Different Lives in the Same Amp

An unbelievably clean ‘49 TV Front Deluxe is on my bench for repair.

Triad transformers, Solar “Sealdtite” capacitors, even the National Union and RCA tubes—all original, in an amp that I happen to know has been gigging steadily for the last twenty-five years.  The tweed covering is lighter than usual, almost cream-colored, and there’s a hint of salt in the grill cloth.  Most of what’s inside this amp has been there since the day it left Fullerton.

Even the electrons are original.  Almost all of them.

You can divide the electrons in an amplifier into three populations, sorted roughly by how interesting their lives are.  Two of them will almost never leave the wire they rode in on.  The third has spent the last seventy-seven years inside a particle accelerator.

The Wall Current Electrons

The first population lives in the wires that carry mains AC: the power cord, the fuse, the switch, the primary winding of the power transformer.  It’s an important job with a very short commute.

When the amp is on, 120 volts at 60 Hz pushes them.  They drift one way for half a cycle, then the other.  Their drift velocity is glacial—on the order of a hundredth of a millimeter per second.  The field reverses every eight milliseconds, so each electron moves less than a single micrometer before turning around.

Less than a micrometer: about the wavelength of visible light.

The same electrons that were in the primary winding the day the transformer was wound are still in there, swaying back and forth sixty times a second whenever the amp is on.  Seventy-seven years on the same beat.  They’ve never seen the rest of the amp.  They’ve never seen the far end of their own piece of wire.  They’ve never been more than a few hundred nanometers from where they came in.

The Signal Electrons

The second population lives in isolated neighborhoods, bounded by coupling capacitors and the output transformer.  They dance to whatever the guitar is doing.

An open low E swings them at 82 Hz—and at 164, 246, 328 Hz…all the harmonics riding on top.  A hard pick attack or palm-mute rattles them at frequencies beyond the audible band, but it’s the same range of motion.  A few microns one way, a few microns back.  A life spent in one neighborhood.

Some of them sit in the grid wires of each tube: fine spirals wound around the cathode.  As these electrons sway with the music, they steer the much larger plate current flying past.  That’s amplification.

The guitar signal simply travels as a wave through this chain, each population oscillating in its own stretch of wire and handing off to the next.  Organized fidgeting.  The electrons don’t go anywhere.

The B+ Electrons

Out of all the electrons in the amp, a small fraction made it into the amusement park: the closed loop of the DC power supply, the B+ chain.  Every tube in the chain is a ride.  The power tube is the wildest.

Take an electron heading towards a 6V6 in this Deluxe.  It starts out drifting through copper at a hundredth of a millimeter per second, just like every other electron in the amp.  The difference is it’s in line, heading one direction.  Eventually, it enters a cathode—a small nickel cylinder coated in a special oxide and heated red-hot from inside, something like fourteen hundred degrees Fahrenheit.

Thermionic emission lifts the electron off the cathode surface and into the vacuum.  Just outside the cathode, hovering in a thin layer maybe a tenth of a millimeter thick, a crowd of electrons is already milling about: the space charge cloud.  Our electron joins this crowd and wanders in thermal motion for a few microseconds before the plate’s field gets ahold of it.  Suddenly it’s in free fall through an electric field, without even air to slow it down.​​​​​​​​​​​​​​​​  By the time it threads past the control grid wire, our electron is moving faster than any object humans have ever launched.

And still accelerating.

How fast does it get?  Let me see if I can calculate it without embarrassing myself.

It starts at rest, gets grabbed by the field, and accelerates toward the plate.  Voltage means energy per charge, so the energy gained is the electron’s charge times the voltage it falls through—in this Deluxe, around 340 volts.  All of that energy turns into motion: kinetic energy, ½mv², where m is the electron’s mass (9.11 × 10⁻³¹ kg) and v is its final velocity.

Setting them equal:

qV = ½mv²

v = √(2qV/m)

Plug in the electron charge (1.6 × 10⁻¹⁹ coulombs), 340 volts, and the electron mass:

v ≈ 1.1 × 10⁷ m/s

Eleven thousand kilometers per second.  Nearly four percent of the speed of light.  From a hundredth of a millimeter per second to eleven million meters per second in a few millimeters of vacuum, in less than a nanosecond.

Bang.  The electron crashes into the plate.  Its kinetic energy scatters into the metal as thermal vibration.  After less than a trillionth of a second, our electron is no longer distinguishable from the plate’s conduction electrons.  It joins the slow drift in the wire on the other side of the plate, travels through the output transformer primary, up the B+ ladder, and in the case of this Deluxe, into the 5Y3GT rectifier tube, where it’s launched a second time across another vacuum gap.  From the rectifier plate, it flows down through the high-voltage winding, out the center tap, through chassis ground, and back to a tube, this time maybe one of the 6SC7 preamps—where it gets in line for another launch.

It’s the same group of electrons, going around and around, every time the amp is on.

Assuming the B+ chain is around a meter in length, a lap takes a day or two.  If this old combo has logged a thousand hours, the electrons in its B+ rail have made the loop something like twenty or thirty times.  A few dozen launches.  A few dozen trips around.  A few dozen sprints across the vacuum at four percent the speed of light, each one ending in hot collision.

Slow, slow… bang.  For seventy-seven years.

The first time I watched my grandfather play steel guitar, I was a kid on his living room floor.  The amp behind him had glass tubes inside that glowed orange.  I’d never seen anything like it.  I sat there and deduced how it must work: plug it in, tubes glow; pluck a note, speaker pushes air.  Electricity must stream through the wires the way water streams through a hose—power leaving the socket, rushing through the amplifier, out to the speaker.

Almost none of that is happening.

The electrons are barely budging.  The brief moments when they do move fast happen inside the tubes, across vacuum gaps millimeters wide.  The electrons that travel fast don’t travel far.  Most in the amp don’t go anywhere at all.  Meanwhile, energy is racing from the wall to the speaker at nearly the speed of light—but it’s not riding on the electrons.  It’s riding on the field around them, which is a different conversation.

Does it matter?

As Hans Camenzind would say, this is “much ado about almost nothing.”*   It’s just where my mind goes when I have an old amp on the bench.  Electrons are indistinguishable.  They don’t have serial numbers.  There’s no way to point to one with certainty and say “this is the same electron that was here in 1949.”  One is exactly like every other.

But the wire is the original wire.  The B+ rail hasn’t changed.  Whatever population of electrons happens to be in those places has been there the whole time—even if we can’t single them out.  So I like to think the electrons in this ’49 Deluxe have led wildly different lives.

Most of them have spent the last three quarters of a century inside a single millimeter of copper, swaying gently every time someone plugged the amp in.  Some ride the signal but never travel any further.  Once in a while, under heavy overdrive, a lucky few may get swept off the grid into the plate current—signal electrons breaking into the amusement park.  Some are retiring tonight: every filter cap I swap leaves with its population still inside.  A small number, the ones in the B+ chain, have actually gone places.  They’ve taken the long way around the loop forty or fifty times, and between every stretch of wire they’ve sprinted through a vacuum at a fraction of light speed.

Different lives in the same amp.  Almost all of them have been here since 1949.  They were there for the first chord this amp ever played.  They’ll be there for the last.

* Hans Camenzind's book on the history of electricity. Published by my friend Ken Coffman at Faraday Press.