No Fluke, only cheapos from Harbor Freight and Jameco. Yes, .62 VOLTS across the 10 ohm resistor in the TubelabSE is 62 mA.
It is far easier to build a resistor into the amp, then measure the voltage across it, than to break a circuit and insert a meter in series with the circuit. The resistor can be left there all the time and the meter clipped in when needed.
Most analog panel meters do indeed measure current. And most are 1 mA for full scale, although other sizes are fairly common. The usual course of action is to use a "shunt" which is a big word for a resistor in parallel with the meter. The shunt bypasses most of the current around the meter. The meter with the shunt is then inserted in series with the circuit. Some meters have the shunt built into them.
To wire an analog current meter into a TubelabSE (for permanent use), simply connect it IN PLACE of the 10 ohm resistor. Remove the resistors from the PC board, and run wires from the PC board over to the meter. Keep in mind that the most small meters are only 5 to 10% accurate, so they may be off a bit.
The screw on the front of the meter is supposed to be used to set the pointer to zero with no electricity applied to the meter. That leaves no easy means to calibrate the meter. The easy way is to connect the meter in series with a digital current meter and a small resistor. Connect all of this to a power supply and adjust the power supply so that the digital meter reads the same bias current that you intend to operate the power tubes at. Then adjust the screw on the meter until it reads the correct current.
This is a fact of life with any unregulated power supply. No power supply is perfect, and all have some "internal resistance", especially with a tube rectifier. As the current goes up, the voltage will go down. The quality of the power transformer also has something to do with this, and Hammonds tend to fluctuate more than others, although the 272JX is better than most. The little (Hammond made) Allied that I use in 45 amps will make 320 volts when running 45's but only 290 with 300B's (operating above the ratings).
Thank you SY and leadbelly. I measured the voltage across 10 Ohm resistor and my previous setting was off by little.
Thank you Tubelab. I measured the voltage across the resistor with digital multimeter first, then removed the resistor and put the analogue ammeter leads in its place. The reading was identical to multimeter. I guess it’s safe to assume that this $9.50 ammeter is ok.
tubelab.com said:
Thank you Tubelab. I measured the voltage across the resistor with digital multimeter first, then removed the resistor and put the analogue ammeter leads in its place. The reading was identical to multimeter. I guess it’s safe to assume that this $9.50 ammeter is ok.
In fact, one of the first recommendations for the Heath IG-18 Signal Generator was to isolate the meter from the amplifier circuitry -- I guess I am old as dirt since I remember this -- the meter coil and mechanical movement would introduce a back EMF into the circuit. It's pretty easy these days to get current measuring diff amps which will work around high voltages -- Linear has a whole series of them.
What you really want is one or two of these babies -- you can set the min-max which operates a relay on the meter.
What you really want is one or two of these babies -- you can set the min-max which operates a relay on the meter.
An externally hosted image should be here but it was not working when we last tested it.
panal meter problem
hello im building a tubelab se 300b amp choked and i added panal meters for biasing on the fly but when i hooked them up my meters read 100-105 ma but when i measured across shunt i get 50-55 ma at that range my b+ drops dangerously low 34v to spare i usualy bias 67ma is there a way to correct this if so how . oh another weird thing when i take my mulitimeter and measure accross shunt the meters read even with each other around 53 ma
my meters are the so-45 pictures earlier in post $10 euphoria
hello im building a tubelab se 300b amp choked and i added panal meters for biasing on the fly but when i hooked them up my meters read 100-105 ma but when i measured across shunt i get 50-55 ma at that range my b+ drops dangerously low 34v to spare i usualy bias 67ma is there a way to correct this if so how . oh another weird thing when i take my mulitimeter and measure accross shunt the meters read even with each other around 53 ma
my meters are the so-45 pictures earlier in post $10 euphoria
ok problem before i took out resisters 29 and 18 i set bias at 69 miliamps per channal with muilimeter after i took them out and hooked up my panal meters they read 98ma and 30ma at were the resistors use to be so i took out output tubes and measured the to smaller pins on the 300b socket and got 460v i think its suppose to be around 390 does anyone know if ajusting bias affects that voltage i think its the plate voltage if so maybe thats how i should bias it out i dont know
Thinking that rather than putting the meter on the cathode side where any non linearity in the DCR of the measurement coil will stuff around the bias, it's better to put it on the anode side where its minimal impedance is in series with half of an output tranny.
Or am I reinventing the wheel (or not noticing some other obvious problem)?
Or am I reinventing the wheel (or not noticing some other obvious problem)?
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