Chokes v. resistor

[greiswig]

I'm curious about something. Doug (er....Randall! With apologies...) Aiken (thank him if you have a chance) as usual has a great explanation of how a choke works amongst his free technical library:
http://www.aikenamps.com/Chokes.html

But suppose you have an amp that has virtually no supply hum, the reduction of which if I understand right is one of the chief benefits of using a choke. I'm not sure that I understand the relative merits of using a choke versus a resistor as in the "Robben Ford Mod."

A choke will usually have a lower overall DC resistance than the resistors used, so there will not be as much of a voltage drop across it. But if one changes the dropping string anyway after installing a choke to compensate, would there still be a difference in sound or feel?

[heisthl]

The "Sag" changes. The choke stores some energy and it's resistance can change with current flow, a resistor does neither.

[Structo]

As I understand it an inductor opposes the change of current.

So if you have one in a guitar amp power supply and you have it cranked up where the power needs are more demanding and you hit a big chord or bass notes, the demand on current will be greater.
The choke seems like it would be a little slower in providing the current needed verses a resistor where the current will be available almost instantly.
Do I have that right?
So a choke would provide just a bit more sag, not as much as a tube rectifier and choke but more than just a large power resistor.

[heisthl]

I'm no expert but I think the choke can impart some of it's energy back as it's field collapses, so you have less sag with a choke.

[talbany]

Chokes serve several functions.. One is to act as a low band pass filter.. The other is to help with the ripple effect.. How I hear it is a choke helps with the smoothness of the low end responce..Amps without chokes have more of a raw type feel to them..You can always put a resistor in series with the choke like 200 ohm should be pretty close to a tube rectifier w/choke.... Typical fender 5H choke about 100 ohms.. Ive used 10H in some 100 watters with good results.. Keep in mind adding a resistor here will drop the b+ a bit around 10 volts...


Tony VVT

[greiswig]

I'm no expert ...

Everything is relative, I suppose.

[Structo]

Yes, I have a 4H 100 ohm choke sitting here that I bought for my 2x6L6 D'Lite that I haven't put in yet.
I was trying weigh the good and bad before I committed to drilling holes.

Right now I have a 330R choke resistor in that spot.

This thread came just at the right time.

Since I would be losing 230 ohms by putting the choke in, I wonder how much my voltage will shoot up with the choke?

So I went digging into Aikens great site and found this:

Q: One intriguing thing about some amps is the 1K resistor used for the screen supply filter instead of an inductor. Does it really work like a small choke? How much inductance would such a resistor really have? Would this inductance be enough to significantly reduce power supply ripple (compared to e.g. a metal film resistor of the same resistance)?
A: The inductance of the resistor is negligible at the frequencies of interest in the power supply. For example, using a typical inductance of 0.04mH for a 1K wirewound resistor, you would get an inductive reactance of 0.03 ohms at 120hz. This, in conjunction with the usual 50uF filter cap, which has a reactance of 26.53 ohms at 120Hz, would give an attenuation of around -0.01dB at 120Hz (the power supply ripple is twice the 60Hz mains frequency because it is full-wave rectified).

By contrast, the 1K resistor itself, which has a reactance of 1K at 120Hz, gives an attenuation of around -32dB at 120Hz in conjunction with the 50uF capacitor, which would reduce the residual 120Hz hum by a factor of around 39 times.

Okay, you might ask, why use a choke? Well, a 6H choke would have a typical resistance of of only 100 ohms or so, so it would drop less voltage than the 1K resistor, for DC. However, for the 120Hz AC ripple riding on the DC voltage, the inductive reactance would make it look like a whopping 4.5k resistor. This 4.5K, along with the 50uF cap, would result in an attenuation of -44dB at 120Hz (ignoring the effect of the 100 ohm choke resistance, which would be negligible compared to the 4.5K reactance of the inductor, but it does help to dampen the overshoot at the corner frequency of 9.2Hz). So, by using a 6H choke, you get 12dB more attenuation of hum (a factor of 4 times less ripple) along with less voltage drop to the screens (more power output) and less preamp hum.

The only thing a large resistor buys you is screen supply sag, which you may or may not like. You can always do both - use a choke and add a largish series resistor for better ripple rejection and screen supply sag.

[greiswig]

So I would get less sag with a choke than the resistor? Well to my limited tube knowledge that doesn't make much sense since an inductor will oppose a change in current. That's why they are used to smooth ripple right?

I thought inductors of, say, 100 ohms DC resistance rating were rated thus precisely because they could had less resistance to low-frequency and DC, hence less sag than a 330 ohm resistor. And they have higher resistance to AC (60Hz and 120Hz), so they reduce the ripple on the supply. Hence better filtering.

I'm not an expert, though.

[mlp-mx6]

When did Randall Aiken start going by "Doug"?

[greiswig]

When did Randall Aiken start going by "Doug"?

Geez...my guitar playing is like that sometimes, too: where did THAT note come from?