"Antonio Vernucci" wrote in message
...
The aim was, guess what? Suppressor-grid modulation! I can
remember
hearing some of these rigs on the air. I don't know just what
percentage of
modulation was possible, but I remember it being pretty weak. Good
strong
signal with very low modulation level.
The aim had been to use what parts one had, and it surely was an
inexpensive
way to join the AM crowd!

Old Chief Lynn, W7LTQ

In the 1957 ARRL Handbook, these particular tubes were also used in a
cathode driven (grounded grid) amplifier.

73, Barry WA4VZQ

Yes, I was aware of the "modified 1625" but for linear amplifier usage.
Now I know that they were also used for suppressor-grid modulated
stages.

But now a doubt come to my mind: the 1625 is a beam-power tube, not a
pentode (actually it is an 807 with 12.6V filament). So, how could it
work in suppressor-grid modulated stages?

73

Tony I0JX

Read Lynn's post again. He said the carrier was strong but the audio was
quite weak. It is quite difficult to get over 95% modulation with a
suppressor modulator. With beam power tubes, you get far less than this.
Also you get lots of distortion because the beam forming plates do not
linearly control plate current, so you have to lower the modulation level
even further to make the audio readable by listeners. Hence the "strong
signal with very low modulation level" makes sense. Fortunately with
pentodes, it is easy to drive the output to zero (negative modulation).
The real problem occurs when the suppressor is driven very far into the
positive voltage region and it starts drawing current. Typically the
suppressor grid can only dissipate a few watts. Since current is drawn
during the positive peaks, an audio driver must present a low impedance.

73, Barry WA4VZQ