Filament chokes for GG amps - how big is too big?

[Ed/KB1HYS]

I have two chokes that I salvaged.  They are heavy wire solenoid wound on stacked iron plate cores  (picture a piece of transformer iron - just a straight bar).  they are stamped with data including the inductance value which is 235mH.   I was thinking that they would make great chokes for the Filament of a GG amp. 

They are self resonant in the 160m band though. ~1.8 mhz +/-  according to the Grid Dipper.

Is that a problem for this application?  I've never had one of the ferrite rod jobs to check out to compare and I really don't see an issue as long as they will handle the filament current.

[k4kyv]

They should work fine for 160m.  Don't know about the other bands, though.  How well they work on higher frequency bands would depend on internal capacitance and core losses. It wouldn't hurt to try them.

[WD5JKO]

I agree with Don, they should work on 160m. Being "self resonant" at 160m suggests that the higher bands could be a problem.

Also consider a filament transformer with dual bobbins such that the filament winding is all by itself. In this case you don't need a filament choke at all since capacitance to ground, and the primary will be much less than a winding over winding type transformer. I bet a dual bobbin filament transformer will be good for 160-40m, and possibly 20m without any choke at all. In fact to get the higher bands, a much smaller choke could be used. My 2 cents.

Jim
WD5JKO

[WA1GFZ]

do not do double bobbins. I tried it once and had hum problems.

[K1JJ]

For a filament choke, all you need is about 8" long of type 43 ferrite beads slid onto BOTH fil wires. (ONE string of beads with both wires fed thru)   I've been using this technique for years and works FB.  Much shorter fil wires and less connections this way.

If the inductance isn't enuff for 160M, just add a few more beads.  You can tell if it's not enuff inductance when the amp is easy to drive on a higher band, (75M) but hard to drive on the lower band.(160M, etc)

T

[Ed/KB1HYS]

Cool, Thanks for the input. 

I wound the filament transformer already (easy enough) It just has a single winding for the fils.  Does this type not need a choke in the filament leads?

[KM1H]

About 15-20uH is sufficient 160-10M. You can always use the inductance as part of the input network on 160 and just use less L in the input coil to compensate if you wind up with less L on the rod.

I dont think an iron core is going to do anything but heat up especially with that L.

The old standby 1/2" diameter, 7.5" long 33 Mix (R-33-50-750) ferrite rod has been around for decades and works well up to 30A and #12. You can stack them side by side for more inductance and filament current.

Carl
KM1H

[WA1GFZ]

Carl, common mode choke so only limit is wire size or a heck a lot of plate current.

[KM1H]

Agree Frank.

Now is that "self resonance" series or parallel?

Carl

[WA1GFZ]

I shunt the choke with a resistor to avoid parallel resonances at VHF.
I've seen amps take off due to parallel resonances in the filament choke. New rig I mounted two pin jacks on the back with .01uf bypass caps on each. Then I'm running a pair of 10K 1 watt resistors to the tube socket to be able to monitor filament voltage as a reference to know where I will need to set the variac.