see that the Bias will be high not horribly but it will be off. Then install a good 1% 10 ohm resistor and set the bias using voltage and then desolder and check it with Ma you will find it is almost exactly the same. I think this is the proper way to end this soon to end up flame war. I have done these tests and to me this proves your theory absolutley wrong and the most inaccurate way to set the bias out of the ones being discussed here. Craig Oh and I check the resistance of my Simpson and it is not 0 Ohm's when shunting the 10 ohm resistor its .7 Ohm and this is why it is not accurate. I tested this with both my simpson and my fluke neither is 0 ohms. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted June 26, 2003 Share Posted June 26, 2003 Ryan, As usual you refuse to learn anything and just keep chucking and jiving. Why do you refuse to open your brain cells and let some concrete information in are you so stubborn that you refuse to take 15 minutes of your time and do a simple test ?? Trust me no one knows everything you can learn too !! Craig PS : My Simpsons leads do not have nearly 1 Ohm of resistance !! Heck my 12' speaker cables don't have that much LOL !! Quote Link to comment Share on other sites More sharing options...
AndyKubicki Posted June 27, 2003 Author Share Posted June 27, 2003 OK! Bias mod completed safely (I think) and it is a breeze to balance and bias now...I love it! Everything above board and with existing connections and switches without burning fingers on hot output tubes! Now for the measurements. I bought 1% tolerance resistors and they measure 10 Ohms on the dot. When I balanced and biased using the volt meter, I biased to .23v (which should theoretically be 46 mA). When I set the meter to measure current, I got 20.8 mA...either the ammeter is not an actual shunt, or the volt meter is off and shows more voltage than exists. ....or...the resistors really aren't 10 Ohms AND my meter is off there too.... Quote Link to comment Share on other sites More sharing options...
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