Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 06:32:58 PM

Login with username, password and session length

Recent Posts

 

Why is "power rail" used in SS but not tube circuits?

Started by UsableThought, January 25, 2016, 06:31:15 AM

Previous topic - Next topic

UsableThought

There's a discussion over on The Gear Page about what "rail" or "power rail" means - the OP asked what aspects of a tube power supply it referred to. In responding I cited some pages defining it in terms of computer PSUs, op amps, and SS audio amps, where basically it seems to be shorthand for "a DC supply voltage."

So now the further question has been raised, does it even make sense to talk about "rail" with a tube amp? After all for a typical tube power supply - let us say for HT - yes, there is a transformer & rectifier & reservoir cap; but thereafter there is also additional smoothing w/ the filter caps for each stage. So is the power supply in a tube amp too distributed for "rail = supply voltage" to be useful? 

I guess another way of asking the question is, are SS and tube power supplies that different in nature? Or is it just historical accident that "rail" gets used more in SS, just because of how op amps evolved or some such?

J M Fahey

Don't overthink it, it's exactly the same, power rail is newspeak , +B is oldspeak, +V or -V is midspeak, if you allow me to make up that word.

UsableThought

Quote from: J M Fahey on January 25, 2016, 07:11:25 AM
Don't overthink it, it's exactly the same, power rail is newspeak , +B is oldspeak, +V or -V is midspeak, if you allow me to make up that word.


Either I've just got a glimmer of understanding or I'm deeper in the mud than ever.

Enzo

I often refer to the "B+ rail".  "Rail" just means power bus.  Back in the days of tubes, I don't remember anyone using the term rail, myself.  I suspect rail is a term sprung of the solid state era, and it just gets applied to tube circuits retroactively.

In SS circuits, in particular power amps and op amp circuits, there are two opposite polarity power sources, and they are often drawn as horizontal lines across the top and bottom of the circuit, not unlike a pair of railroad track rails.  That is where I think the terms comes from, resemblance to train track, but it could be something totally different.  In any case, the meaning stays the same.

Power bus would be appropriate, though few say that.   I have never heard anyone refer to a "ground rail", though I suppose one could and be understood.  "Ground bus", yes.

It refers to any power supply source distributed through a circuit.  I suppose if you had a four banger tube output stage and wired individual bias controls, you might say each adjusting circuit is connected to the "bias rail", though I have never seen that usage.  "B+ rail" would confuse no one.  Many tube amps these days also have some op amps - either in the signal path or not - so +/-15v rails are more common.

B+ is an old and traditional name for high voltage supply in a tube circuit.  No one says V+ for that, though I suppose it would be OK in context.  I tend to use V+ and V- when discussing SS power amps.  I use that as separate from +15 and -15, which would be more for preamps.

Juan is right though, don't overthink it, rail just means power supply.

g1

  To speculate a bit about the etymology of "rail":
in electric railway (and subway) systems, the power is taken from the "third rail".  This goes back over 100 years.

Loudthud

Quote from: g1 on January 26, 2016, 11:59:27 AM
  To speculate a bit about the etymology of "rail":
in electric railway (and subway) systems, the power is taken from the "third rail".  This goes back over 100 years.
Good point! Same with Lionel toy trains where I got my first electric shock. Really just a tingle from 24VAC.

There was a discussion on MEF a year or so back about a "Split Rail" tube power amp. In that design the screen supply is about half the main B+ Voltage like many of the Music Man amps.

Enzo

And for that matter there are a few designs that bring the cathode of some stages down to a negative supply, I assume to provide increase headroom.

I used to restore old Jukeboxes, and certain era Seeburg boxes had control centers running on tubes, and the plates were at ground, and the cathode sides were running off -300v.  I have no idea the rationale behind that, but it worked.

teemuk

QuoteIt refers to any power supply source distributed through a circuit. 

This.

And in some systems the distribution scheme involved using an actual metal rail for the task. In typical electronic schemes wires, circuit board traces and such tend to replace a genuine "rail" but nevertheless the function, distribution, remains.

For example, a generic breadboard. It should be pretty obvious to spot what "rails" in this case are. In practice various subcircuits that draw current need physical connections to power supply. Conductors distributing those currents can be considred "power rails".

teemuk

Quote from: Enzo on January 26, 2016, 06:09:20 PM
And for that matter there are a few designs that bring the cathode of some stages down to a negative supply, I assume to provide increase headroom.

Or to allow DC coupling between stages.