I have a very old valve radio from a company called Lafayette.
It has 6 valves which are
6v6 gt
Audio Power Pentode. Powers the speaker. Weak tube could cause distortion.
Power Supply Rectifier. Provides High Voltage for tubes. Weak tube could cause low Voltage which
causes distortion.
First Audio Amplifier and Detector. Weak tube would likely cause low volume.
Pentode RF/IF Amplifier. Weak tube usually reduces sensitivity.
Oscillator and Mixer. Weak tube usually reduces senstitvity.
This radio was working fine but recently has developed the following
problem, when the volume is increased the sound is distorted
completely,
it can only be heared ok when the volume is very low, and putting the
ear onto the speaker. Any ideas?
If you can find the Model No. on the cabinet or chassis, try to find the schematic and service/parts
information in Sam's Fotofacts or Ryder Service Manual. Sometimes, you can find these in the local
Public Library. Or there are sources on the WWW.
There are a few sources of old valves on the internet. Also you can find vendors at flea markets
and Ham Swap Meets. Probably the easiest way to determine if a tube is worn out is to substitute a
known good tube. In the old days, there were tube testers in drug stores and your local radio
repair shop. If you have a schematic and know vacuum tube fundamentals, you can make determinations
of tube or circuit (Resistors, Capacitors, Etc.) by making voltage and resistance measurements.
The circuits for radios with your tube lineup were all very similar and were referred to as the "All
American Five". After repairing a few, most radio service people could visualize the schematic in
their head just by looking at the under chassis wiring. They could sometimes diagnose failures by
touching grid circuits with a screwdriver or test lead and listening to the audio change.
Be careful in trouble shooting. The power supplies for tube anodes could be from 150 to 300 volts.
And capacitors can retain a dangerous charge even with the power switched off.
Good Luck,
Ken