juniper8 Posted July 21, 2012 Share Posted July 21, 2012 Finally found a staco variac locally, to reduce my wall outlet voltges from 124.6 to 110, and tried biasing my pilot sa-260 for the first time, and the meter is jumping around a bit. What would be the acceptable range for this type of amp in millivolts as far as the meter jumping. My technician says I should bias at 900 mv opposed to 1volt like the sams photo fact reccomends, for tube life and other things (he speakes egyptian to me). Also how will biasing it lower affect the sound? Thanks any help will be appreciated. Quote Link to comment Share on other sites More sharing options...
Guest " " Posted July 21, 2012 Share Posted July 21, 2012 if you want to permanetly run gear at alower wall voltage, you should use a correctly rated ISOTAP which is a box with different outlets marked at different voltages. using a variac is problematic since the nature of their design (the brushes) can introduce noise. Quote Link to comment Share on other sites More sharing options...
juniper8 Posted July 21, 2012 Author Share Posted July 21, 2012 Thanks Speakerfritz, I will look into it... are there any brands you would reccomend? Are there specific brands made for audio? Or should I be looking for an industrial or medical grade unit? Quote Link to comment Share on other sites More sharing options...
Guest " " Posted July 21, 2012 Share Posted July 21, 2012 they pop up on ebay quite a bit. Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted July 22, 2012 Share Posted July 22, 2012 Finally found a staco variac locally, to reduce my wall outlet voltges from 124.6 to 110, and tried biasing my pilot sa-260 for the first time, and the meter is jumping around a bit. What would be the acceptable range for this type of amp in millivolts as far as the meter jumping. My technician says I should bias at 900 mv opposed to 1volt like the sams photo fact reccomends, for tube life and other things (he speakes egyptian to me). Also how will biasing it lower affect the sound? Thanks any help will be appreciated. If you go by the sams photo fact manual and the unit they used as a reference you should note the voltage readings were taken with the ac line voltage maintained at 117v. Take channel A as an example: Using the sams voltages as a reference when you bias at 1 volt the EL34 tubes are drawing 50ma each (ie: 1 volt/ 20 ohms of R19 and R23) and the plate dissipation is 21.2 watts for each EL34. Now if you adjust the bias instead for 900mv the current is 45ma ( 900mv/ 20 ohms) each tube and the plate dissipation will be 19.08 watts. The watts I've just quoted you is based on the plate B+ voltage (425v in this case) not changing as you lower the bias but it isn't uncommon for the plate voltage to rise slightly as you load the power supply less (ie: less current through the tubes) so you will still have reduced plate dissipation at 900mv but it might be slightly higher than I calculated in the real world. I also will often try a slightly lower bias on some amplifiers as your tech suggested and if I hear no difference (most common for me) or prefer any difference I do hear then the lower bias is where I will run the amp. You just don't want to under bias any amp to the point of current starvation which a 10 percent reduction in this case shouldn't be any problem. As far as the meter jumping around give us an example? Just in case you don't know there should be no signals (ie: music) running through the amplifier when your making bias adjustments. The bias readings should become pretty steady after 30min to 1hr but they will change with any AC Line Voltage variations from the power company. Have the electrolytic capacitors in the amplifier's power supply been replaced especially in the negative bias supply and the old selenium rectifiers? If not considering the age of the amplifier it might be something to consider if you plan on keeping the amplifier long term. miketn Quote Link to comment Share on other sites More sharing options...
juniper8 Posted July 22, 2012 Author Share Posted July 22, 2012 Thanks Miketn didn't know it should be biased with the variac set at 117v so this evening I will redo the procedure. As far as the fluctuations, the multimeter was moving up and down about 5mv. As far as the rebuild all electrolytics in the unit were replaced in the bias, rectifiers and power supply. Thanks Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted July 22, 2012 Share Posted July 22, 2012 As far as the fluctuations, the multimeter was moving up and down about 5mv. As far as the rebuild all electrolytics in the unit were replaced in the bias, rectifiers and power supply. Thanks 5mv variations out of a 900mv set point is insignificant and most likely reflects noise, small ac line variations and even your meter's resolution coming into play. Sounds like your good to go juniper8 By the way if you or anyone uses an isotap type variac always make sure you actually measure it's Output Voltage and not go by any written values on these units because those values are based on a certain Input AC Voltage (ie: with 120v ac input for example) and if the AC Input Voltage from the power company runs higher or lower this will be reflected in the different Tap settings output voltages also. miketn Quote Link to comment Share on other sites More sharing options...
juniper8 Posted July 24, 2012 Author Share Posted July 24, 2012 Thanks Miketn, Speakerfritz and Tubefanatic for all of your help. I was able get readings of 899 and 900 on A/B channel and 00.5 and 01.2 for balance. Thanks gentlemen!!!! Quote Link to comment Share on other sites More sharing options...
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