Small value resistor in series with a choke/inductor in the pwr filter rail???

[smackoj]

Gentlemen; Is there any benefit i.e. smoother ripple, smoother amp tone, to adding a small value, larger wattage resistor on one leg of a choke  in the 'A' node of the power filter rail?  I have a couple small chokes that may be inadequate for the amplifier by itself, so I'm wondering if I can use the choke anyway with a resistor in the filter circuit? If there are no ripple reductions and no tone differences, this would be a mute issue of course.

thanks brothers in tone

 

[HotBluePlates]

It probably wouldn't hurt.

What are the ratings of the chokes you're considering? The current rating of the choke is probably very relevant, as it impacts where in the power supply it can go, and also implies a limit above which the choke core could saturate and effective inductance drop.

For what it's worth, I first thought you might be asking about a snubber network (which would also include a lower value cap, maybe 0.1uF or the like). However, the resistor and cap are usually in series and the whole thing in parallel with the choke. A snubber like this is probably not relevant to typical guitar-amp use of a choke, but I did once see them in a choke-input power supply that was fed by a solid-state rectifier.

[kagliostro]

I was wondering if the resistor to be put in series to the inductor

is better suited before the inductor or after the inductor 

I don't think that the position of the resistor, with respect to the inductor, is insignificant

K

[smackoj]

<what are the ratings of the chokes>?

I have two newer ones purchased from "triodestore" that I have the specs on. They are: 40ma/4H and 90ma/4H (both used on SE amp)
I have a few others salvaged from vintage radios that are bigger but I don't know the specs on them?

thx,  jack d     

[HotBluePlates]

Then probably no resistor needed.

Why use a choke at all? Because a choke will reduce a.c. ripple while presenting a fairly small d.c. resistance, resulting in less voltage drop especially in the face of changing current.

So why care about that? Because the output tube current varies more with screen voltage than it does with plate voltage, so to some degree your amp's output power depends on a solid, unvarying screen voltage. This is the real reason why most old amps use either a low-value screen resistor or none at all, so screen voltage doesn't drop with increased screen current and result in lowered max output power (of course there are exceptions, where the resistor is protection for the screen).

So amps use a choke to feed the screen power supply node to reduce ripple and potential hum in the output stage (the screen can also be used as a low-mu input).

The inductive reactance of a choke is 2*pi*f*L, with L in henries. Assume f=120Hz due to a full-wave rectifier and U.S. 60Hz wall voltage.

So at 120Hz a 4H choke has a reactance of a bit over 3kΩ. A 20uF cap has a reactance at the same frequency of about 66Ω, so ripple sees a voltage divider for a.c. giving a 45:1 reduction of ripple. The d.c. resistance of the choke will be small compared to its 3kΩ reactance, minimizing the voltage drop between the plate and screen nodes.

So why did you feel the need for an added resistor? I think it will be hard to get materially better ripple reduction over a 4H choke using a resistor without a potentially significant voltage drop. Also, screen current varies with output signal (more than the preamp stage, but less than the output tube plates), so you might get some added power compression.

If you want that effect, you could just as easily do it with bigger screen resistors (I think Tubenit has used up to 3kΩ screen resistors before).

[smackoj]

<so why did you want to add a resistor> 

ans. I think it is kind of a two part ans. one, I thought if I had a small choke not sufficient for a larger, more powerful amp, that an added resistor would 'beef' up the choke and make it control the voltage to the screens better. and 2, I thought that aside from smoothing ripple, that a choke added a bit of tone shaping, or maybe tone enhancement? and I wanted to use that as an advantage on a larger amp without having to buy another bigger choke. Use the smaller one I already have with a resistor to bring up it's ability to control voltage.

thanks amigo

 

[HotBluePlates]

Well current is current no matter what you do. The choke and the resistor would be in series, so if the current through the pair exceeded the choke's rating, its inductance would still fall.

But you can't really put a resistor in parallel, because current will take the path of least resistance through the choke; again causing its inductance to fall if current is too great.

So I guess it wouldn't hurt, but doesn't seem likely to help either.

[smackoj]

gracias for your help and knowledge senor hot blue.