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Old 9th May 2012, 07:38 PM   #11
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The cathode resistor is shared so, your numbers would be in half, yes... And that's assuming my resistor is actually 750 ohms. So that formula would then read: 46/1500=.0305 @ ~15mA per tube.

That is strange, the first resistor I used was 250 ohms and got over 100mA. It took awhile to get to the 750 value and figure out that was good. But that was with other tubes that I mentioned. These tubes must be what is causing this change...

But I did not know that formula for figuring it out, thanks.

To test current, the probes are on the B+ and the plate pin.

I will have to assume that the tubes are not a GC equivalent, so they'll be used for the next few projects.
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Old 9th May 2012, 07:40 PM   #12
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Before I try it, can I test screen current in the same way: B+ to screen pin? I've blown too many fuses in my meter to want to even test it... I'd rather someone just say yes or no.

Thanks
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Old 11th May 2012, 07:51 PM   #13
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by dscottguitars View Post
Before I try it, can I test screen current in the same way: B+ to screen pin? I've blown too many fuses in my meter to want to even test it... I'd rather someone just say yes or no.

Thanks
A meter on the right setting should take 750 volts (max for most meters).

But you cannot measure with a signal on tube,
since the swing could be 100 or more volts +- the supposed value.

One way to measure screen voltage (and current) is to
measure between the B+ and the screen pin.
There would / could be a voltage drop across the screen resistor (if there is one).

This will be much smaller than the total B+ powersupply.

Lets say the difference was 20 volts.

If you know the B+ (HT) is 500 volts, then the screen is 480 (or 520! check polarity).

This is safe for the meter but NOT FOR YOU: (!!!)

Remember that there is still 500 volts to ground on your meter clip/ prod, or lead.

You should be on a rubber mat,
wearing rubber soled running shoes,
and ALWAY HAVE ONE HAND IN POCKET
AWAY FROM ANY GROUND:

Its NOT safe for your left hand for instance to be touching a chassis, or leaning on a grounded bench or power supply, etc.

The multimeter leads should be shielded,
and you should be careful not to slip or short them to anything else like the chassis.

Also, when you look up to read meter,
and your eyes are away from the meter lead,
stay perfectly rigid and don't move.
Then look back to the circuit quickly.

Try to use leads that are mostly insulated, with no bare metal hanging out. Clip your leads on firmly if possible,
and don't just poke around with your meter lead:
that's how accidents happen.

Anytime you clip your meter to HV to take a reading,
hurry it up, then shut power off and remove the clip.

NEVER leave the clip of a meter on a HV connection,
because the whole meter (including the other lead) will be 'hot'!!!
Even after you shut off the amp.

Last edited by nazaroo; 11th May 2012 at 07:53 PM.
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Old 11th May 2012, 09:19 PM   #14
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Yes, for voltage I don't have the fuse problem, it was for testing current that I would blow fuses.

Thanks for the safety tips too.

So, do I test the screen current the same way as plate current?
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Old 12th May 2012, 05:07 AM   #15
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by dscottguitars View Post
Yes, for voltage I don't have the fuse problem, it was for testing current that I would blow fuses.

Thanks for the safety tips too.

So, do I test the screen current the same way as plate current?

To be clear, the only way you can test the B+ is to connect between it and ground.
You should do this on three locations:

(1) the powersupply positive cap (be careful!)

(2) the output transformer center tap.

(3) the Plate pin on the tube(s).

They should all be about the same voltage (to ground).


The screen voltage can be measured at the pin
when there is no signal on the amp,
either :

(1) from ground (in your case less than 750 volts).

(2) from the B+ (then subtract the difference).

Never forget the one hand in pocket rule until you have it as a fixed habit.
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Old 13th May 2012, 01:30 AM   #16
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Nazaroo: CURRENT!!

Not voltage. I know how to measure voltage. I was asking about current. Milliamps not volts.

To test the plate CURRENT I touch the B+ with one probe and the plate pin with the other.

Do I do the same with the screen CURRENT?
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Old 13th May 2012, 04:54 AM   #17
nazaroo is offline nazaroo  Canada
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I think I understand now.

For your information, most people don't ever attempt to measure current in High Voltage circuits directly.

You should instead measure the voltage-drop across a known resistor,
and calculate the current from Ohm's Law.


Furthermore, you should always do this from the LOW VOLTAGE end
of the circuit,
namely a resistor connected to ground,
or near ground, such as in the cathode of a tube circuit,
NEVER at the HIGH Voltage end, where death is immanent.

One of the main reasons for this is that the incredibly cheap and crappy insulation material on the leads of your multimeter
WILL NOT PROTECT YOU FROM HIGH VOLTAGE.
This 'insulating' material will instead conduct electricity and kill you.

So E.Engineers do this for safety reasons among other things.

I think I know what you are trying to do now,
and I should warn you immediately,
that most multi-meters are not designed to measure current in High Voltage circuits at all,
and to try to do it is foolish and an expensive way to wreck meters and burn money, as well as risk death.
(1) You shouldn't be trying to measure current with a multimeter by connecting it between B+ and plate.
This is an accident just waiting to happen.
(2) The nature of AC current is that it is much more slippery and esoteric than D.C. current.
It is always in the form of AVERAGE current, and this is a mathematical average, an APPROXIMATION, based on the idea of 'work done' or 'heat generated'.

Let me explain: We only say that '30 volts RMS (Root Mean Square) is equivalent to 30 volts D.C., because over relatively long time periods (far greater than the cycle-period) it will produce the same heat on average in a given fixed resistance as 30 volts DC would.
That is where the similarity STOPS.
Other than this convenient ad hoc heat calculation, there is nothing whatever similar between AC currents/voltages and DC currents/voltages.

Although your multimeter can measure AC current, it makes this same 'heat-equivalence' calculation, but for the purposes of tracking screen currents its just about useless, and also downright dangerous!

Here is why:

(1) If you are measuring the screen current during 'idle' (no musical signal), then this can be done by simply measuring the voltage-drop across the screen resistor.


But what is it you think you are measuring?
Its not A.C. current! Unless in idle conditions the tube is actually conducting electrons from the screen to the plate, there can be no true A.C. current. Instead, the only possibility is a pulsed D.C. current, but the most likely occurance will be a steady D.C. current from the cathode through the screen to the B+ either directly or through the Output transformer. The only A.C. will be a faint powersupply ripple from the B+ and the heater element.

(2) If you want to measure the screen current during the amplification of a musical signal, you are risking death!


Why? Because suppose you pump in a healthy sine-wave and it is amplified to 30 watts RMS. This huge voltage swing (say 250 volts) is dancing on the plate and screen, and it will ride on top of the already High Voltage B+ (say 500 volts).
Now not only will the voltage easily exceed the 750 volt maximum your meter can handle, but it will also blow through the 600 volt insulation on 99% of the wire used in amplifiers and in house-wiring.

(3) The multimeter will also lie to you about the voltage, and the risk
: It takes samples and displays an AVERAGE signal. Thus it could read "300 volts RMS" on your meter, while in fact, for short periods the actual voltage on your leads could be 1 Kilovolts, enough to easily kill you. The most important thing you must understand with High Voltage and A.C. currents is that your multimeter cannot and will not ever tell you the peak voltages or warn you of the danger lurking in your circuits.

Last edited by nazaroo; 13th May 2012 at 05:00 AM.
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Old 13th May 2012, 05:05 AM   #18
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by dscottguitars View Post
To test the plate CURRENT I touch the B+ with one probe and the plate pin with the other.
NO NO NO!

You should NEVER measure the current this way!
This is the most foolish thing you can do, especially
if there is a potential music signal or test signal in the amp.

Measure the Plate current during idle by measuring the voltage across the cathode resistor or load,
and calculate the current using Ohm's Law.

Never use the "current" function on your multimeter for High Voltage.


Quote:
Do I do the same with the screen CURRENT?
No NO NO NO NEVER!


If you are trying to measure screen current during idle,
measure the voltage across the screen resistor and calculate the current.

NEVER MEASURE IT DIRECTLY.

If you are trying to measure screen current during full amplification of a signal,
You have to CALCULATE it by subtracting the Plate current
from the Cathode current!
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