S. O. 765244
INSTALLATION
Subject:
Date
Nov. 12, 1935
AND
MICROPHONE HOWL
Number
3.156-B
SERVICE DIVISION
MODEL D 22-1
Classification
Instructions
RCA Manufacturing Company, Inc
and
NATIONAL OFFICE
Activities
Camden, N. J.
INSTRUCTIONS AND INFORMATION
The high level of sound energy within the cabinet of the D 22-1 may
occasionally cause acoustic regeneration or a sustained howl. This
condition occurs only during record reproduction and is due to vibrating
elements in the 6L7 of the audio expander stage.
Improvements have recently been effected in the manner of supporting the
6L7 elements so that the tendency to howl is eliminated. D 22-1
instruments and tubes of the 6L7 type received lately should, therefore,
be free from ”howling" tendencies.
The "howl" produced by the origina1 type of 6L7 is usually at a
frequency of about 2000 cycles. Any "howl" experienced with the later
type will be at a frequency of approximately 400 cycles.
The improved 6L7 is being shipped in the first detector socket of the
D 22-1 receiver chassis as well as in the expander stage so that they
may be interchanged if "howl” occurs.
W.J. Zaun
Installation and Service
Camden, New Jersey.
