[Vintage-Audio] Scott LK-72 biasing?

[JM/CO]

Scott was notorious for "neglecting" to make this info available. If you can
obtain an original copy of the "service data" ( not just a schematic ) you
should find out what you are looking for. If not, the following method(s)
will work with ALL tube amps that use "twiddle" pots. This procedure assumes
that all tubes, coupling capacitors and other components are in good
condition.
To adjust output tube idling bias:
FIRST THING--Clean all adjustment pots carefully. These have probably not
been "twiddled" in decades.
1) Consult a tube manual. What you are looking for is plate current in MA at
idle ( static ) conditions. NOT exceeding this spec. is what you are trying
to accomplish.
2) Tie a 1.0 ohm, wirewound ( preferably 1 or 2 % ) resistor, 3 or 5 watts,
in series with the cathode of the tube in question. Once installed, you can
leave these resistors in the circuit permanently. Your results will only be
as accurate as the spec on your resistor. Use a digital ohmmeter, and select
carefully.
3) Attach a digital voltmeter across the resistor. Set on 100 Millivolt
range.
4) adjust bias pot. 1.0 Mv on meter  equals 1.0 MA. of cathode current,
which will be equal to plate current ( for all practical purposes ).
To set DC "balance":
1) Attach Milliammeter ( 0 to 50 should work, or use multi range, and adjust
to lower ranges as you approach the final reading ) to the two plate
terminals of the tube pair under observation. Turn amp off to do this, or be
VERY careful with the HV present here.
2) carefully adjust "balance" pots until reading is ZERO MA.

Things to keep in mind:
1) If possible, maintain AC line voltage at that which will actually be
applied when the amp is in service. If this is not known, maintain 120 volts
with a Variac.
2) These adjustments will inter-act. Go over your settings until no further
improvement is possible.
3) All this assumes good, reasonably matched tubes. Sometimes, even with
good tubes, it is not possible to obtain proper readings. Try switching
tubes with those of the opposite channel. Sometimes it is necessary to have
six or eight good tubes available in order to get four that will set up
properly.
4) If using NOS or newly manufactured tubes, readjust all settings after 100
hours of operation. Adjust yearly after that. Keep 'em cool--use small fan.

After setting Bias and DC Balance, you may ( depending on the particular
amp ) wish to adjust the AC balance. To do this:
1) Perform all DC adjustments as per above instructions.
2) Attach signal generator to AUX input. Volume pot to zero, Generator
output to zero, all tone controls, filters and loudness comp controls to
"flat" position.
3) Set signal generator to 1.0 KHZ, sine wave, output to zero.
4) Amplifier output to be terminated into proper resistive load (4 or 8
ohms).
5) Load to be paralleled with oscilloscope and AC voltmeter ( HP 400
series )
6) Watch your grounds. Some H.H. Scott and Fisher amps ground their 4 ohm
terminal, not the "common" one.
7) Always observe AC conditions with a Variac with voltmeter and ammeter.
Slight adjustment of Variac will be necessary as amp swings between full and
no load conditions.
8) Bring volume pot on amp to maximum.
9) Bring up output of signal generator until output waveform, as observed on
the o'scope just begins to clip ( peaks start to flatten out )
10) Adjust pot. Proper AC balance is obtained when clipping of both top and
bottom of waveform is equal ( symmetrical ).
11) At this point, both channels are at maximum operating conditions. Reduce
generator output until peaks are, again, rounded. Voltage across the
resistive loads can now be measured with the HP 400 series meters, and
computed for full RMS power output. Output of both channels should be the
same. If a distortion analyzer is available, now is the time to use it.
Significant differences in distortion between channels should be
investigated. First thing to check are the phase inverter tubes.

After successful completion of all of the above:
1)Turn output of signal generator and volume pot to zero. Amp is now in
"idling" state.
2 Let it sit on the bench for 5 minutes to restore normal idling
temperatures and conditions.
3) re-check DC bias and balance settings, and re-adjust if necessary.
4) congratulate yourself on a difficult job well done.

Chuck N1LNH "The Scott Doctor"
-----Original Message-----
From: Philip Atchley <k06bb@elite.net>
To: vintage-audio@mailman.qth.net <vintage-audio@mailman.qth.net>
Date: Sunday, June 23, 2002 12:10 AM
Subject: [Vintage-Audio] Scott LK-72 biasing?


>Hello
>I'm overhauling a Scott LK-72 for Duane.  I downloaded the owners manual
and
>schematic from the Scott site but didn't see any procedures/specifications
>for adjusting the bias and DC balance pots.  While the preliminary work
>isn't finished yet I realize that proper adjustment of these controls is
>essential to prevent output tube damage and distortion.
>
>This unit has the Bias select switch and two RCA jacks labled as Bias test
>points.  What Voltage am I looking for at these test ponts.
>
>Help!
>
>73 de Phil  KO6BB
>
>
>
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