DYNACO ST-70 Assembly Manual - Page 9
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rectifier tube shows arcing after a few seconds check carefully all of the QUAD CAP
CONNECTIONS against the pictorial.
6. If the amp plays with HUM disconnect all associated equipment and short the input terminals.
Plug in an RCA input cable and connect a wire across the POSITIVE and NEGATIVE terminals. If
the hum goes away, the problem is with your associated equipment. If the amp still hums, check
ALL GROUND CONNECTIONS inside the amp. A bad tube (that still allows the amp to play OK)
can also cause hum.
7. ALWAYS BE CAREFUL WORKING ON THE AMP with the power on. This amp has 400 + volts in
certain areas.
Always adjust bias with no signal going through the amplifier and the SPEAKERS CONNECTED to the
amp. Without a load (the speakers), bias will jump around because of circuit instability. Bias will vary
with line voltage and tube changes. Don't try to be a fanatic about getting the bias "perfect" - Expect
bias to go up and down as your line voltage goes up and go down. Subtle changes in bias have no
effect on sound. Checking the bias about once a month is fine. As the tubes wear a little bias will
change slightly.
If you cannot bias ONE output tube properly then THAT tube may be defective. REPLACE that tube. If
two or three or ALL tubes will not bias properly or all tubes have NO bias voltage try replacing the GZ34
rectifier tube. A common symptom of a weak rectifier tube is LOW bias on all or multiple tubes with the
bias control turned all the way counterclockwise. If a 5U4 tube is used in place of the standard GZ34
rectifier tube then bias will have to be readjusted. Always use a GZ34/5AR4 rectifier tube if possible as it
is indirectly heated and is slower to apply full voltage. This allows the amplifier's components to warm
up at a slower rate.
The Stereo/mono switch on the front has been bypassed for better sound. The input jacks connect
directly to the driver board.
Specifications of the ST-70 Amplifier with VTA Driver board
Power output . . . . . . ...... 35 watts RMS per channel at 1 KHz at 1% THD with both channels driven
Frequency response . . . . 10 Hz to 30 KHz +/- .1 dB at 1 watt
Power bandwidth . . . . ... 17 Hz to 30 KHz +/- .3 dB at 35 watts
Distortion . . . . . . .......... Less than .15% THD from 20 Hz to 20 KHz at 1 watt, less than 1 % THD at 35
watts 20 Hz to 20 KHz
Hum and noise . . . . . ..... More than 90 dB below 35 watt output
Sensitivity . . . . . . .......... 500 millivolts for 35 watt output
Bias setting ............... .400 volts DC per EL34 or KT77 output tube
Feedback ..................... 13 dB total loop feedback
Tube complement . . . . ... (4) EL34 (3) 12AT7 (1) 5AR4/GZ34 or 5U4
Size
. . . . . . . . ............. 13" x 9.5" x 6.5"
Weight ......................... 30 pounds (shipping weight = 35 pounds