Replacing power supply resistors with chokes

[pro_crip]

I've heard mention again and again that replacing resistors in the power supply is a good thing. What exactly are the guidelines for doing this, and if it's too lengthy to respond here where would I read up on it. I don't suppose it's as easy as pulling out the resistor and dropping in a triad c-7x. Is there some sort of formula to follow, like replace x ohms with y mH? Thanks for listening.


Rich

[Grainger49]

It was often done to the FP 2 (no specific kit mentioned here).  In the case of the FP 2 you replaced a 1k ohm resistor in the power supply, or both, with Hammond 155J choke (15H, 1026 ohms, 30mA, 400V).  The choke rejects noise that the resistor will simply pass.  It also passively regulates current.  The 1026 ohms (DC resistance) drops the same voltage as the 1000 ohm resistor.

I don't know the specs on the Triad C-7X.  I have used the Hammond.

I'm not sure where to put the choke if you choose only one.  Maybe PJ will weigh in.

[Paul Joppa]

The C7-X has a resistance of 270 ohms, and 270 ohm resistors are used in several Bottlehead amps. The current rating of the C7-X is 90mA, more than enough for any of our current products. The Paramount power supply board has holes for a choke such as this one to replace the 270 ohm 5 watt resistor. The out-of-production Paramour II also has a 270 ohm resistor, and since it had the same chassis plate as the Paramount, there is a space behind the power transformer to mount a choke; this was part of the iron upgrade, re-purposing the original plate choke. The Stereomour in 2A3 form uses two 270 ohm resistors, one per channel. The Extended Foreplay III uses four 270 ohm resistors in the power supply (though the C7-X is larger than necessary in that application).

A caution is in order; if the choke is not positioned and oriented optimally, it can pick up 60Hz hum from the power transformer magnetic field, even while reducing 120Hz ripple in the high voltage. I have no data on how severe a problem this might be in practice, since I've never tried (at least, not on purpose!) using an inappropriate location. The Paramour II is the only case where a suitable location and orientation is built into the design.

[pro_crip]

Would the correct orientation be at right angles with the power transformer? It looks like I could also use one in the eros as well. Thanks.


Rich

[Paul Joppa]

Placement and orientation trade off. The easy way is to make the coil axis perpendicular and the location symmetric - look at the SEX amp for a fairly clear example. But other locations will still have an optimal orientation, though it may not be "square" on the chassis.

Yes, I forgot Eros.

[Paully]

So if the Paramour II is the only one with a suitable location specifically designed into the amplifier, is it a good idea or not to replace the 270 ohm resistor on the Paramount power supply board with a choke?  I.e. is it more likely to pick up 60hz hum in that application than the 120hz ripple it reduces?  I know you said you haven't tried it so this is probably an unfair question, but did you not try it because you thought the negative effects would be likely or maybe the potential upside isn't worth the cost?  Just wondering what your best guess would be knowing full well we can't know for sure without sticking one in there.

[Paul Joppa]

Back when Paramount was being developed, I had in mind the cheapest possible 300B amp. Later, it looked like perhaps a very high end amp - like, a $5000 amp. Eventually of course we settled on the middle ground. The high-end version was to have had that choke, which would sit where the current output transformer sits - there was to have been a much larger output transformer mounted on top of the chassis. The PC board was designed around that time, and it retains the optional terminals for a choke. The current chassis plate however is not designed for that option.

So yes, I expect the change would be an improvement. Not a big one like better coupling and parafeed capacitors, but probably audible to most careful listeners.

The thing is, to make it practical requires some experimenting and forethought as to where to place the choke - especially, keeping in mind that it is at the full B+ voltage and the wires - even if they are insulated - should not be accessible. A simple slip of a screwdriver, or a cat's claw, could punch through the insulation, for example.

[pro_crip]

Safety's the easy part. My listening room is a closed room and I'm the only one allowed in there. No pets, no quadruped infants, no one. It's my sanctum so to speak.

Rich

[johnsonad]

I remember a post by Jim where he had a loaner set of Paramounts and the C7-X was mounted above the filiment choke on the back side of the transformer.  Seems like the mounting holes would be the same or close to it.  Would only need to drill one hole for the leads.  Not sure if this would be the ideal location but it would be the easiest.

There were no comments as to any 60 Hz noise or not. 

[Paully]

Looking at the picture in the manual of the completed amp, I was thinking you could super glue it on top of of the filament choke.    But seriously, it seems way to crowed on the bottom of the chassis, mounting it on top seemed like another good option if it wouldn't be sitting too close to the power transformer itself.

[Paul Joppa]

Being symmetrical and with a perpendicular axis, on top of the filament choke would be a good position for low hum pickup - though it might pick up some from the filament choke, which has the same axis. Only one way to tell!

[johnsonad]

Nice!  Just ordered two from Mouser.  It will be a tight fit but worth a shot.

[Paully]

I ordered my pair as well.  We can meet back here and compare notes...

[debk]

I like to modify my equipment.  So I ordered one to try in the Eros


Deb

[xcortes]

My Paramounts have top mounted opts so I will place the chokes in the original opts space. Some small spacers may be required to clear the opts wire and hardware.

Deb, where will you put the one in the Eros?

Anyone trying em in a FP3? I would think the adequate position here would be on the sides of the PT. In the same positions as the Sex amp parafeed chokes. Some spacers would be useful too to clear the wires coming to the front of the chassis.