The AM Forum

I added a second GS-35b to my home-brew linear amplifier, in order to improve the performance on positive peaks.

http://mysite.verizon.net/sdp2/id20.html

Although the GS-35 has more than enough plate dissipation capability to produce 1500 watts of peak output in AM mode (375 watts of carrier), I noticed that the output rf envelope seemed to be "flattening" on positive peaks.

The attached .jpeg shows that a Class AB2 (operating close to Class B) linear amplifier requires a peak plate current that is pi x as big as the average plate current. Thus with 800 mA of average plate current on 100% modulaton peaks (i.e., 400 mA of average plate current at carrier), I need more 2.5 amps of peak plate current.

In addition, the peak plate current occurs when the plate voltage is at the bottom of the rf cycle.. i.e., a small fraction of the 3000 volt B+ I am using.

Looking at the published curves for a GS-35b, I found that a GS-35b simply cannot deliver 2.5 amps of plate current, at that low a plate voltage, unless you assume some unreasonably high value of grid voltage and grid current.

I had room on the chassis (moving the existing tube socket a bit) to add a second GS-35b in parallel. My filament transformer (rated at 6 amps) was able to handle the second tube (each GS-35b requires 12.6 volts at 3 amps) without any heating problems.

The amplifier now works great at 1500 watts peak output, with excellent linearity on AM.

Stu

Hi Stu,

Good job, OM.

Yes, saturation in its many forms can ruin a potentially clean amplifier's specs.

I'm a big fan of improving amplifier linearity and coming up with ways to keep the side trash levels down.

I'm working on a project here myself to try to grab another 10db 3rd order IMD improvement.

What you've just accomplished sort of goes along with something both Jay/W1VD and Frank/GFZ (my amplifier advisors) have been telling me....   Run an amplifier at 1/2 it's maximum power (-3db) and improve the 3rd order cleanliness by 9db.

That's a HUGE improvement. So, I'm attempting to do the same thing and designing up a super clean driver to compensate.  The standard FT-1000D and many ricebox drivers are -31db 3rd IMD at best.    I am attempting to tap off the super clean 100 mw tap and amplify up with a cleaner -45db ++ 3rd  class A driver and then run the amp at 1/2 power of max peak.  This overall scheme should produce -40db 3rd or better, I'm hoping.


Again, congrats on your efforts to help clean up the environment, OM!    ;)

T

for the more non technical types = more maul = betta

Or the Rick Measures approach use a lot of tube with plenty of resting current. When you need to clear a path you grow taller and wider.
I'm going to be starting with my HPSDR module at 1/2 watt and third order crud at -50 dB. Not easy to hold -40 through a chain with power at >.12 KWH

Tom

Thanks!

Stu  :)