Poor mans variac

[dai h.]

is there anything detrimental dropping the AC with the diodes like that? Hmm...I seem to recall seeing some big ass 1kV 12A at a surplus place and wondering what the heck those would be useful for.

[tbryanh]

Why not put several 12v lightbulbs paralleled, in series with the amp's AC?

If the amp drew a constant current, you would get a fixed amount of voltage drop accross the resistor, and it would work, but the current through the resistor changes depending on how much power is going to the speakers, so the drop would change too which is not what you want.

[tbryanh]

If the amp drew a constant current, you would get a fixed amount of voltage drop accross the resistor, and it would work, but the current through the resistor changes depending on how much power is going to the speakers, so the drop would change too which is not what you want.

To be more correct, it would work on a single-ended amp because single-ended amps draw constant currents. However, if there is a standby switch, the drop would go down in the standby mode which is not what you want. Most single-ended amps do not have standy switches, so in general, it will work for most single-ended amps.

It will not work with push-pull amps because the current they draw varies with the amount of power they supply to the speakers.

[bob p]

R.G has a page on how to build a vintage voltage adapter:

http://geofex.com/Article_Folders/vintvolt/vintvolt.htm

i think that the bucking transformer is a great solution to the OP's problem. i built one. i love it.

(1000th post. couldn't resist.)

[Enzo]

Single ended amps have a constant draw because they run in class A. Any class A amp will run that way, even push pull.

[Steve Conner]

The diode trick is an old and really cheesy one. A diode drops about 0.6V in the forward direction. So to drop 10V you can just connect 16 in series.

You don't want to do any actual rectification (as those "power saver" things did) because the DC component would destroy your P.T. So you connect another string of 16 diodes across your first string, facing the opposite way, and the result is a sort of vile kludged-together "zener" that drops 10V in either direction.

It's nowhere near as nice as a bucking transformer, but it's better than a resistor or lamp.

However, resistors worked fine for many applications. You used to see American "AC/DC" table radios that got imported here. We have 240V mains, so the American 120V sets would catch fire or explode if they were plugged in directly. The solution was to replace the AC line cord with one made of resistance wire! This would be considered very unsafe nowadays: if the guts of the radio shorted out, the resistive line cord would probably set fire to your carpet. :-O

[tbryanh]

Single ended amps have a constant draw because they run in class A. Any class A amp will run that way, even push pull.

Yeah, I forgot about those.

the DC component would destroy your P.T.

The DC component on the rectified half cycle is 0V, so no harm would be done to the transformer.

[TD_Madden]

anybody got pics of a completed R.G.'s "vintvolt"?

[Bruce / Mission Amps]

Yeah, I forgot about those.

The DC component on the rectified half cycle is 0V, so no harm would be done to the transformer.

Not arguing the "harming" part but isn't rectified AC... DC? Anything rectified above or below the zero line in the sine wave would be DC, wouldn't it?

[Old Tele man]

...rectified AC is indeed DC, with a "ripple" component riding on the DC-level (ie: DC-offset), whereas AC-voltage (normally) has a 'zero' reference.

[tbryanh]

Not arguing the "harming" part but isn't rectified AC... DC? Anything rectified above or below the zero line in the sine wave would be DC, wouldn't it?

The rectified half cycle is DC. DC means Direct Current which is interpreted to mean current that flows in one direction only.

Here the current is pulsating which means there is an AC component riding on top of a DC component.

The DC component is the lowest voltage that the AC component dips to. Here, the pulse dips to 0V, so the DC component is 0V.

If a capacitor is added across the primary, the pulse will not dip as low, and the DC component will increase in value (be greater than 0V). Now constant current will be flowing through the primary along with the pulsating current.

Too much of either type of current (constant or pulsating) or too much of the combined currents (constant and pulsating) will harm the transformer.

[Enzo]

Oh, we've been down this semantic path before.

Pulsing DC is still DC. And i am not sure how you came to the lowest point a DC voltage dips to is its DC "component. A pulsing DC will have an average voltage that will be more than zero and less than peak.

That "AC component" is where you will get arguments. AC implies alternating current, and that means current flowing one way, and then the other. Pulsing DC flows only one direction, so it is not AC. It is pulsing DC.

It is an informal use of the term to call that the AC component, I even do it myself, but that is merely to describe something to someone, it is not really AC.

Ripple is not AC. We measure it with an AC meter, since the meter blocks the DC. And we informally refer to the ripple as AC volts sometimes, but it is ripple, and that means changing levels of DC, not AC. AC would require polarity reversals. The only polarity reversals you will see are when you artificially strike a new reference for the measurement

[tbryanh]

i am not sure how you came to the lowest point a DC voltage dips to is its DC component

1. Take a mixer that has a frequency response from 0Hz on up.
2. Put 10V of DC that is filtered smooth (no ripple) into one of the inputs of the mixer.
3. Put 5V peak-to-peak AC into another input of the mixer.
4. Note that the ouput signal is pulsating DC.
5. Note that the average value of the signal is 10V. This is the DC component. (Your right, the DC component is not the lowest point the pulse dips to.)
6. Note that the pulsing portion is 5V peak-to-peak. This is the AC component.

That "AC component" is where you will get arguments. AC implies alternating current

“AC” implies alternating current, “AC component” does not imply alternating current.

A pulsing DC will have an average voltage that will be more than zero and less than peak.

Correct.

the DC component would destroy your P.T.

Correct.

[Enzo]

“AC component” does not imply alternating current.

There are those who would not agree with that.

1. Take a mixer that has a frequency response from 0Hz on up.
2. Put 10V of DC that is filtered smooth (no ripple) into one of the inputs of the mixer.
3. Put 5V peak-to-peak AC into another input of the mixer.
4. Note that the ouput signal is pulsating DC.
5. Note that the average value of the signal is 10V. This is the DC component. (Your right, the DC component is not the lowest point the pulse dips to.)
6. Note that the pulsing portion is 5V peak-to-peak. This is the AC component.

I know what you are referring to when you say AC component. The intellectual debate is about if that is "correct" or not.

We were talking about power supply, so a signal with a DC offset might not be the best example.

And impressing a sine wave on DC, in my personal view would not constitute pulsating. I could be in the minority, but I reserve "pulsing DC" for rectified AC left unfiltered. I differentiate between pulsing DC and signal with DC offset. You could make a case that the intense ripple of unfiltered DC is a "signal," but I would not use the term that way.

When we talk about the signal at the plate of a tube, obviously the average DC voltage remains the same, signal or no if the signal is symmetrical, so if you want to talk about the DC component of that as opposed to the signal voltage of that, fine. But when talking about unfiltered DC, then "DC component" kinda leaves me scratching my head.

[Steve Conner]

The terms "DC component" and "AC component" come from Fourier analysis.

The DC component is just the average value of the waveform over one cycle. Any waveform that isn't symmetrical has a DC component, and that includes the half-wave rectified waveform under discussion. I know that the DC component of this waveform will make transformers saturate and buzz horribly, because I've tried it. The DC component is also what the DC ranges on your DMM should measure.

The "AC component" is just the vector sum of all the other Fourier components: everything else but the DC component. You can think of it as the waveform after it's been through a DC block capacitor. Also, the AC component is what the AC ranges on a DMM should measure, on a true RMS unit at least.

The term "pulsating" to me implies a rectified waveform that dips to zero, but can't be called "alternating" because it never actually goes through zero and becomes negative.