Tektronix 585 - Page 33

Circuit Description — Type 585
of the distributed amplifier principle permits amplification of
frequencies much higher than conventional amplifier sys 
tems; d c coupling allows amplification of signals down to
d c . All triode sections are neutralized to prevent spurious
oscillations. Gain of the overall vertical amplifier is set in
the Delay-Line Driver Stage cathode lead by means of the
VERT. GAIN ADJ. control, R1015.
Stabilization of the Delay-Line Driver Stage
Overall distributed amplifier stabilization involves many
details normally disregarded in lower bandpass systems.
The Type 585 vertical distributed amplifier has been sta 
bilized by several special methods.
As the push-pull signals enter the grid lines of the Delay-
Line Driver stage they pass through a toroidal pulse trans 
former, T1014. T1014 serves to open the common-mode grid
oscillation path to aid in the stabilization of the amplifier.
(The “ common-mode oscillatory path ” applies to the char 
acteristic of distributed amplifiers that they sometimes os 
cillate with all grids in phase, and all plates of the opposite
phase.) Another toroid, T1046, located in the plate lines
at V 1 044, also aids in stabilizing the amplifier to prevent
common-mode oscillation.
Additional stabilization is provided by capacitive neutrali 
zation of each section, with the fourth section adjustable
to allow for minor differences in tube and stray capaci 
tances.
The signal velocity of propagation is identical in the grid
and plate lines of the Delay-Line Driver stage. Since the
amplifier is paraphase, the cathodes must exhibit a similar
velocity of propagation. This is provided by placing small
ferrite beads around one of the leads of each cathode cou 
pling capacitor.
Finally, the shield between triode halves of each 6DJ8
has a 150-ohm resistor to ground to reduce the shield Q
and increase the isolation between plates at high frequencies.
Plate Line Termination
The Delay-Line Driver Stage plate line is a 186-ohm bal 
anced line terminated at each end. In instruments below
Serial No. 2585, the termination resistors are not adjustable.
The reverse termination resistors, R1008-R1 011 and R1009-
R1012, are above the required 93-ohms each (186-ohms in
series). The amplifier tubes ’ parallel plate resistance brings
the value down to 93-ohms each, 186-ohm total.
In Instruments after Serial No. 2585, the reverse termina 
tion has been improved permitting more accurate adjust 
ment of the termination and minimizing reflections to an
optimum amount.
The fact that the reverse termination is adjustable and
quite closely matches the line impedance, makes it unneces 
sary to make the termination at the other end of the plate
line adjustable.
The Delay Line Driver plate line sends its output signal
through the fixed delay line, through the Output Amplifier
grid lines, and is terminated at the far end of the Output
Amplifier grid line. Current and Voltage supplied to the
Delay Line Driver stage is supplied at the far end of the
Output Amplifier grid line and is carried to the tubes V 1 014
through V 1 074 via the fixed delay line.
DC Shift Compensation
A common fault of most high current d c coupled ampli 
fiers is temporary instability following a sudden current
change called DC Shift. This has been compensated in the
Type 585 Oscilloscope. The DC Shift compensation network
is located next to the plate load resistors at the input end
of the Delay-Line Driver stage. R1004, C 1 004 and R1005
make up the compensating time constant to effectively can 
cel the DC Shift effect. The DC Shift compensation is not
adjustable.
Trigger Pickoff
At the output of the Delay-Line Driver stage a push-pull
tetrode amplifier receives signals for the triggering circuit.
Tubes V1084 and V1094, amplify and invert the vertical
signal and drive two cathode followers that drive both
Time-Base Trigger circuits and the vertical Beam Position
Indicator Amplifiers. The Trigger Pickoff circuit is a wide
bandpass amplifier capable of sending frequencies to the
Time-Base A Trigger circuit that are considerably above the
vertical amplifier 3 db point. Two 1000 ohm resistors, R1086
and R1096, isolate the stray capacitance loading of the
leads to the Time Base B chassis from the Time Base A
leads.
Delay Line
The push-pull output of the Delay-Line Driver stage is
applied through a 186-ohm balanced delay line to the
Output stage. The delay line is a specially braided line
which delays application of the vertical signal to the de 
flection plates until the crt has been unblanked and the
horizontal sweep started. This delay allows the leading edge
of fast rising pulses to be displayed as much as 60 nseconds
after the sweep is started. The delay line does not require
adjustment because of the nature of its construction.
Vertical Amplifier Output Stage
The Output stage is a distributed amplifier consisting of
five triode sections driven by the signal from the delay line.
This stage is similar to the Delay-Line Driver stage with all
triodes neutralized to prevent oscillations. The major dif 
ference is that each stage has an adjustable compensating
capacitor between opposing plates permitting the tran 
sient response to be adjusted. Toroidal pulse transformers
(1) at the input to the grid line and (2) in the output leads
to the grids of the output pentode amplifier, open the
common mode oscillatory circuits aiding stabilization. The
output of the distributed amplifier is applied to a push-pull
power output stage which supplies the necessary power to
drive the vertical deflection plates of the cathode-ray tube.
In instruments before Serial No. 2585 the Output Stage
plate line reverse termination was fixed. After Serial No.
2585 it has been made adjustable. Step 45A of the Calibra 
tion Procedure details the proper adjustment procedure.
This change, with the Delay Line Driver changes is avail-
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