GG Amplifier AM operation

[wa3dsp]

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.



 

[Steve - K4HX]

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

[wa3dsp]

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.

[Steve - K4HX]

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.

[kb3ouk]

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.