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FStephenMasek
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Post subject: Bias measuring, settings? Posted: Jun Fri 28, 2019 8:04 pm |
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Joined: Mar Sun 11, 2007 6:55 am Posts: 11054 Location: Mission Viejo, southern California
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How do you measure bias and how do you determine the desired voltages? In a device with a power transformer, from the tube pins to the chassis, or from the tube pins to the cathode of that tube?
_________________ many of my radios http://s269.photobucket.com/user/FSteph ... t=3&page=1
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JnTX
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Post subject: Re: Bias measuring, settings? Posted: Jun Fri 28, 2019 8:24 pm |
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Joined: Nov Tue 14, 2017 5:09 am Posts: 3289 Location: Austin, Texas
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When a bias voltage is specified for a tube, it is measured from the control grid to the cathode of the tube.
Jay
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FStephenMasek
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Post subject: Re: Bias measuring, settings? Posted: Jun Fri 28, 2019 9:01 pm |
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Joined: Mar Sun 11, 2007 6:55 am Posts: 11054 Location: Mission Viejo, southern California
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n3uvt
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Post subject: Re: Bias measuring, settings? Posted: Jun Fri 28, 2019 9:32 pm |
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Joined: Sep Tue 15, 2015 1:16 am Posts: 886 Location: 18424 PA
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Lou deGonzague
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Post subject: Re: Bias measuring, settings? Posted: Jun Fri 28, 2019 10:13 pm |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 9383 Location: Latham NY 12110
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Are you using self bias with a cathode resistor or fixed bias from a DC supply voltage?
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JnTX
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 2:35 am |
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Joined: Nov Tue 14, 2017 5:09 am Posts: 3289 Location: Austin, Texas
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Lou deGonzague wrote: Are you using self bias with a cathode resistor or fixed bias from a DC supply voltage? Either way, the bias is measured between the cathode and grid. Jay
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FStephenMasek
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 2:48 am |
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Joined: Mar Sun 11, 2007 6:55 am Posts: 11054 Location: Mission Viejo, southern California
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Thanks for the replies to what is surely a very basic question to many people!
I'm pondering my 6L6GC amplifier and measured the following today:
6L6GC grid to cathode -2.28VDC, with 409 VDC on the plates
_________________ many of my radios http://s269.photobucket.com/user/FSteph ... t=3&page=1
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JnTX
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 3:06 am |
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Joined: Nov Tue 14, 2017 5:09 am Posts: 3289 Location: Austin, Texas
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FStephenMasek wrote: Thanks for the replies to what is surely a very basic question to many people!
I'm pondering my 6L6GC amplifier and measured the following today:
6L6GC grid to cathode -2.28VDC, with 409 VDC on the plates That tube must be close to melting. You are supposed to have about -30 volts on the grid at that plate voltage. Jay
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FStephenMasek
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 3:56 am |
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Joined: Mar Sun 11, 2007 6:55 am Posts: 11054 Location: Mission Viejo, southern California
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Norm Leal
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 4:25 am |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 30750 Location: Livermore, CA
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Stephen
Plate to cathode should be much higher. Maybe a bad meter connection?
Have you replaced the coupling capacitor to grid #1, pin #5? How about grid resistor on this pin?
Cathode to ground shows there is excessive bias. May be caused by grid #1 being slightly positive from bad cap or resistor. Try another 6L6. Internal tube leakage could also cause this problem. Be sure cathode resistor , pin #8, isn't open.
_________________ Norm
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JnTX
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 5:03 am |
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Joined: Nov Tue 14, 2017 5:09 am Posts: 3289 Location: Austin, Texas
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FStephenMasek wrote: More measurements:
6L6GC Plate to cathode 11.69VDC plate to ground 409VDC grid to cathode -2.28VDC cathode to ground 39.1VDC Something is wrong with the measurements since the Plate to cathode plus the cathode to ground should equal the plate to ground. 11.69 plus 39.1 does not equal 409. Jay
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westcoastjohn
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 6:00 am |
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Joined: Dec Sat 24, 2011 9:17 pm Posts: 7185 Location: Vancouver Island, Canada
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This site describes ways to measure bias. https://sites.google.com/site/yourtubea ... nt-methodsI recall in self biased amp designs adding the 1 ohm cathode resistor and then measuring voltage across it. A well matched pair of output tubes will have near equal bias currents.
_________________ Watch the doughnut, not the hole. Burl Ives, RIP, oldtimer. [:l>)
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Marcc
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 3:39 pm |
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Joined: Feb Sun 01, 2009 2:56 pm Posts: 11756 Location: Victoria, Australia
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The bias of 6L6 does vary substantially according to class & situation. The screen is often run at lower volts than the plate. Control grid bias is always in respect of the cathode. The voltage across the self bias resistor is the grid bias. Where this varies (general) is where there is back bias and the cathode has no resistor, the voltage being developed by all of the cathode currents passing through the back bias resistor ct to chassis and the OP tube grid being on the centre tap. If the cathode resistor fails, that normally results in the destruction of any bypass electrolytic. It either goes short, explodes, or goes open. Open is indicated by all grids going substantially positive. Grid 1 resistors on many of those OP tubes 6V6 especially have a high attrition rate, normally on the high side. http://frank.yueksel.org/sheets/021/6/6L6.pdf This is a fairly informative sheet. To hopefully alleviate things? Marc
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Dutch Rabbit
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Post subject: Re: Bias measuring, settings? Posted: Jun Sat 29, 2019 6:14 pm |
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Joined: May Sun 13, 2012 8:12 pm Posts: 14849 Location: Central PA 16801
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when calculating the bias and static dissipation in watts, I use the cathode resistor method. the key is to use precision 1 ohm resistors. they can be a bit expensive for a little one ohm resistor, but well worth it in the end. I use 2 watt resistors b/c the leads are thicker and more sturdy for the probes. http://www.duncanamps.com/technical/lvbias.htmlhttps://www.mouser.com/ProductDetail/Vi ... 5VRW6K8%3Dthis method has got me extremely close when matching tubes and for the standard 6V/K/L/F6 hifi or pa amplifier, this worked just fine. steve
_________________ You have enemies ? Good. That means you've stood up for something, sometime in your life. -Churchill
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Marcc
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Post subject: Re: Bias measuring, settings? Posted: Jun Sun 30, 2019 12:02 am |
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Joined: Feb Sun 01, 2009 2:56 pm Posts: 11756 Location: Victoria, Australia
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Ohms law does apply to a cathode resistor as it does for the rest. The voltage across it will tell you the current through it. That is a method (where cathode resistors are fitted) which is where "Duncan's amps" article is going; A measurement of voltage across a cathode resistor, will actually give an indication as to a tube working, or not.
A diagnostic often missed.
Marc
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Dutch Rabbit
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Post subject: Re: Bias measuring, settings? Posted: Jun Sun 30, 2019 4:48 am |
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Joined: May Sun 13, 2012 8:12 pm Posts: 14849 Location: Central PA 16801
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i use it all the time. it also helps identify when a tube will begin to run away and draw too much current..
i've seen that too.
steve
_________________ You have enemies ? Good. That means you've stood up for something, sometime in your life. -Churchill
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FStephenMasek
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Post subject: Re: Bias measuring, settings? Posted: Jul Mon 01, 2019 12:18 am |
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Joined: Mar Sun 11, 2007 6:55 am Posts: 11054 Location: Mission Viejo, southern California
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n3uvt
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Post subject: Re: Bias measuring, settings? Posted: Jul Mon 01, 2019 12:51 am |
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Joined: Sep Tue 15, 2015 1:16 am Posts: 886 Location: 18424 PA
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FStephenMasek
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Post subject: Re: Bias measuring, settings? Posted: Jul Mon 01, 2019 1:03 am |
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Joined: Mar Sun 11, 2007 6:55 am Posts: 11054 Location: Mission Viejo, southern California
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Norm Leal
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Post subject: Re: Bias measuring, settings? Posted: Jul Mon 01, 2019 1:14 am |
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Joined: Jan Thu 01, 1970 1:00 am Posts: 30750 Location: Livermore, CA
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Stephen
Voltage drop across the 350 ohm resistor gives you bias. You have 39 volts and a 350 ohm resistor.
39/350 = 111 ma current being drawn by the two 6L6 tubes. Is this what you are looking for?
_________________ Norm
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