CATHODE CURRENT MONITORING PROBE
The
Bias Probe output tube current monitor/bias sensor enables the technician or
musician to directly monitor the DC current flowing through an output tube,
allowing the correct adjustment of the output tube bias without disassembly of
the amplifier..
The
Bias Probe will work with many common output tube varieties using the standard
8 pin configuration. These include the 6V6, 6L6, 5881, EL34, KT88, KT90, 6550
and others using pin number 8 as the cathode.
CAUTION:
The Bias Probe contains an epoxy insulation that may soften with extended
exposure to high temperatures found in tube amplifiers. While the epoxy
insulation is rated to an excess of 250 degrees, some softening might occur if
the Bias Probe is inserted for extended times reading idle current, or shorter
times reading current at actual playing volumes. It is therefore
suggested to keep the heat exposure to Bias Probes to less than 15 minutes of
continuous idle current measurement. High volume monitoring of current should
be kept to the minimum time needed to get the current indication. Should
the epoxy soften in the Bias Probe, allow it to cool without disturbing the
relationship of the parts. Once cool, it's performance will be renewed.
With
the power off, allow the amp to cool. Remove one output tube.
Output tubes are labeled 6L6, 6V6, KT66, 5881, EL34, 6550, KT88, KT90,
etc. Do not confuse the output tubes with a rectifier tube.
Rectifier tubes are labeled 5U4, GZ34, 5AR, 5Y3, 5V3 etc. Insert a Bias
Probe into the output tube socket. Replace the tube by installing it into
the Bias Probe socket.
Insert
the leads of the Bias Probe into the "milliamps" input range of your
mulitmeter. Set your multimeter to read milliamps, with the smallest
maximum scale greater than 50 milliamps. Usually this is 200 milliamps on
modern meters.
Power
up the amp, leaving the standby switch in the "standby" mode for at
least 30 seconds. Switch the standby switch to "play" and
observe the milliamp indication on your multimeter. Allow the indication
to settle by letting the amp warm up and additional five to ten minutes.
Adjust
the bias such that the current (milliamps) indication on your multimeter is the
desired level.
Power
down the amp and allow the tubes to cool before removing the Bias Probe, and
restoring the tube into its original position.
Two
Bias Probes may be used simultaneously to observe the current indication on a
set of tubes. A DOUBLE FEMALE ADAPTER can be used to join the leads of
the unmetered Bias Probe if only one multimeter is being used.
Remember:
The indication shown on the multimeter is the correct "per tube"
current, with no doubling or halving of the value required. The same goes
for one, two or four Bias Probes being used at the same time.
Tubes
should be matched for idle current and transconductance. Idle current is
the current that the tube conducts with no signal input. Transconductance
is the amount of change in current that is exhibited with a given change in
grid voltage (either bias or signal voltage).
While the Bias Probe can not directly measure transconductance, if two Bias
Probes are used, the effect of transconductance can be observed in the
following manner.
1.
Set the bias on the amplifier to the point that gives an idle current somewhat
lower than what would be normally used for the tubes being tested. For
instance, setting the idle at 20ma instead of 35ma would be fine.
2.
Observe and record the current reading of each tube. Note the difference
in current, such as 3 milliamps.
3.
Set the bias of the amplifier to the point where the idle current is somewhat
higher than what would be normally used for the tubes being tested. For
example, setting the idle current to 50ma instead of 35ma would be fine.
CAUTION - do not run the amp for extended periods of time with higher than
normal current settings.
4.
Observe and record the current reading of each tube. Note the difference
in current, such as 4 milliamps.
5.
Reset the idle to the normal indication.
If
the difference in current conduction stayed about the same as the current went
from extra low to extra high, then your tubes have a good transconductance
match. Normally there will be some variation. Consider the tubes
matched when the difference on conduction varies by no more than a few
milliamps in the low, medium and high idle current ranges. For example,
tubes may show 20.5 and 23.5 at low idle current, 34.0 and 36.5 at medium idle
current, and 49.5 and 50.5 at high idle current. The difference has
changed only two milliamps, and that would be acceptably matched.
Your
indications may vary considerably from this example.
Some
knowledge of the operation of the amplifier is needed to actually adjust the
bias level of the output tubes. If in doubt, refer to the manufacturer's
instructions, a reputable text, or seek advise from a qualified tube amp
repairman.
Most
blackface Fenders and most Marshall's have a bias level control on the bottom
of the chassis. This control alters the overall level of the bias for the
entire output section.
Silverface
Fenders usually have a bias balance control. Do not confuse this with the
hum balance control found on the back panel of some silverface Fender amps,
which is for the elimination of heater hum.
If
you have a silverface Fender, it is suggested that you have a qualified
repairman rewire the control for level operation
Some
older Fenders and some other newer amps may be cathode bias designs.
In
this design, there is no bias control. The bias is set by the
relationship of the grid to the cathode, which in turn is set by the cathode
resistor. Cathode bias amplifiers are self biasing. The Bias Probe
will verify whether the amplifier is working correctly by showing the cathode
current. If the amplifier is out of spec, the correct solution is to
replace the cathode resistors.
Some
amplifiers, such as tweed and brownface Fenders have a non-adjustable grid bias
(as opposed to self adjusting cathode bias). These can usually be
converted, as described above, to a potentiometer accessed without pulling the
chassis from the amplifiers case.
All
Bias Probes made on or after 9/15/1999 are using a new improved epoxy, which we
feel is an improvement in strength as well as heat resistance. All Bias Probes
using the improved epoxy compound will be identified with a plastic stick on
label as opposed to the older paper label. During the manufacturing process,
the epoxy is trimmed from the Bias Probe and some small flakes may occasionally
be seen inside the units. These are just a few stray bits still remaining
for some reason after being dusted with compressed air, and represent no failure,
nor do they detract from the Bias Probe performance. The contacts of the
base and socket may be cleaned periodically with denatured alcohol to eliminate
any oxidation or contamination that may have been picked up during
operation. The socket pins seem to be cleaned well with a Q-Tip whose end
has been broken off. The paper shaft of a Q-Tip fits well within the
socket contact. The banana plugs on the Bias Probe are high quality, industry
standard plugs. We have found that some meters may have undersized jacks.
This normally presents no problem when using the Bias Probe. Should the spring
metal contact of the banana plug be deformed through use in a very tight
socket, it may be restored by gently prying the spring back to shape using a
suitable tool (jewelers screwdriver works well).
|
Tube Series |
Cathode Current |
Notes |
|
6L6 |
32ma - 37ma |
In Fender's, usually best results do not exceed 35ma |
|
5881 |
35ma - 40ma |
The Russian large base (WXT) tube is usually good at around
37ma |
|
6V6 |
25ma - 35ma |
Fender Deluxe and Princeton amps at around 30ma, maybe a
little less |
|
EL34 |
30ma - 40ma |
Wide ranges of acceptable currents. Maybe even higher than
40ma in some amps. |
|
6550 |
35ma - 45ma |
Some amps will work best at higher than 45ma |
|
KT88 |
35ma - 45ma |
Some amps will work best at higher than 45ma |
|
KT66 |
35ma - 45ma |
This will vary depending on whose KT66 you use |
|
KT100 |
35ma - 45ma |
Some amps will work best at higher than 45ma |
These cathode current settings are intended as starting points. There is no one absolutely correct bias level or current level. The range of acceptable cathode current levels is fairly wide. Lower current levels will sound weaker, thinner and the amp will run a little cooler. Extremely low current levels will sound distorted (terrible sounding crossover distortion). High current levels will sound louder, punchier and fuller. The tubes will run hotter. Extremely high current levels will cause the tubes to overheat (start to glow then short out).