WhitakerAudio 60 W Stereo Power Amplifier, User And Assembly Manual

Page: 111 / 154

111
60 W Stereo Power Amplifier
This can be most readily observed with IMD measurements, where small differences in power
output (due to low ac line supply voltage and/or tube performance) can have a large impact on
measurements at full power.
In the design of any amplifier, there are a number of variables that can be adjusted to trade-off
key parameters. For example, the distortion performance of the amplifier may be reduced by
increasing the negative feedback. This, however, also lowers the gain of the amplifier. Distortion
can also be reduced by driving the output tubes harder through reduction in bias (making the bias
less negative). This reduces distortion, but also tends to shorten tube life. The design trade-offs for
this amplifier are intended to provide 30 W per channel peak output with the expectation that
long-term operation will be considerably less than that.
7.9 Thermal Considerations
Running performance measurements on the 60 W stereo amplifier as described in this section
requires at least one hour (when using manual test equipment), during which time the typical
heating pattern for this unit can be observed. It will be noted that the output transformers run at
essentially room temperature. The power transformer and choke run warm, but not too hot to
touch. The vacuum tubes are, of course, quite hot and are capable of inflicting burns on the skin.
No other areas of the amplifier should run hot.
Since the shelf life of a receiving tube is essentially unlimited, it is good practice to purchase
at least one spare tube for each type used. In the case of the power output tubes, which for best
performance should be matched, a pair of tubes is recommended.
7.10 Final Touches
The 60 W stereo amplifier has now been completed. Remove power from the unit and carefully
place the Plexiglas cover over the top. Secure with the four #6 screws and flat washers provided.
Use of the cover is recommended as it serves to protect the tubes from users (and users from the
tubes). It also aids cooling somewhat by creating a “chimney effect” that collects cool air from the
edges of the chassis and exhausts heated air from around the tubes and transformers. Do not
overtighten the mounting screws.
If the bottom chassis plate has not already been screwed on, do so now.