Vox Ac30 choke rewind

[Tubetec]

Someone gave me an  old vox  choke that had shorted turns on it , in fact the the insulation breakdown was so bad ,its was practically a dead short across the windings  .  Anyways I stripped down the bobbin and cleaned it up then today spun 5000 turns of .18mm(33awg) magnet wire onto it , came out at 580 ohms . Im wondering whats an appropriate gap for the lams , 20ma or so current is all I need .Heres the original spec 20 hy @100ma 419 ohms ,

http://www.classictone.net/40-18043.pdf

I'll rig it up to the lcr meter later and find out what Hy it gives ungapped , I guess I can experiment until a find a convienient value ,but maybe someone in the know could tell me a more  scientific way to go about it .

 

[CJ]

square stack of what?  100 EI ?  (1" tongue)

that core wants a 3 mil gap for max perm,

3000 turns would be fine ,  maybe a gauge bigger and DCR goes way down, unless you want high DCR to drop screens down below meltdown, those 6BQ5's run hot!

 

[Tubetec]

Its around 1 inch square the core , the usual dark grey  metal I see in almost all vintage Uk made amps .
The amp this piece came from was a late 60 Ac30 , its been replaced now with a new one ,so the old core is mine to play around with , it wont be going back into the ac30 .
I might end up using it as a screen filter on my reverb unit though , 6au6 and 5881 for drive , 12ax7 and 12au7 as recovery amp/reverb output .
I actually seemed to have misplaced the E core section ,so I'll need to look around for it ,but it'll turn up alright , I was aiming for a little more HY's than the standard Ac30 choke with less current capabillity ,maybe 20-30 ma would be around the right mark .
I need to drop some volts on the screen supply in any case so dcr isnt an issue  , dropping  and stopper resistor will be included in any case. 

 

[Tubetec]

The actual spec of the vox choke varied a bit over time ,
I found another with 520 ohms and 19 Hy , 100ma ,
Mine came out 545 ohms dc ,25 hy with the same gap as was in there already ,shows no major variation in  value between  100 or 1000 hz on the LCR meter . I did get some other military screened chokes they measure 6Hy at 1khz but at 100 hz they measure around 8 hy , not sure why this is .

 

[JShaffer]

Its around 1 inch square the core , the usual dark grey  metal I see in almost all vintage Uk made amps .
The amp this piece came from was a late 60 Ac30 , its been replaced now with a new one ,so the old core is mine to play around with , it wont be going back into the ac30 .
I might end up using it as a screen filter on my reverb unit though , 6au6 and 5881 for drive , 12ax7 and 12au7 as recovery amp/reverb output .
I actually seemed to have misplaced the E core section ,so I'll need to look around for it ,but it'll turn up alright , I was aiming for a little more HY's than the standard Ac30 choke with less current capabillity ,maybe 20-30 ma would be around the right mark .
I need to drop some volts on the screen supply in any case so dcr isnt an issue  , dropping  and stopper resistor will be included in any case.

This is super random, but do you still have this choke and would you be interested in selling it for an AC30? '60 AC30s are very rare ya know haha

 

[Tubetec]

I still have the choke , it remains on the shelf unused.
I guess in the interests of bringing a nice old amp back to life I'd be willing to consider selling it to you.
Of course we'd need to agree a price, figure out postage and a way to transfer the money .
I suppose pm me and we can see if it will work out economically viable to ship .

 

[abbey road d enfer]

I did get some other military screened chokes they measure 6Hy at 1khz but at 100 hz they measure around 8 hy , not sure why this is .

Core always has perm varying with frequency, some in a 10:1 ratio between 50Hz and 1kHz.
When gapped, there is less variation, because air perm does not vary with freq.
Large gap yields almost zero variation (because air is dominant), smaller gap varies, but less than ungapped.
Imagine a resistor and an inductor in series; the total impedance varies with frequency, a lot if the inductor is dominant, very little is the resistor is dominant.

 

[Tubetec]

Thank you for the extra info Abbey,

 

[trobbins]

The inductance will also vary with excitation voltage, and of course with DC current.  Often vintage choke specs used 5V or 10Vac, which an LCR meter is not going to apply. Nor will a typical LCR meter measure the inductance at the rated DC ma.

 

[Tubetec]

Its only the basic Chinese Ebay LCR meter I have , 700mV excite voltage, still Ive measured a few chokes with the spec written on them and its not far off what it says on the tin , maybe 10% high , most certainly good enough for psu choke work.
Good to hear from you Trobbins, hope all is well .