Owner`s manual

2) The Output Stage – There are two output stages, one for each channel. Each

output stage consists of :

a. the output transformer,

b. output tubes (2 per channel), and

c. cathode bias resistor (the original value was 15.6 ohms and had the

appearance of a white ceramic tubular device connected to pins 1 & 8 of

both output tubes).

d. The output tube sockets should also be considered a “component” in the

output stage – they have a higher incidence of failure than the typical

mechanical component due to the constant high temperature exposure.

That’s it – really simple. It is important for you to know that the output stage

tubes are meant to conduct a little “bias” current when the amp is idle. This bias

current is controlled by placing a negative

voltage at the control grid (pin 5 of the

EL34 output tubes). Note - if this negative voltage were to disappear, the control

grid will fall to zero volts and at this point the tubes will conduct way too much

current causing the familiar glowing red plate elements (from the heat).

Eventually the fuse will trip or, if too late, the tubes will be destroyed. The

negative voltage needed to keep the tubes conducting the desired current is

produced in the bias supply (located in the power supply section) and is delivered

to the control grid (pin 5) via the tube socket. Therefore should the tube socket

corrode or otherwise fail to connect to the tube control grid pin (5), the voltage

appearing at the control grid (pin 5) will fall to zero causing the tube to conduct

excessive (and destructive) current. You may want to think of the negative control

grid voltage as the adjustable safety valve. Because each tube differs from unit to

unit, it is necessary to make this voltage variable and user adjustable. This

adjustment

is made via the BIAS ADJUSTMENT pots (located on the amp

between the rectifier tube and Quad filter capacitor). As the user adjusts the

voltage on the control grid, the current flowing through the tube (from plate to

cathode) flows into the Cathode Bias Resistor (the 15.6 ohm resistor discussed

above) causing a voltage drop to appear across this resistor (via ohms law). The

user then measures

the voltage across the resistor as an indicator of the current

flowing through the tube (via ohms law Voltage = Current X Resistance). Since

the current flowing through each tube should be about 50mA, and since both

tubes (in one channel) have their cathode current flowing through a single

common Bias Resistor, the total current through the bias resistor will total 2X

50mA, or 100mA. By Ohms law, 100mA X 15.6 ohms = 1.56 Volts (DC) – and

this is the Bias Voltage specified by Dynaco. Failures in the output section usually

manifest themselves as bias difficulties (too much or too little) usually the result

of problems in the bias supply, weak or shorted tubes, defective tube sockets

(loose or corroded terminals), or (rarely) defective output transformers.

3) The Driver Amplifier & Phase Splitter – This is perhaps the most complex part

of the Stereo 70 power amplifier. It is the 7” X 4 “ (approximate) PC board

located in the front center of the amp between the two pairs of output tubes. It