The AM Forum

Gonna build my first GG linear amp and would like some suggestions on what kind of filament choke to use. I've gone through most of the previous threads here and have some of the basic concepts figured out, but I'd still like to hear thoughts on what others would do. My amp project will be a simple one band linear amp for 160 meter SSB. I plan to use a single 813 with around 2000V on the plate. Hoping to get around 500W PEP input. The exciter will be a modern transceiver. I found a very simple circuit at w4npn.net that puts all three grids directly to chassis ground.

For a filament choke, I have several options. RF Parts has an Ameritron FILRFC30 choke that is listed as a 15uH choke usable on 1.8-30MHz. 15uH seems a bit light for 1.8MHz, and I was wondering if anybody has experience with this choke on 160 meters.

RF Parts also has a choke listed simply as "RF Pillar Choke 55uH". The picture of this choke sure looks like a bifilar wound coil on a ferrite rod. 55uH sounds better than 15uH for 1.8MHz. Maybe somebody could take a look at the pic and let me know what you think.

Lastly, I could roll my own. Looking for suggestions on ferrite mix, wire gauge, and number of turns. I could also do the ferrite bead thing. Again looking for suggestions on ferrite mix and number of beads.

Thanks in advance for your comments.
73, Ron W8ACR

Here's a pretty good thread on filament choke designs that may be of benefit.

http://amfone.net/Amforum/index.php?topic=28171.0

I tested chokes using #43 beads...results are on page 2 of the  thread "FILAMENT CHOKE CONSTRUCTION"  Good luck Ron....

Above 7mhz, I use 43.  Below I use 31.

--Shane
WP2ASS / ex KD6VXI

Thank you for the comments. After further study, I have ordered a 77 mix rod 7.5 inches, some 75 mix beads, and a large 31 mix toroid. That should give me adequate options for a good filament choke. I think that 20ga enameled wire filament leads should be adequate for a single 813. Is there any reason that I should avoid cloth or plastic covered filament leads?

Does anyone know where I can find published data on operating conditions for an 813 in GG service? Specifically, what kind of input impedance can I expect? I guess I should shoot for a choke impedance 10X the tube input impedance. Is that correct?

One more thing, I want to leave the plate voltage on continuously. Can I do this with all the grids at DC ground, or would I need to put the control grid at RF ground only and add some negative control grid bias? Plate voltage will be around 2000.

Thanks again,
Ron

Are you going to run it as a tetrode or strapped as a triode?

As a tetrode, yes.  You'll need to put some cut off bias in.  How you do that is up to you.


As a triode, you lift the cathode connection on rx.  Usually the CT of the filaments.  10k resistor to ground, shorted out on TX.

With tetrode connected you'll need to either lift the cathode or increase the negative grid bias.

--Shane
WP2ASS / ex KD6VXI