Thanks everyone! It works!

[ka1tdq]

I asked for advice a few weeks ago about a neutralization circuit for the new RF deck that I was building and got lots of responses.  I almost made a faux pas with my original decision to use a pair of KT-88's when someone suggested using a pair of 7984's.  Well, I built it, hooked it up and it worked first try without any modifications or anything! 

Finished product is an RF deck driven by 10 watts of RF (I knock it down with an attenuator) using a pair of 7984's for 12 watts output, plate modulated.  Pretty PW but I'm using a linear after it.  I'm using a Kenwood TS-570 to drive it.  I'll put it on the air as soon as I get a functional 75 meter dipole up.

Thanks again everyone!
Jon KA1TDQ

[WD5JKO]

Jon,

   That was me suggesting the use of 7984's at HF as an experiment. Those tubes have pretty good RF ratings for  VHF use, and are readily available, and inexpensive. If they can be stable at HF, then we might have another tube type to play around with on HF AM.
   Your description of 10 watts giving you 12 watts output makes me wonder. You said you have an attenuator, so maybe you take 10 watts, and reduce that to say 1 watt, and then make 12 from that?
   I'm curious just how well the thing modulates, and what plate voltage / plate current, screen voltage, bias, etc. you are using with those tubes.

   Do you have a picture or a schematic you can post?

I was thinking of using two of those 7984's as a class AB2 linear amplifier in the Central Electronics 20A. I currently use a pair of 7591's, and these are great tubes, but the NOS ones are very expensive. The new manufactured 7591's (EH & JJ) are not as good as the old stuff. BTW, with 550v B+ and 400v G2, I can get 60 watts PEP (15w AM) out of a pair of 7591's acting as a linear amp.

You give me hope that the 7984 will be a viable option.

Regards,
Jim
WD5JKO

     

[ka1tdq]

Yeah, I did have an RF problem from the attenuator pad I had installed originally (poor workmanship) and replaced it with an RF tight external one.  Now I take 5 watts through a 7db attenuator (which makes, I don't know 1.5 watts-ish) and run that into the grids.  Now I have 8mA of grid current for 25 watts output.  Respectable.

I don't know, didn't meter, didn't care about plate current.  I just figured I'd use a wattmeter to tune for maximum output.  But I do know that I'm using 425 volts on the final.  Rough numbers should put plate current at 84 mills. 

I fed each screen seperately with a big 25k resistor.  I think I calculated 9 mils at a voltage drop of 225. 

I put it on the air and got great audio reports from it!  Something is wrong with my linear so I was PW but both Mark KA2QFX and Bruce KF1Z could hear me fine and both said it sounded good. 

This part is done but I still need to fix my linear and there's something wrong with my dipole... vizzwarrr is way up.

I don't have a schematic and if I drew one up it wouldn't be pretty.

Anyway, thanks for the advice on the tube selection.  Worked great!

Jon
KA1TDQ