GS-35b linear amplifier: much-improved linearity with two (2) tubes

[AB2EZ]

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

[K1JJ]

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

[N3DRB The Derb]

for the more non technical types = more maul = betta

[WA1GFZ]

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

[AB2EZ]

Tom

Thanks!

Stu