Choke versus cement resistor: Why use one over the other?

[Leo_Gnardo]

How does one determine the appropriate value of a choke for a given circuit?
I have come across a few and would like to use them in future builds, but not sure how to work out which to use.

Typical choke in the classic Fender amps is @ 3 or 4 Henries & 100 ohm DC resistance. You can always up the value for better filtering. I've done so, but rarely. Also "pre-filter" the hi voltage supply as in some early tweed Fenders and the standard Vox AC-30 power supplies. That works a charm, but costs, and makes the amp that much heavier. Made some dead-silent Champs that way.

Although voltage rating has nothing to do with operational characteristics, it DOES have to do with safety. Don't put a choke rated for 400V service into an amp that has a 500, 600, 700 V supply. Last thing you need is a short or arc to chassis thru your choke. And that ain't no joke.

[overtone]

On choosing the choke:
I stand to be corrected, but as chokes do not operate linearly the mA rating of the choke should be chosen to be as close as possible to the actual current draw according to the location in the circuit in question.
I have a supplier here in Germany (Reinhöfer) that advises against going too much higher with the mA rating, because it can reduce the efficiency.

They publish their performance data to show the non linearity,
e.g. M85 62.83 135 Ohm 12H 200mA Choke as follows:
5,7H@60mA,
14H@150mA,
12H@200mA,
10H@240mA,
7H@280mA

[kanons]

Hello. I am trawling around for possible upgrades to my builds. I have just finished my Ampmaker N5X 5w single end amp. Great sounding Marshallish amp, but, as i have it full out at all times with the VVR controlling the WAF (Wife acceptance factor), it is a bit noisy and prone to the vagaries of the Norwegian mains. Rectifier is ss
Would a choke help filter out unwanted interference?
Any idea where I should put it
And: What data am I looking for if I should buy a choke?
I am also thinking of replacing all the caps in the power supply with known high quality stuff. Is this a waste of money/time?

[roberto]

I swear it makes a difference in some amps, but I don't have an understanding of why.

I do know I prefer Marshall circuits with a choke.

Sorry for the old post quote, but this is the focus of what i would like to focus on.

While a resistor is a passive component that can only decouple supply stages and give a voltage drop proportional to the current flowing through it, a choke has the capacity to store energy, and this give a different behaviour to the screens (that are supplied through it).

While the voltage drop during high current demanding times (when you hit the string) is lower with the choke, resistor will increse it until it recharges the supply cap again (RC factor for screens is very important to change the behaviour of an amp, expecially at mid to high SPLs).

This can be positive or not, depending on what you are looking for and how you haved esigned the rest of the amp, and so and so.

As an example, I always use chokes on my amps, but I prefer JCM900s with resistors (I actually increase the value of this resistor, to increase the "warming" effect of an higher voltage drop during peaks.
Mid content radically changes.

[Leo_Gnardo]

"Chokes have ratings of voltage, DC current ( measured in milliamps), resistance (measured in ohms) and inductance (measured in Henries). A typical choke for a 50 watt guitar amp is rated for about 500 volts and 50 milliamps. The resistance measures in the 250 ohm range and inductance between 10 – 20 Henries. The inductance of the choke coupled with the capacitance of the filter capacitors determine the bass response of the amplifier. A larger inductance value choke will have a better bass response than a smaller value choke"

Hoss shit, bull shit, chicken shit. Where DID you find this? Geral Dweeber? Dam' Torres? Sneezer Diaz? Wikipedia?

Practically everything in that statement is flat wrong.

Amps without chokes have NO bass response? WTF ???

This has been sitting here since last August? I should have replied then. Don't worry I'm not mad. But it DOES give us somethin' to laugh at.

[overtone]

Hello. I am trawling around for possible upgrades to my builds. I have just finished my Ampmaker N5X 5w single end amp. Great sounding Marshallish amp, but, as i have it full out at all times with the VVR controlling the WAF (Wife acceptance factor), it is a bit noisy and prone to the vagaries of the Norwegian mains. Rectifier is ss
Would a choke help filter out unwanted interference?
Any idea where I should put it
And: What data am I looking for if I should buy a choke?
I am also thinking of replacing all the caps in the power supply with known high quality stuff. Is this a waste of money/time?

1. Personally I always use a choke in a single ended amp because I find it much more efficient in smoothing out the ripple current - which I guess is the noise you are refering to.
2. On an SE I would use a CLC (Cap-Choke-Cap) filter set up before the B+1 OT / plate supply.
3. Maybe the best route for you is to order Merlin Blencowe's power supply book. After reading that you should be set.
4. You choose the mA rating to be at least and as close to the maximum mA that your circuit will draw.That amp can use several different tubes, so we cannot pull a number out of the hat for you right now.
5. There is a free programme called PSU designer which will let you get an idea of the results between different power supply designs before you bite the bullet. You can see the effect of different choke inductances, resistances etc. Expect choke inductance to be between 3H to 20H for guitar amps.
6. I also usually use SS rectification in a single ended design because there seems little point to me in an SE going for tube rectifier sag etc., etc.
7. The caps that Barry at Ampmaker supplies are fine for the task. If you sourced all from him then you may be wasting your spending changing them out. If you really need to, then go for the first reservoir cap - maybe throw a cool PEG Rifa in there or something like that.

hope that covers all the questions,
sorry cannot fix the power supply in Norway right now. Maybe they need a big choke up there.
tony

[eniam rognab]

uhoh..

im gonna hide now

[pdf64]

Also "pre-filter" the hi voltage supply as in some early tweed Fenders and the standard Vox AC-30 power supplies

Leo, you may have been thinking of the 60s AC15?
I think the AC30 has always taken the OT CT from the reservoir cap / rectifier node.
Pete

[Leo_Gnardo]

im gonna hide now

Naah, just reveal the source of this dezinfomatzia. Or we'll spear your chickens.

pdf64 - yes early AC30's. To bump up power for those who think they need it I've moved the CT to first filter node at the expense of noise. Apparently more recent versions of Vox did the same.

On single-ended amps, with that pre-filter, notes tail out to a "fade to black" instead of "fade to buzz." One less noise to edit out on recordings. Yaaaaay!

[eniam rognab]

i dont know officer i swear, i was smokin lotsa resistors hanging around some transients....

dont believe me though...

[pdf64]

Hoss shit, bull shit, chicken shit. Where DID you find this? Geral Dweeber? Dam' Torres? Sneezer Diaz? Wikipedia? Twisted Evil

Ha, that's inspired Leo!

Just to move back to the OP, if using a resistor verify that it has a suitable voltage rating (element voltage rating, max overload voltage etc), eg equal to the VB+.
At power up the resistor will have to charge the second cap up to VB+, so for a fraction of a second will have to withstand almost full VB+ across it.
I've had 100 ohm 7 watt resistors fail in this application. When I checked the spec of the particular type, it was only rated up to an max voltage of 200V.
Pete

[David Root]

+1 on a choke for SE operation. I have found 10H gives a very quiet amp.

Also there is a good reason not to use a tube rectifier in the power supply, with SE operation but I don't recall the details right now. I'll probably wake up at 2:00 AM with the answer. Don't call me then though.

[talbany]

The best explaination of the difference between a resistor and a choke is explained well by Randall Aiken..
Here it is!

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.

Tony

[martin manning]

For the purpose at hand, I'd be inclined to get a 4H, 90mA choke, such as for a 100W Fender (cheap and easy to get), and put it between C16 and C17. That will reduce the ripple voltage dramatically. If its still not quiet enough, I'd increase the size of C16 and C17 to 47uF or even 100uF. Use at least 400V caps there, too.

Re using a tube rectifier, it's a class-A amp, so the average current is constant, i.e., no increase to produce voltage sag.

[Leo_Gnardo]

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.

To quote the mighty Warren Zevon: "Yeah, and I done that too." Only in this case the results are pretty darn nice! Took a big ol' Traynor YBA1A with the big transformers, KT88's & turned it into the crush monster. Hoo-boy!

Thanks for your post Tony!

For a pre-filter in Champs I just use the choke for a Twin @ 4 Hy. No need to go 10 but suit yourselves amp meisters. It's all good!