Audio research d 79b d 79 pwr biasadjust

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D798 ADJUSTMENT PROCEDURE AND DISCUSSION

The D798 utilizes very high quality,commercial grade components and this,
together with conservative operation of all components and tubes, should

provide long service life.

After long service, or after vacuum tube failure and replacement, or in a
location with consistently low line voltage, it may be desirable to readjust
the amplifier for optimum performance.

CAUTION: The following internal procedures should not be attempted by the
owner unless he is technically Qualified. There are high voltages and
currents within this unit which can be lethal under certain conditions.
Refer all internal adjustments to a qualified individual.

There are three parameters which may be adjusted (in the following sequence)
in the D798. The DC and AC balance adjustments are internal, requiring
removal of the rear cover.

l. OUTPUT TUBE IDLE CURRENT ("BIAS")
2. DC BALANCE
3. AC BALANCE

l. OUTPUT TUBE IDLE CURRENT (BIAS")

The output stages of the D798 are partially cathode coupled "push-pull
parallel Class A81, utilizing our tightly-coupled output transformers
which provide low distortion and sonic accuracy.

As shipped from the factory, the output bias adjustments are set for a
nominal 55mA cathode current per tube with a stable power line of 120 Volts.
Under these conditions the tubes are dissipating a proximately 33 Watts of
their 48 Watt rating (42 Watt plate, 6 Watt screen?. This point of
operation provides "enriched" Class A8], and will satisfy most critical
listeners.

Make sure adequate ventilation is provided to prolong tube life.
lA. "BIAS ADJUSTMENT PROCEDURE

For best results operate and adjust the D798 at lZOVAC line voltage, or at
the line voltage that is typical in the final installation. Adjustments
should be made under zero-signal conditions after at least 15-20 minutes of
uninterrupted stabilization time. There may be a slight interaction between
the 4 output tube bias adjustments, so recheck the first tube current after
adjusting the other three.

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Select the desired tubes (V15, V16, V17, V18) with the Bias/Operate switches
and adjust at the corresponding control at the lower edge of the front panel
for identical readings between the 2 tubes on each channel, within about
1/16 of meter deflection. Use the plastic alignment tool supplied with the
D798. Each reading should be within f1/4" of the top of the green band on
the meter scales. This adjustment usually corresponds to lowest lkHz total
distortion (typically less than 0.1% at 75 Watts) into 16 ohms. It may be
trimmed for lowest measured distortion after completing the DC and AC balance
adjustments. If identical cathode currents or low distortion cannot be
achieved, change to a new matched pair of output tubes (matched within 5%).

CAyTION: Always return the Bias/Operate" switches to "Operate" after
completing the adjustments, before applying input signals to the unit.
This will minimize meter "pinning during large signal operation.

DC BALANCE

Because of the nature of the push-pull, direct-coupled input circuit the bias
of the driver stage following is determined by the DC balance of the input
stage. Best sonic operation occurs when these DC input plate voltages are
the same within 0.lVolt DC or better. The actual voltage is not critical at
about 108 to llZVDC. It is the balance that is important within each channel.

The balance adjustments are located on the rear vertical circuit board as
follows, viewed from the rear of the unit:

V6 R18 R17 v2 v1 R18 R17 v5
6FQ7 E83CC E83CC 6FQ7
(AC) RV8 (AC) Rv7
(DC) RV4 (Dc) Rv'3
(DC) RV6 (DC) RV5
Channel 2 (right) Channel l (left)

Channel 1 DC Adjustment RV3 (upper
RV5 Klower

affects the voltage on R18, and
affects the voltage on Rl7.

Vaffects the voltage on R18, and
affects the voltage on R17

Channel 2 DC Adjustment RV4 (upper
RV6 (lower

vv vv