Menu
This is 'close' but not entirely correct. Rather than point out the
numerous errors in your post, I'll refer you to this link:
which answers most any question you could have with regard to tube
amplifiers.
And BTW, a transformer is an 'active' component. There even have
been transformer 'power amplifiers' manufactured in the past where
the transformer was the active power element.
regards,
--john
--- In , "unlunf" <unlunf@y...> wrote:
>
> Hmmm, the whole idea of an article that is tech oriented,
> and completely off topic (surf music).... is this a good
> thing? So let's cut to the chase, and eliminate all the
> blathering. We'll also eliminate the math portion of our
> curriculum. <g> Since we started talking about impedance
> in the power amp section, we'll stay there for now. In a
> few paragraphs we'll deal with low signal level impedance.
>
> First, every circuit in electronics has two impedance
> factors, the input and the output. Some components
> in those circuits are impedance-neutral, such as resistors,
> diodes, batteries, etc. Others are very impedance critical,
> these would be capacitors, inductors such as transformers, etc.
> Impedance is frequency sensitive.
>
> Let's just take it for granted that a tube has fairly high
> input and output impedances. OTOH, a speaker's impedance
> is low. We use a transformer to convert from one to the
> other. So, what happens if we don't match the speaker to
> the transformer?
>
> Well, the transformer doesn't do anything in and of itself,
> it's not an active component. What comes in, goes right
> back out, after the conversion. If we hook up a 4 ohm
> speaker to a 4 ohm output transformer, then the tubes will
> be happy. If we hook up an 8 ohm speaker to that same
> transformer, what happens to the tubes? They 'see' twice
> the number of ohms of impedance. Is this a bad thing?
> Depends on the tube, but for the most part, no. Lots of
> tolerance is built into both tubes and transformers, and
> even most speakers can handle some mis-matching of
> impedance values.
>
> Some guidelines that have been accepted by the amp design
> and repair industries over the years:
>
> You can usually feed a lower output impedance into a higher
> speaker impedance without causing grief to the tubes, but
> there are limits. You shouldn't go over twice the rated
> number of ohms. Remember, as the ohms went up, the tubes
> tried to overcome that, and tried to draw more power from
> the power supply. Go too high, and the power supply will
> crap out on you. You'll end up with less power output at
> the jack, and probably a muddy sound.
>
> Pretty much the same holds true in reverse, but that way
> is a bit more dangerous. All the good amp designs, not
> just Fender, can handle a halving of the impedance (2 ohms
> instead of 4), and the tubes should survive. As has been
> pointed out before, always look at your tubes for awhile
> after doing this. If they start glowing red, STOP! The
> tube is not sufficiently loaded down (too few ohms), and
> is drawing too much power. This is something that can't
> be fixed by twiddling the bias, you'd have to reduce the
> power supply voltage (the B+). Moreover, the tubes will
> very likely clip in the second harmonic range, and that
> will pass right through the transformer and take the
> speaker out like a snuffed candle. Too much of this,
> and the transformer will also buy the farm.
>
> Final rule of thumb: You can go one step up or down, but
> no further. If you're going down (speaker is lower than
> what the amp is rated for), then always observe the tubes
> for at least 5 minutes after warming up. And re-check them
> from time to time, they may not get stressed out until
> later in their lives.
>
> in re: low level signals......
>
> The same rule of thumb holds true here, except that we don't
> have to worry about blowing tubes or speakers. In fact,
> what we now worry about is fidelity. You can always go
> from a low impedance source to a high impedance load, and
> it's very likely that you will suffer little or no signal
> degradation. The reverse is -not- true, you will definitely
> suffer degradation if you do more than halve the load's
> input impedance compared to the source's output impedance.
>
> And that's it in a nutshell. Without any math, even!
> The rest of the lesson is best taught by example, and that's
> where you come in. Ask your questions, and we'll see what
> would happen (good or bad) if we do this or that or the
> other thing.
>
>
> unlunf
>
