Power supply - choke resistance

[Kermit]

I bought a Triad C-7X to replace one of the power supply resistors.  It's resistance measures a bit on the low side - 240 ohm.  It's within spec for the part, but seems low compared to the stock resistor.

Is this safe to use, or should I buy another one and hope for a part with higher resistance?

Thanks!

 

[Tom-s]

It's ok. Go ahead. Use it in place of the second PS resistor.

 

[Deke609]

Ditto. By my rough PSUD estimate the 240 DCR of the choke in place of a 270R resistor will only increase B+ by 2-3 volts - which is pretty small. The mains voltage coming out of the electrical outlets in your home probably bounces around by this much or more.  So you should be fine.

cheers and happy new year, Derek

 

[Kermit]

Just checked my voltages - they're a tad high (before installing the choke).  B+ is 181, and the big Speedball has OA and OB at ~108.5.  Still ok?

 

[Paul Birkeland]

That is within the acceptable range.

 

[Kermit]

Great, thanks all!  Cheers to unnecessary but fun mods!

 

[Deke609]

Cheers to unnecessary but fun mods!

I'm a big fan of these types types of mods too.

cheers, Derek

 

[fromnowon]

I'm curious how and where you will decide to mount the new choke.
Considering this myself;
would be great to see a picture if you get a chance!  Thanks and good luck.

 

[Thermioniclife]

This is how I did it.

 

[Kermit]

I have a piece of sheet aluminum arriving tomorrow.  I intend to cut and mount a sub-chassis using 1" standoffs on three of the power supply screws.  My intention is to mount it below the power supply, rotated 90* from Therm's mounting position.  The lugs on the choke will go front to back, rather than side to side.

Pic attached of me holding it approximately where I intend to mount it, and a mock up of the sub-plate.

I am open to input!

 

[Paul Birkeland]

If you start connecting the transformer screws on the bottom side of the amp with metal, you will likely cause some operational issues.  The solution is to either use nylon standoffs or use a piece of sturdy plastic instead of aluminum.

 

[Pingu]

Another place too mount is on the side either next to the valve nearest the transformer. Double sided tape to hold the choke to the base plate, I use servo tape, it’s used in radio module aircraft to hold the control servos in place and is incredibly strong. You can then add a small wooden support to the inside of  base if you want more support. Very easy, no cutting or drilling extra holes, insulated from chassis so a small earth required, and much less chance rf from the transformer.
Here’s a good explanation although he has used metal stand offs screwed tho chassis.

https://www.headphonesty.com/2019/04/upgrading-bottlehead-crack-headphone-amplifier/#comment-5889

You  could also add a small mains filter next to the input with a VDR and Rifa x/y either two or three legged cap to smooth the mains even more. Not sure if make a difference but only a few pennies and a friend who knows more than I do always adds one to all his hi fi bits he works on

Martin

 

[fromnowon]

Thanks for the pictures!

The picture attached must be the location in the Headphonesty article.
- If you were to attach with tape, I suppose a ground wire would be necessary.
- If the choke is attached with metal stand-offs bolted to the plate, then no ground wire is needed.
Is that correct?
(That Headphonesty write-up is excellent by the way.)

 

[Paul Birkeland]

- If the choke is attached with metal stand-offs bolted to the plate, then no ground wire is needed.
Is that correct?

Our preference is to control any potential magnetic eddy currents by isolating chokes and transformers from the chassis plate, then providing a separate ground wire back to the chassis ground lug. 

 

[Kermit]

Thanks all for the input and info.  Revising my plans for mounting...

 

[Pingu]

Kermit and Fromnowon
yes that’s the picture and I found the write up clear and helpfull with sensible advice about upgrade bits glad you liked it too.

Paul
thanks for the advice yes I intended to earth the choke separately but will now make sure it’s insulated from the base plate too, thanks

My first job is to sort out several dry joints. No not my build I inherited the crack from a friend so just giving it a check over before running it and then after that trying the caps and choke upgrades
Martin