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Just got a used Music Reference RM-10 tube amp direct from the manufacturer, Roger Modjeski. Hard to believe this is the third tube amp I've ever owned, but the first one I've ever had to bias (the others were self bias or auto bias). Actually, I don't have to bias it as Roger was kind enough to set the bias before he shipped it. Nonetheless, I want to everything I can about the amp and be able to keep an eye on the bias periodically. I went and got a Radioshack digital voltmeter. Roger indicated that he set the bias to 280 mV or 28 mA. When I took the readings, I got the 28 mA, but the millivolt readings were 30 to 40 mV higher, and read approx. 315 mV on each side. The question are: is the discrepancy in readings normal? Is the voltmeter inaccurate? Which is the most reliable readings to take, mVs or mAs? Any guidance in this matter is appreciated.I'm sure there has been enough said about Music Reference amps, but if anyone cares to know, this amp is fabulous. Too premature to say definitively, but it may be the best amp I've ever owned (out of tube and solid state). Anyway, thanks again.
Follow Ups:
you gotta find out if you're supposed to measure millivolts and TRANSLATE millivolts into milliamps, or whether you've been given true milliamp reading points.If you're measuring across a 1 ohm resistor, then one millivolt of voltage drop (difference, end to end) indicates or translates to one milliamp of current, because E=IR; if the current is 280 milliamps, then 280 millivolts (measured across 1 ohm resistor) times 1 ohm = 280 milliamps.
If you're reading across a 1 ohm resistor, then a millamp READING is meaningless; you're measuring a shunt current and you have no way of knowing what it represents.
Now, if the two measuring points are really giving you a current measuring link - and it's a little hard to know how that would be done unless there's a switch that puts your millammeter in series with the current, although a shunt arrangement could be set up - but in that case you're not actually measuring the current through the cathode, you're measuring a shunt current that corresponds to the cathode current in some predetermined ratio - all in all, not bloody likely - but if you are, then the millivolt reading is of no particular meaning.
My guess is that you are supposed to be measuring millivolts, and you're reading a voltage difference across a 1 ohm resistor, and thus whatever reading you get in millivolts is the same number as the cathode current in milliamps.
So, without knowing whether you're supposed to be measuring millivolts or milliamps in the first place, or without knowing the schematic, it's pretty close to impossible to figure your question out.
P.S.: I was also assuming you're measuring individual EL84 cathode current; if these are in push-pull pairs, and each pair shares a single cathode resistor, then the current you determine is actually double the current per tube.........
not enough data.
best, charles
"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.
On the RM10 if I am not mistaken, you have two holes per channel where you insert the leads from your volt meter, right? You can only take one type of reading from those test points. If Roger has it set up to take a mA reading, mA readings is all you are going to be able read from those test points. So your job just got easier, check the bias using the mA setting on your meter and all is well.I had that amp for a period of time, I got to tell you if you have the right speakers that EL84 tube is one of the best kept secrets in Audio, plus the tubes are cheap and you only have four in your amp so you can have some fun tube rolling. Good luck with it, that amp will out live you, Roger makes some of the most reliable and durable amps, solid state or tube..........Bob
To do so you'll have to break the loop and get in series. So unless you were given a parallel tap to measure on, you would have to measure Mv. To do so usually you would measure voltage drop(Mv) across both sides of a resistor. A simple formula (Ohm's law, I=E/R) will give the current reading.
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