The AM Forum

I have a question about using a grounded grid amplifier for AM phone operation.

In the 1966 ARRL HB page 167 under grounded grid amplifiers the statement made is:

"In plate modulated phone operation the driver power fed through to the output is not modulated"

on pg. 169  - " In a.m. operation, however, the fed-through power prevents the grounded grid amplifier from being fully modulated (100 per cent).

This is a little confusing because they are talking about a linear GG amplifier but I presume they are talking about actually plate modulating the GG tube. Doesn't a GG linear amplifier obviously properly adjusted just amplify the applied AM signal carrier at the same modulation percentage? Which would be a percentage of fed through driver signal plus the amplified signal.



 

I'm pretty sure they are talking about plate modulating a GG amp. You can make it work, despite claims to the contrary.

http://amfone.net/Amforum/index.php?topic=16683.msg115713#msg115713

OK, but just to be clear my reason for asking this question was not to high level modulate a GG amplifier but rather to use a linear GG amplifier with a fully modulated driver. The HB statement just confused me but I see now that it must have applied to trying to modulating the GG amplifier directly vs. using it as a linear amplifier for AM.

IF the GG is linear, it doesn't matter what signal type is put into it (AM, SSB, etc.) you'll get an amplified exact replica out. The power fed through a GG amp is in phase, so it just adds to the output power.

Using a grounded grid amp as a linear for AM will work, that is referring to using a GG amp biased to class C them plate modulating it. And really, that statement is a little misleading, since the tube can actually make 100% negative modulation, it's just that the positive peaks become compressed due to lack of drive. When the GG tube is being modulated towards 100% posiive, the input impedance is half of what the input impedance is if the amp is putting out a dead carrier, so the driver cannot deliever full power into the load. At 100% negative, theere is an infinite impedance at the input, but since 100% modulation is carrier cut off, it doesn't really matter. So to take care of the compressed positive peaks, you have two choices: drive the GG amp with more power than is really needed that way there would still be sufficient drive at 100% modulation, or modulate the driver in phase (at about 20% modulation) so that the driver would be putting out slightly more power on peaks to compensate for the lower impedance.