I want to add a meter to be able to check the bias on my amp ( yes I know its not adjustable). I bought a pair of small 100ma meters. Is the attached drawing the right way to connect them? I will use a momentary make before break switch to check the bias. The meter will not be in the circuit except when checking it.
Attachments
Do not do it that way.
When you change the position of the momentary switch, the tweeter is going to go ping!
And the woofer may go bang!
And the output transformer may go fizz.
And the 300B might go zap.
Instead:
You have a 1k 300B self bias resistor.
Connect the bottom of the 1k resistor to ground (no switch, no ugly transients)
There will be 1V/mA across that resistor (for example, 60mA = 60V).
Use a 55milli-Ohm 0-1 mA current meter, and put a 100k Ohm resistor in series with the meter.
Then, connect the combination of the series resistor and series meter across the 1k Ohm resistor.
The series 100k and 1mA meter creates a "0-100V meter".
How to read the current:
If the meter reads 1/2 of full scale, the 300B current is 50mA + 0.5mA = 50.5 mA.
If the meter reads 7/10 of full scale, the 300B current is 70mA + 0.7 mA = 70.7 mA.
(The extra current, is the small current through the meter )
Done!
It took me a while to edit.
Chris has the better solution, you already have the meter movement (0-100mA I hope).
If you ever do a stereo amp, you can use a L / R switch, a single 100k, a single 0-1mA meter, and never hear a transient when switching L / R, the meter current is so low.
When you change the position of the momentary switch, the tweeter is going to go ping!
And the woofer may go bang!
And the output transformer may go fizz.
And the 300B might go zap.
Instead:
You have a 1k 300B self bias resistor.
Connect the bottom of the 1k resistor to ground (no switch, no ugly transients)
There will be 1V/mA across that resistor (for example, 60mA = 60V).
Use a 55milli-Ohm 0-1 mA current meter, and put a 100k Ohm resistor in series with the meter.
Then, connect the combination of the series resistor and series meter across the 1k Ohm resistor.
The series 100k and 1mA meter creates a "0-100V meter".
How to read the current:
If the meter reads 1/2 of full scale, the 300B current is 50mA + 0.5mA = 50.5 mA.
If the meter reads 7/10 of full scale, the 300B current is 70mA + 0.7 mA = 70.7 mA.
(The extra current, is the small current through the meter )
Done!
It took me a while to edit.
Chris has the better solution, you already have the meter movement (0-100mA I hope).
If you ever do a stereo amp, you can use a L / R switch, a single 100k, a single 0-1mA meter, and never hear a transient when switching L / R, the meter current is so low.
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So I can really just leave them in circuit? Even though its in the signal path?
won;t they bounce with the music?
And yes I already have the meters. Nice looking little round retro looking ones.
won;t they bounce with the music?
And yes I already have the meters. Nice looking little round retro looking ones.
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jwags818,
You are correct. The changing Class A signal current will cause the meter to vibrate.
Depends on the meter mass, springs, ballistics, etc.
If you use the 0-100mA meters, then connect the bottom end of the bypass cap Directly to ground. That way, the bypass cap will bypass almost all of the changing current.
Done.
Correction of my Post # 3, anybody using the 100k and 0-1mA meter . . .
Both the 1k and the bypass cap need to be connected directly to ground (again, let the bypass cap take the changing current, not the poor little meter movement).
You are correct. The changing Class A signal current will cause the meter to vibrate.
Depends on the meter mass, springs, ballistics, etc.
If you use the 0-100mA meters, then connect the bottom end of the bypass cap Directly to ground. That way, the bypass cap will bypass almost all of the changing current.
Done.
Correction of my Post # 3, anybody using the 100k and 0-1mA meter . . .
Both the 1k and the bypass cap need to be connected directly to ground (again, let the bypass cap take the changing current, not the poor little meter movement).
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The safest way is to permanently ground the resistor and capacitor. Then install a voltmeter in parallel. Since your resistor is 1k, your voltmeter will read milliamps (1V=1mA). You can leave the voltmeter connected permanently or switch it on and off at any time.
The meters do not bounce around with signal. There's a big capacitor across the cathode resistor bypassing signal around it. Besides, meters don't move that fast.
Really, this is the way folks have be doing it forever. No drama.
YOS,
Chris
Really, this is the way folks have be doing it forever. No drama.
YOS,
Chris
I have actually seen a "blurred" meter needle.
Blur is vibration.
Depends on the meter ballistics; resonance of the needle, and frequency of the music note.
Not all meters are created equal.
Blur is vibration.
Depends on the meter ballistics; resonance of the needle, and frequency of the music note.
Not all meters are created equal.
If the series'd cathode bias resistor and meter are bypassed by a suitable capacitor, there's no signal there. The meter will read an integrated cathode voltage that doesn't vary much in class A.
I've done it lots, other folks have done it lots. There's no drama.
YOS,
Chris
I've done it lots, other folks have done it lots. There's no drama.
YOS,
Chris
Must say though, that the post #1 schematic doesn't have the bypass cap to ground. As 6A3sUMMER has posted, signal up to a few Hz is visible, so don't do that - connect bypass cap to signal ground.
YOS,
Chris
YOS,
Chris