6L6 current-voltage problem
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 9th May 2012, 06:38 PM #11 dscottguitars   diyAudio Member   Join Date: Mar 2011 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.
 9th May 2012, 06:40 PM #12 dscottguitars   diyAudio Member   Join Date: Mar 2011 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
nazaroo
Banned

Join Date: Feb 2012
Quote:
 Originally Posted by dscottguitars 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,
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,
that's how accidents happen.

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 06:53 PM.

 11th May 2012, 08:19 PM #14 dscottguitars   diyAudio Member   Join Date: Mar 2011 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?
nazaroo
Banned

Join Date: Feb 2012
Quote:
 Originally Posted by dscottguitars 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.

 13th May 2012, 12:30 AM #16 dscottguitars   diyAudio Member   Join Date: Mar 2011 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?
nazaroo
Banned

Join Date: Feb 2012
Quote:
 Originally Posted by dscottguitars 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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