Hi again
New question.
Without buying or building a bias cureent checker from tube sockets, How
does one measure bias current?

Thanks
Mike M

in article et, mike m at
wrote on 5/22/05 9:21 AM:

Hi again
New question.
Without buying or building a bias cureent checker from tube sockets, How
does one measure bias current?

Thanks
Mike M


For fixed bias amps, install a 10 ohm 1/2 watt resistor between the cathode
of each output tube and ground.

Measure the voltage across the resistor. E.g. 0.3 volts corresponds to 30
ma by ohm's law.

The 1/2 watt resistor becomes a safety fuse should the tube draw too much
current, preserving tube and tranny.

Jon

Jon Yaeger wrote:

in article et, mike m at
wrote on 5/22/05 9:21 AM:

Hi again
New question.
Without buying or building a bias cureent checker from tube sockets, How
does one measure bias current?

Thanks
Mike M

For fixed bias amps, install a 10 ohm 1/2 watt resistor between the cathode
of each output tube and ground.

Yes, the measurement across a cathode R is the simplest way to measure cathode
current.
a separate measurement should be taken across the screen resistors at idle
to find the screen current, which should be within maker specs, usually
from 5% to 10% of the cathode current.

In UL amps the sure way to measure the Ia of each tube is to measure the
DC resistance of the primary OPT winding between each UL tap and its
nearby anode connection with the amp cold and turned off.
Each side of the OPT will have slightly different DC resistances.
When the amp has warmed, measure the voltage across the winding sections above
and I = E / R.
Ia for each tube should be within 3% of being equal.
Screen currents should also be within 15% of each other.

A 10 ohm 1/2 watt R allows only 0.225 amps to flow before it
reaches its rated hot limit, and perhaps it does not go open until
a watt is reached which is at 0.316 amps, which may be 10 times the
idle current of a tube.


Measure the voltage across the resistor. E.g. 0.3 volts corresponds to 30
ma by ohm's law.

The 1/2 watt resistor becomes a safety fuse should the tube draw too much
current, preserving tube and tranny.

But a saturated tube with some fault could sit there with 316 mA
for some time before the 1/2 watt 10 ohm R decided to blow,
and with 225mA, the tube dissipation could be 500v x 0.225 = 112.5 watts
which would wreck the tube before anything fuse or resistor blew,
although perhaps the OPT winding would get hot.
If the cause of the overheating tube was a loose socket gripper on a grid pin,
or a stuffed coupling cap, some other protection would seem to be required,
lest we unnecessarily let a tube blow up for no justafiable reason
when it may be some surrounding component or fault causing excessive
Ik, such a shorted speaker wire or failed coupling cap.


I usually place wire wound 5 watt 10ohm R in cathode circuits to save
nuisance blowings of such R, since I don't think these R should have a role
in offering protection.
But then I use active over current protection circuitry
in the event of a saturated tube which tries to
exceed the idle bias by more than twice for longer than 4 seconds.
So if the tubes are set up for 50mA x 500v, or 25 watts dissipation,
its rare that with music the DC input current will double in a class AB1 amp.
The tubes will easily handle 50 watts dissipation for a short time.
This would happen if only 100 mA flowed with Ea = 500v.

The 50 watt dissipation in 6550/KT88/EL34/6L6 etc caused by music signals
rarely lasts for more than a split second.
Its usually a DC fault condition that will make Pd rise by 100%,
and so I say 4 seconds is the time to allow the protect circuit to decide if it
is DC overload,
and turn off the B+ supply.

A couple of signal bjts and an SCR and R&C bits does the job for protection.

Patrick Turner.



Jon

"mike m" wrote in message
ink.net...
Hi again
New question.
Without buying or building a bias cureent checker from tube sockets, How
does one measure bias current?

Thanks
Mike M

The cathode resistor method seems to be the one most commonly mentioned, but
there is an other method I see mentioned in old radio books, that I have
never seen mentioned on the net or in audio books, so I thought I would
mention it here. It is the method I use now.

It is to simply put a dc milli-ammeter across the output transformers
primary, and read the plate current directly. The meter is of such low
resistance relative to a transformer primary it shorts out the transformer
winding, and draws virtually all the current. Obviously this is done with
zero signal.

When I first read of this, I thought "that wont work, the bias point will
change since the plate circuit resistance is changed". But the difference
in plate voltage will only be a few volts for the size of amps I fool around
with, say 25mA and 150 ohms so 3.75V, and with a pentode a few plate volts
on top of a couple of hundred make very little difference. A bigger amp with
50 or 50mA bias and a few hundred ohms of OPT winding may be up in the 15 or
20V range. Still not much difference when you look at plate curves, 20V is
just a blip on the horizontal scale.

The same answer applies to the objection that the sides of a PP transformer
have different winding resistances.

Anyway, I have compared the readings done this way, to actually cutting the
leads and inserting an ammeter, and the method works fine.

There is a safety issue, in that you need to be aware that you are poking
around at the B+ side of things, and not the nice safe low voltage of the
cathode, but this applies to much of tube work.

I dont know how it would work with UL connection.

The advantages of this method, in my view, are
1) no mods required for existing amps that do not use a cathode resistor.
2) in the case of pentode amps, you actually measure the plate current, and
not the cathode current which is plate + screen. It is the plate current you
want to have balanced, since that is what is affecting the transformer core.

Robert McLean wrote:

"mike m" wrote in message
ink.net...
Hi again
New question.
Without buying or building a bias cureent checker from tube sockets, How
does one measure bias current?

Thanks
Mike M

The cathode resistor method seems to be the one most commonly mentioned, but
there is an other method I see mentioned in old radio books, that I have
never seen mentioned on the net or in audio books, so I thought I would
mention it here. It is the method I use now.

It is to simply put a dc milli-ammeter across the output transformers
primary, and read the plate current directly. The meter is of such low
resistance relative to a transformer primary it shorts out the transformer
winding, and draws virtually all the current. Obviously this is done with
zero signal.

When I first read of this, I thought "that wont work, the bias point will
change since the plate circuit resistance is changed". But the difference
in plate voltage will only be a few volts for the size of amps I fool around
with, say 25mA and 150 ohms so 3.75V, and with a pentode a few plate volts
on top of a couple of hundred make very little difference. A bigger amp with
50 or 50mA bias and a few hundred ohms of OPT winding may be up in the 15 or
20V range. Still not much difference when you look at plate curves, 20V is
just a blip on the horizontal scale.

The same answer applies to the objection that the sides of a PP transformer
have different winding resistances.

Anyway, I have compared the readings done this way, to actually cutting the
leads and inserting an ammeter, and the method works fine.

There is a safety issue, in that you need to be aware that you are poking
around at the B+ side of things, and not the nice safe low voltage of the
cathode, but this applies to much of tube work.

I dont know how it would work with UL connection.

The advantages of this method, in my view, are
1) no mods required for existing amps that do not use a cathode resistor.
2) in the case of pentode amps, you actually measure the plate current, and
not the cathode current which is plate + screen. It is the plate current you
want to have balanced, since that is what is affecting the transformer core.

There isn't anything wrong with your method here.
I rarely think of using ammeters and maybe its because i zapped couple voltage
sources when i forgot to
change the leads on the meter for voltage measurements.
We get addicted to measuring voltages rather than currents in tube circuits.

But one can measure current anywhere if its safe to shunt a highish resistance
element.
So UL circuits could be measured across the 470 ohm G2 stopper R,
or across the 43% of turns in the primary which share the screen and plate
current,
then measure the anode current, and subtract total Ia from Ia + G2a, and you get
G2a.

I must try my Fluke meter's current ability on a few things.

Patrick Turner.