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

[pdf64]

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.

Has anyone verified this yet?
I accept that's the theory but reality will inevitably differ, if only because the theory ignores overdrive, which tends to be a key part of guitar amp usage, even for ostensibly clean tones.
From static to max square cranked, my guess is that the current draw will increase a fair bit, eg 25%.
It will vary according to the particular amp, eg operating conditions (eg biased hotter or colder than mid point), whether the power tube is a pentode or beam tetrode, how saggy the plate and screen grid supplies are, signal asymmetry when overdriven in earlier stages.

At the weekend I found a virgin transformer set and chassis that I bought over a decade ago for an SE EL84 amp, and had totally forgotten about.
I'll put it together sometime, honest, and make my own measurements, but in the meantime would be grateful if someone could check their amp?
Pete

[iknowjohnny]

Does the choke's H value affect the tone when the amp is loud ? If it does how would you say very large values like say a 20H would sound or feel compared to a 5H?

[talbany]

Does the choke's H value affect the tone when the amp is loud ? If it does how would you say very large values like say a 20H would sound or feel compared to a 5H?

To answer your question, a larger inductance will result in a cleaner DC supply, but will also change the transient response of filter network. (Usually, the reason for going to an inductor is to get the extra filtering without the DC voltage drop of a resistor.) However, you replace the resistor with a choke of the same DC resistance, it will have the same voltage drop, but the transient characteristics will change. The purely resistive filter will generally rise with no overshoot, but the large inductance may tend to make the supply overshoot or ring on transients, depending upon the size of the filter caps in relation to the inductance value

Because the preamp supply is relatively constant, the extra filtering won't make that much difference in touch response. This is why some amps get by with no choke, only a resistor for preamp supply filtering. If you were to change from a resistor to an inductor with much lower DC resistance, you would have better filtering and a higher preamp supply voltage (if you don't compensate the dropping string), which would increase headroom, giving a slightly different feel.


Tony

[iknowjohnny]

This ringing you speak of, does it sound like an acoustic guitar ringing like if you hit a chord with the amp loud and a acoustic in a stand in front of the speaker? I've heard that when cranked under certain circumstances.

Does the choke's H value affect the tone when the amp is loud ? If it does how would you say very large values like say a 20H would sound or feel compared to a 5H?

To answer your question, a larger inductance will result in a cleaner DC supply, but will also change the transient response of filter network. (Usually, the reason for going to an inductor is to get the extra filtering without the DC voltage drop of a resistor.) However, you replace the resistor with a choke of the same DC resistance, it will have the same voltage drop, but the transient characteristics will change. The purely resistive filter will generally rise with no overshoot, but the large inductance may tend to make the supply overshoot or ring on transients, depending upon the size of the filter caps in relation to the inductance value

Because the preamp supply is relatively constant, the extra filtering won't make that much difference in touch response. This is why some amps get by with no choke, only a resistor for preamp supply filtering. If you were to change from a resistor to an inductor with much lower DC resistance, you would have better filtering and a higher preamp supply voltage (if you don't compensate the dropping string), which would increase headroom, giving a slightly different feel.


Tony

[jml77]

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.

Has anyone verified this yet?
I accept that's the theory but reality will inevitably differ, if only because the theory ignores overdrive, which tends to be a key part of guitar amp usage, even for ostensibly clean tones. .........

......
Pete

Pete, I did these tests a few years ago on an epiphone VJ with reduced interstage attenuation, with a 500mv pk-pk 500 hz i/p, with the amp cranked into an 8ohm dummy load, the increase in current draw from quiescent was 73% mods were re-evaluated!! I should repeat the tests sometime.

[roberto]

Because the preamp supply is relatively constant, the extra filtering won't make that much difference in touch response.

But screens' current will change alot, and so the response of the whole amp.

[talbany]

Because the preamp supply is relatively constant, the extra filtering won't make that much difference in touch response.

But screens' current will change alot, and so the response of the whole amp.

Roberto
An inductor, also called a coil or reactor, is a passive two-terminal electrical component which resists changes in electric current passing through it..It also helps to provide some voltage regulation as well..The purpose of the choke in a capacitor type input supplies, (which is what we use it for) is not just for for filtering and voltage regulation, but just for filtering the DC supply to the screen grids of the output tubes and the preamp section. The screens typically take around 5-10mA each, and the preamp tubes draw about 1-2mA or so..Of coarse the current/voltage will change due to the supply sag and transformer regulation under heavy loads!

Tony

[roberto]

Thanks talbany, no need to explain me the whole story starting from Adam and Eve. I've just learned the Noah story with the electrons flooding the whole amp, and soon I'll be ready to split the electron's flow like Mose did.

The choke is between plate and screens, and screens current is not constant, but has big variations from zero signal to overdrive, so a different choke will react differently to the screens' current, and so we will obtain different feels from the amp.

[iknowjohnny]

on a side note, If the amp's output section feels rather stiff even with small filters, i think larger screen resistors will soften that, right? What would be considered the largest value to use in a marshall 50 watt that you think would be considered within reason? I use 1k now.

[Jana]

IKJ,

Go ahead and try larger values. You can go as large as you want (greater than 1K) and you won't hurt anything. You will have to reset the bias when you change these resistors.

Try 10K and see what happens. It will be massive compression in the power amp.

Try 2k7--that gives a bit of compression and a smoother sound without the overboard compression that 10K would give. I suggest trying 10K just so you can see (and feel) what the effect really is. I doubt you would want to leave those values in the amp.

Before you ask, can I make this switchable? I repeat that you have to readjust the bias.

[iknowjohnny]

I'll see what values i have laying around and try for around 3k. Thank you.

[roberto]

Larry Novosibir, in his first amps, used a separate supply for screens, formed by a reostat and a dedicated supply cap, to change the feel of the amp.

I remember that another person, misunderstanding the novosibir mod, placed the reostat between the screens cap and the four screens resistor, but it worked.

Basically, instead of changing four screens' resistors, you can add one single reostat in series (taking care of dividing the value by four and calculating the right power).

Then you have to accept a certain amount of bias variation.