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I have been going over some documentation for some of the VTA amps.
I see bias in the instructions....rated in MV and located the test points. Easy....and I have one of those long plastic screwdrivers from a past life.
But I also see bias CURRENT referred to on some threads Bob Latino contributed to.....
So? What's it gonna be? MV or MA?
Too much is never enough
Follow Ups:
Take a look at item #6 in my FAQ post. That may help you to understand it.
I printed your list.
When I worked in semiconductor manufacture.....a 'wafer fab'......I had to know a LITTLE about test and probe to see effects of stuff I did in the fab and to see how any possible rework I did actually worked out in practice.
So I know that, at least for semiconductors, you have a BUNCH of measurables. But If I reworked certain steps, I knew to look at maybe Threshold Voltage or a breakdown voltage.....
And even look at the gross numbers of 'good' vs 'bad' and the failure modes of the bad devices.....
Too much is never enough
When measuring a voltage across a resistor, the voltage measured divided by the resistor value will tell you the current.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/30/21
Most tube Amp Designers who offer adjustable bias have done the calculations so that you the operator can measure voltage to adjust the bias current.
Given that the Resistor chosen is a fixed value - and often of sufficient quality that it will not drift appreciably, they can provide mV values that will effectively manage the bias current.
Happy Listening
Thanks for good answer......and it makes good sense so I won't worry.....just make sure my meter is on the right range....
Too much is never enough
Use the mV setting on your voltmeter.
You are effectively reading mA.
The amp test points have a resistor wired in series so that you get a 1:1 reading of mV=mA.
Now THAT makes good sense. Need to look at the schematic.....
Too much is never enough
First, please let me put something clear,
1. mV - millivolts
2. MV - megavolts
3. mA - milliampere
4. MA - megaampere
mV is not equal to MV
mA is not equal to MA
As for your question, please follow the recommended bias setting according to the amp's manufacturer and no one else, then you should be fine.
I don't need a spelling lesson.
I see references to Millivolts AND Milliamps when dealing with bias. I just want to know which is correct.
Most of the Latino docs call out a VOLTAGE while in some posts he talks CURRENT.
Is that better?
Too much is never enough
In most setups you measure a voltage across a resistor and then use ohms law to calculate the current.Setting the "bias" on output tubes means that you are adjust a negative voltage, connected to the grid of the tube, to control the idle current through the tube. Measuring the voltage across a small resistor connected from the cathode of the tube to ground is an easy way to determine how much idle current is flowing through the tube.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/30/21
I need to see a 'sample' sketch-matic so I can LOOK at what you are talkng about.....
than I'll expect no issue understanding and hopefully LEARN something.......
Too much is never enough
On the power supply schematic there are two 10K pots. These are the bias pots and control how much negative voltage there is at the grids of the output tubes. One is for the left channel output tubes and one is for the right channel output tubes. The more negative voltage there is at the grids of the output tubes, the less idle current will flow.On the circuit schematic the red arrows point to where the negative voltage from the bias pots go. The negative bias voltage goes to the bottom of the grid resistors. Because there is no current flowing through the grid resistors (or the grid) the negative bias voltage makes it's way through the grid resistors and the grid stop resistors (the 1K resistors) to the grid of the output tubes (pin 5) without being reduced in value.
The 15.6 ohm resistors underlined in blue are the cathode resistors for the output tubes. There is one cathode resistor for each pair of output tubes. When you setup the bias current on a stock dyna st70 power amplifier you use a volt meter and set the bias pots until you have 1.56 volts across the 15.6 ohm cathode resistor. The only reason that there is a voltage across those resistors is because there is current flowing through those resistors.
1.56 volts across 15.6 ohms is 100ma. of current. That would be 50ma. per tube of idle current.
Ohms law, voltage divided by resistance equals current.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/31/21 10/31/21 10/31/21
Nt
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