Grid chokes

Just wondered if anyone could tell me a bit more about grid chokes . I see they are used on Tele stuff . Some look like pot cores ,some more like mumetal ,
Theres a few on offer here , quite pricey too ,
http://www.sacthailand.com/

Ive looked over CJ's blueprints and found the anode and cathode load inductors used in the V series , obviously these need to carry dc , the grid load choke doesnt ,appart from grid current ,which is probably tiny under normal circumstances .
Will we still need to Pi sectionalise as well as neatly layer the windings of a grid inductor ,to balance stray capacity ,just like we do in a transformer ?

Seems like the Silk inductor is two sections , opperable either as se or push pull grid drive .

I have a couple of big grid chokes from the driver stage of  old Vortexion pa amps ,fed a pair of EL34's , mu metal core and screening can ,been meaning to dig them out for a test .

Located the one of the grid chokes ,
It appears to be centre tapped ,two identical sections as far as inductance, resistance and capacitance . Centre tap to each end measures 250  pf ,end to end its about 65pf  . Dc resistance is 1.3kohm exactly on each half , Ac 1.6meg @100hz,2meg@1khz and 740kohms @7.8khz  .each half is 47HY ,which gives around 185HY in series @100hz , so not much better than a 1meg resistor over most of the range , maybe the capacitance causes the load resistance  to dip at higher frequencies  .Amp was from the 50's rated to about the usual 15khz . As far as I remember it was driven off a dual triode maybe ecc32 ,both sections in parallel cap coupled off the anode ,centre tap of the choke to ground , power tube grids either end ,probably gave good balance and strong drive to the power tubes ,There was also tertiary feedback possibly into cathode of the driver stage.EL34's were cathode biased.

Its potted up in ganky wax , part animal, part vegetable, part chemical, part crap  Id prefer to leave it alone for now ,just seal it back up,but I'd like to know about the bobbin construction , what do the measurements of the capacitance infer ? bobbin divided into two(or more) seperate sections side by side just like we'd expect to see in an anode load choke ?

Looks like small signal grid load chokes need the finest grades of magnet wire  ,very delicate stuff ,doesnt work on a hand cranked winder ,you need nice stable speed to wind fine wire ,not jerky and all over the place .

I want to put in an order for copper especially the finer grades  , so maybe 250 or 500gr spools would be the most appropriate size  ,
Whats the range of wire gauges most used  for  audio transformers and inductors  40's down to 20's AWG?
                                   

Sounds like another brand of snake oil to me.

Cheers

Ian

Tubetec said:

Will we still need to Pi sectionalise as well as neatly layer the windings of a grid inductor ,to balance stray capacity ,just like we do in a transformer ?

Click to expand...

Not really. Most of the choke capacitance will be swamped by the following Miller capacitance. You're sure to get resonance in the audio band from that, at least. Funny way to spend your money, but no one asked me!  ;D

Dont worry I wont be  shelling out over 150 dollars plus shipping for these Silk permalloy grid chokes  at all , really just wondering if theres anyone who tried them or other grid chokes in their designs .

Never heard of a grid choke before but was looking at a big class b  p.a. Amp schematic and by the way it was drawn, I thought “ ha! There’s a grid choke “ but it was actually the plate choke of the previous stage

I usually only see them in 1920-30’s circuits.  Though i think there are some DISA broadcast stuff from the 50’s with grid/plate/cathode chokes. 

they use them in Ham Radio a lot,

It's a beautiful solution to a non-existing problem.
Current types of vacuum tubes do not need that for satisfying operation.
Indeed some poorly designed circuits that see their bias point varying with signal could benefit from that.