Some general thoughts on
I test all tubes in a radio before I begin
restorations. I am looking
for hard shorts, heater shorts (when hot), weak tubes and plain old dead
tubes. I do not want to
restore a radio to have a shorted tube put B+ on circuits it does not
belong and take out perfectly good parts.
This is obvious. A
heater to cathode short will typically not do too much damage but will
send you in circles chasing poor performance and hum.
A dead tube is good to know.
The three above mentioned tubes get replaced to avoid damage,
aggravation and tube sub guessing. The
weak tube is the condition needing consideration.
A weak class A wired audio output tube can cause significant poor
performance. This distortion
at low volume usually leads to general irritability (in the owner).
This may or may not present a problem in an AA5.
But it certainly will lead to general irritability if left in a
high end Zenith or Grundig. And so
will a Push-Pull configured tube. A weak audio tube will
suffice for restoration activity and alignments.
But weak audio tubes should be replaced before returning the
radio to the cabinet.
Sometimes IF or Detector tubes show weak or questionable on the
tube tester. I put those
back into the chassis, complete the restoration and a full alignment.
The final alignment and general performance (within spec or not)
will indicate if the tube needs immediate replacement.
Also, I find many detector tube diodes will show weak in the tube
tester but perform to specifications.
Check the AGC & tube bias voltages and observer the audio
output. If good then use the
A lot of times a weak RF and first stage RF tube will critically
degrade reception sensitivity. I
replace those with fresh strong tubes.
Some aficionados may be reluctant to spend the money or
want the original branded tube but that leads many times to “This
radio does not receive good or sounds like crap!”
I don’t usually take the chance on RF tubes or audio output
High voltage, high power AF/RF tubes need special consideration.
After testing for a damaging short circuit, a strong or weak
indication in a tube tester means little.
The true test of a high voltage high power tube is under full
For more reading on tube testers there is a great site here: http://www.tone-lizard.com/Tube_Testers.html.