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Old 18th December 2004, 04:11 PM   #1
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Default bias meter

hello again,
got me a bias meter that goes "inline" between octal types to measure the cathode current.

my question is, i measure the voltages with only the rectifier.
this shows to be ~530 volts on the plates of my 6l6GC!
i realize that the plate voltages will drop when the tubes are installed. is there a formula to calculate the "actual" plate voltage while the tubes are installed, or should i just install the meter(fluke 73) in series with the plate?
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Old 19th December 2004, 06:45 AM   #2
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Default Re: bias meter

Quote:
Originally posted by el capitan83
is there a formula to calculate the "actual" plate voltage while the tubes are installed
The amount the B+ will fall when the valves are conducting depends on lots of things like the type of power supply (choke-input power supplies will rise rather dramatically as they revert to cap-input when the current draw is too small), and the regulation of the power supply itself.

Anyway, it isn't really a good idea to fire up the power supply without the valves installed. You could have had the electrolytics explode!

Quote:
Originally posted by el capitan83
or should i just install the meter(fluke 73) in series with the plate?
The Fluke 73 is a DMM, so it can measure either voltage or current. Install the valves, and measure the plate voltage then.

When you measure voltage, put the DMM into VDC mode, place the black lead to ground, the red lead to the voltage you want to measure (you measure voltages by putting the meter in parallel, not series). But of course you knew that
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Old 19th December 2004, 07:14 AM   #3
nanana is offline nanana  Sweden
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hi,
i'll try to help you but we have to clear up some things first...
whatever this "bias meter" is, it might be useful to have an idea of what it will be measuring and what good that will do you. if the output tubes are 6L6's, then we know it is a *multi-grid* tube. that means there will be more than one path for the current to flow into, providing the tube is connected as a beam tetrode and not a "strapped" triode. there will be a path from cathode to plate (plate current) and there will be a path from the cathode to the screen grid (screen current). if the tube is "self biased", or "cathode biased" as it is also known, then there will be a resistor (a few hundred ohms is common) between the common negative power supply return (ground) and the tube('s) cathode. if the tube is "fixed biased", there may be no resistor at all or else a very small resistor (typically 1 to 20 ohms) in the same place. a look at the control grid (grid #1) will reveal if the amp is self or fixed biased: if the grid resistor shunts to ground (a reference of 0 volts), its obviously self biased; and if it returns to a negative supply, its "fixed" (although fixed is almost always variable... confusing right!). anyway, if either of these situations is encountered, it is very unlikely that there will be a path of current from the control grid to the plate, so you won't have to worry about that! so, if you were to look at the cathode again, you would find that to measure the current moving through the tube you would see the sum of the plate and screen currents together. this could be measured by calculating the currents with ohms law and the voltage drop across the cathode resistor, or by inserting an ammeter in series with the cathode and reading it directly. OK, thats interesting. but lets say that you were only interested in the plate current because you were trying to balance the two sides of a push-pull class AB output stage for maximum watts at low frequencies and maximum common mode rejection (probably for best AC operated heater hum rejection...). well you wouldn't have much of a clue what that was from the cathode now would you? that would be because some of the current at the cathode goes to the plate, and some to the screen. but how much of each? another problem is that as the screen voltage is raised, the current drawn by it increases and progressively changes the plate characteristics, as in the case of most guitar amps that use the plate supply voltage to feed the screen. the variations between tubes will be exaggerated more and more. the plate currents might be very close but the screen currents very different or (much more usually the case), vice versa. no, the only way to measure the plate current is at the plate... and then the screen current can be disregarded (as it almost always is). it is a very common mistake to measure the plate current of a multigrid tube at the cathode... nearly ALL technicians and DIYers do it. this can only be done with a triode or triode connected multi-grid tube. in that case, the cathode and plate are one path only. to measure plate current in your 6L6 amp, you can take your fluke multimeter and turn it to the 200milliamp setting and connect the postive probe to the output transformer center tap (or mains B+ if single ended) and then put the negative probe on the plate (shorting out the transformer primary). this will give you a much better measurement of the plate current. you can alternately check each plate in a push pull amp this way and adjust the bias balance to balance the two halves. your inline bias meter will be worthless for 6L6 tubes (6V6, KT-66, EL-34, EL-84, 6550, KT-88, etc.) if it only measures current at the cathode and the screen grids are not tied to the plate. now some will probably say that "at idle, the screen current is too small (typically 5% - 10%) of the plate current and so its close enough". this is probably true enough for conservatively designed older stuff. modern hifi and guitar amps and any class B design will definitely not fall into this category. maximum rated screen voltage for a 6L6GC is 400 volts (450 absolute max). the screen current drawn at 450 will be much much higher than at 350 volts. you could have a pair of really well matched tubes that will not appear to be matched at all at the cathode... and yet, at the plate they could be pretty close. anyway, i have gone on long enough... hope this helps.
jc
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Old 19th December 2004, 07:28 AM   #4
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jc, that was a monster of a post 813 words!

elcapitan83, re-reading yours and jc's posts, I was wondering what exactly you were asking...

You have some sort of ammeter that is designed to measure cathode current, yes? Why octals specifically? Anyway, as said by jc, unless the 6L6s are in triode mode, plate current cannot be measured this way.

Are you asking about cathode current measurement or your high B+ voltage?
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Old 20th December 2004, 06:03 AM   #5
Enzo is offline Enzo  United States
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El, if your bias meter is like the ones I know, it simply adds a 1 ohm resistor in series with the cathode and takes a voltage reading across it. This results in the voltage in millivolts equalling the current in milliamps. Thus you read the current through the tube. Granted there will typically be a couple milliamps of screen current included in the total, so if you get say 38ma reading, you can assume the plate current is maybe 36ma or thereabouts. Close enough for rock and roll. There are ways to read just the plate, but you won't be far off this way.

DOn't run your amp without the tubes. It won't hurt the tubes to be on a minute with incorrect bias. Set the bias to the coldest end of the scale to start if you are concerned about that. As you increase the current, your plate voltage may well drop. Get the amp in the ball park, then recalculate your target current and fine tune the adjustment.
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