[Vintage-Audio] Scott LK-72 biasing?

Mon Jun 24 02:12:00 2002

Hi Chuck
Thank you, lots of good info here.  Yes, I know about using cathode
resistors to read cathode current etc.  But I sorta figured that since Scott
did put a bias select switch and two test points on this amp that there must
be an easier way, at least for this particular amplifier.   And, I no longer
have a scope, Audio Oscillator or dummy load etc (except down at the shop)
to do "dynamic" testing.   (I'm actually trying to do less "equipment work"
and more music making than I've done in recent years).

73 de Phil  KO6BB

REAL Radio is heard on Boatanchor Receivers.
REAL Music is played through  'Valve' Guitar amps!
[email protected]
Merced, Central California

> 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 <[email protected]>
> To: [email protected] <[email protected]>
> 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!