As I said I would about eight years ago, I will relate to the NG how I
use the scope/notch method to set the bias on guitar amps.
This method requires that you have an oscilloscope, a signal generator and a non-inductive
power resistor as a dead load. If you plan on doing amp servicing, you need these tools
Heres how I do it:
I set the signal generator to deliver a 150mv, 600 cycle sine wave signal to the input of
the amp. I set the master control, if there is one, on full and use the gain control to
vary the intensity of the signal to the output stage.
I set the tone controls at their
mid-point. I set the presence control full down, where applicable. I hook up a dead load
to the output and if the opt is multi-tapped, I usually use eight ohms. The scope is
hooked up across the load resistor so I can monitor the output. Assuming that I am
starting with a known good amp with new tubes, I will set the bias control, if there is
one, to give the highest negative voltage at the grids of the output tubes.
I then power the amp and once it is operating and stable, increase the gain/volume
control to give a sine wave at the output. I set the scope to see usually three sine waves
on the screen, but I may want to isolate just one notch as I complete the bias procedure.
The sign wave will have a horizontal "notch" on the rising and falling portions
of the wave, the sides you could say, at the middle. When the output is high enough the
top and bottom will start to clip.
When this happens I reduce the gain/volume until the sine wave is fully rounded at the
top and bottom. At this point I begin to reduce the negative bias voltage. As I do this,
the sine wave will increase in size and at some
point it will begin to clip again. I simply reduce the level of drive signal again until
the sine wave cleans up and continue. I continue reducing the bias voltage and adjusting
the drive signal level until the sign wave is
smooth on the sides. At this point the amp is "biased" and ready for testing.
As an example, I pulled out a Fender Bandmaster Reverb that I had in the garage. It is
black-faced and has new filters but is otherwise stock. I performed my usual bias
procedure outlined above on it and then used a one ohm resistor in the cathode circuit to
measure the idle current. It was 34 ma.
Not exactly the "cold" setting some people associate with this method. But
this method is not as simple as some have made it out to be. It is VERY subjective. It
takes some experience if you hope to have repeatability and not all amps can be biased
this way. Let me explain.
The crossover notch made by some tube types are more apparent than others. EL34s notch
sharper than 6L6s, for example. The notch in the above BR was really a gentle curve. How
do you bias with that? I have seen this type of curve in Fenders many times and can deal
with it but it is as a result of experience that I know where is "biased" (
32-36 ma idle current in a Fender with 6L6s) and where is not. How do you get this
experience? How do you know when to stop cranking that bias pot? There is a part of this
procedure not written in the "book" that you need to know about.
The check is a way to confirm what you have done. You can check the idle current with the
one ohm resistor or by the transformer shunt method or by just looking at the current load
present at the ammeter in your Variac, if you have one. For example, the above BR, at 120
vac and with the output idling at 34 ma pulled 625 ma of current from the wall. Most 50
watters will do that, with EL34 amps getting up near 750ma. 100 watters will go from 1.25
amps to 1.5 amps. So, what is this check, cheating? Sort of. After a while you will know
where to bias familiar amps without it but it is the one thing that was not written about
that is the most important. You can not know where the amp is biased without measuring it
in some way, right? Some amps give a notch that is so vague that there is no way to tell
if you are biased or not. The check is a way of being sure.
So, why use the scope at all? I do it to check on the amp. I look into the amp to confirm
its condition. Biasing in this way puts a strain on the amp, a lighter strain than
that encountered when playing but it works the amp and I can see if the filters are happy,
if the output tubes are happy, the opt, etc
But I will not say that this is the
right way or that there is one right way. If anybody tells you there is ONE right method
or ONE right bias point,
please understand that that person is wrong. I was wrong on this topic once, too.
Customer is right.
Some folks like to run their tubes at high currents. I consider the normal idle current
for EL34s with 440-480 volts on the plates in guitar amps to be within 34-40 ma. Typical
for hifi amps is around 50 ma. Some folks like their EL34s in their guitar amps to idle at
45ma. Thats ok. The higher idling current means higher currents throughout the
operating range, as well as higher operating temps. This means more wear and a decreased
life for the tube. If the player is willing to make the trade of less tube life for more
of the sound that he likes then ok.
Going in excess of 45 mills in the typical guitar amp can stress the tubes or power
transformer to the point of premature failure. I could see up to 50 ma in an amp that was
never pushed to clipping, but most fixed-bias guitar amp power transformers are not
designed to deliver that much high current continuously, so I like to encourage that
people stop at 45 on the above types of amps. To do a "custom" bias such as
this, you will need to use the cathode resistor method or the trans shunt or insert an
ammeter in the plate circuit- something other than the scope method as reasonable accuracy
is important. And if you have the amp biased past the point where the notch disappears,
how ya gonna know where you are?
Still, I urge the player to try a bias setting a bit lower that the
"preferred" if that setting is kind of high. I.E., If he likes 45 ma, I say try
40. If that sucks to him we try 43. If at that point hes happy, he has given himself
a little more tube life. If hes not happy, 45 it is.
Common problems with using the scope method are:
Not using a dead load. I have heard from people saying they can not get the amp to sound
good using this method no matter what they do. After going over the procedure many times
they finally admit that they are using a speaker for the load, even though they have read
to use the dead resistive load. You can not substitute a reactive device, be that a
speaker or an emulator for the dead load. 50 watt resistors do not cost that much.
Dont use an old voice coil either. Its not a load.
Getting a good sine wave. You need a good, even sine wave for this. On some amps, this
means adjusting the tone controls to reduce any non-linearitys or odd curves. If the
amp has several channels, use the clean one. If it has a loop, use the loop return and
increase the signal generators output to about half a volt ac.
Where do you stop? This is the hardest and is the reason for the "check". On
some amps removing all of the sine wave is just right. On others, removing all the sign
wave may be cold or hot. That BR still had some curve to it at 34ma/idle. Having fun yet?
Most amps are happy with the 150 mv input signal. But if you see that the signal is
clipped at the output even when you have the gain/volume low, try reducing the signal
generators output level.
So what do I use?
If you are just getting started or are a player who just wants to keep a eye on his gear
or change tubes without going to the local repair shop, I would suggest using the one ohm
resistor in the cathode circuit. Its easy and safe. Better yet is to use a plug-in
probe of some sort, although I have no sources. I did mention previously the Bias King
that was a digital meter and probe all in one. It was sold by R&G International (the
Audio Glassic folks) but I dont know if it is around anymore. The Bias Probe from
Hunt Dabney is or was, at over $100.00 too expensive. And as far as the techs out there,
use what method you are comfortable with and which works for you.
Thats easy enough, isnt it?