Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bub on February 21, 2016, 11:56:38 am

Title: Choke question
Post by: Bub on February 21, 2016, 11:56:38 am

Hi Gents,


I have a KOC YBA-3 Custom Special schematic. It calls for a Hammond 159M choke, 15H, 100mA, 500V, 256 ohms.
I would like to stay with Hammond but use a enclosed choke. The closest spec I can find are,
193B, 12H, 100mA, 600V, 155 ohms.
193C, 20H, 100mA, 600V, 181 ohms.
Is this a case where close enough is good enough?  I'm thinking of the 193C


Thanks for any help


Rob
 

Title: Re: Choke question
Post by: eleventeen on February 21, 2016, 12:22:58 pm

I don't see any problems. Don't forget that (and I have not looked at your amp schematic) in almost all cases, the choke is installed so that the B+ fed to the output tubes and hence the plate current of those tubes does not flow through the choke. What flows through the choke is the power supplied to the preamps (which is very, very low) to the PI which is only a bit higher than your typical preamp tube and maybe a reverb driver. So let's just say, crudely, that 80-85% of the total B+ consumed by the amp does not go through the choke. So maybe 20-25 mils flows to the non-power tube sections. The difference in DC resistance between the stock and your proposed replacement choke is 75 ohms. Times 25 mils is 1.87 volt which is nothing. The stages of your amp downstream from the choke will see 2 volts less B+. Hardly even a rounding error in tube work. 

Title: Re: Choke question
Post by: Bub on February 21, 2016, 12:32:33 pm

Thanks eleventeen,


That was the answer I was hoping for.


Rob

Title: Re: Choke question
Post by: jjasilli on February 22, 2016, 01:27:20 pm

Determining choke values:  http://www.valvewizard.co.uk/smoothing.html (http://www.valvewizard.co.uk/smoothing.html)