[nnck]

In laymans terms please:

What is the function of a Rectifier tube (such as the 5AR4/GZ34 found in my tube preamp)?

Why/How would the Rectifier tube affect the sound created?

How would this differ from the function of a typtical Driver tube (such as a 6SN7, 12AX7, 12AU7)? Also, would a driver tube typically have more affect on the overall sound (i.e. Which tubes would you roll out first if you wanted to change the sound of your preamp?)

Also, what is a "Getter" tube?

[Mark Korda]

Hi NNCK,The rectifier tube was essentially the first tube made for audio.Tom Edison invented the light bulb.He installed an extra metal electrode in the light bulb he called a plate.He observed a flow of current using an ammeter between the plate and the light bulb filiment.He called this the (Edison Effect).he did nothing with the idea.In 1901 O.W.Richardson looking into the Edison effect added heat.When the metal was heated,the electrons gained velocity in a vacuum.He called it (Therimonic Emission).The electross would break away from the metal.Sir Joseph Thompson from England came up with it about the same time and called it (Electron Theory of Matter).Then in 1904,J.A.Fleming assembled a tube called a(Valve).His valve (tube) had a heated electrode called a filament or cathode,and a non heated electrode called a anode or plate.This was all in a vacuum.When he energized the valve with a current he noticed current would flow from cathode to anode,but no current would flow from anode to cathode.The current flow was shut off in the opposite direction like a valve on a water pipe,hense the name valve,English for tube.This is called a (Diode) from dielectrode,2 active elements.When higher power levels are used it is called a (Rectifier) tube.So in your amp,which is pugged into the wall socket,the AC power coming from your outlet goes first to a Power Transformer,which then goes to a Rectifier tube changing the AC to DC current.A Diode tube with 2 rectifiers in one tube is called a Full Wave Rectifier.Older tube amps worked this way.Today a little thing called a silicon diode does the same thing as the Rectifier tube.Some peoplle think the Rectifier tube contributes to the warm sound of tube amps and might save tube life by slowly bringing the current up to levels more slowly than the silicone diode.I'm reading this out of a book called (How to Sevice Your Own Tube Amp)by Tom Mitchell.I'm horrible in math,but this book does not dwell in that,which I should know,but this book was made for a Geiko caveman such as myself,and has gave me a better grasp on vacuum tubes and how they work.In 1906 Lee Deforest came up with the Audion tube,the triode,which added a third electrode between the cathode and plate(Anode) called the grid.This made amplifacation possible and is the answer to your power tube question.I am going to let some one else take over now,but it's all in this book you can get from Parts Express,or just Google it.I hope I did not confuse you...take care...Mark Korda

[Elizabeth]

Great response mark!! Bravo!!   
Added:
The rectifier tube is used to create a direct current from the alternating current of the incoming A/C power. The rectifier tube is a diode. That is it transmits the current only in one direction. The plate and cathode charges are responsible for the allowing of current in one direction only. The resultant sine wave of the A/C source is just chopped off on the negative side of the cycling waveform. So the power becomes direct current, but it still is going from zero to full back to zero just as the peak of the alternating current did, with a gap of staying zero when the negative AC part of the current is being blocked.
The negative portion can also be 'rectified' AND inverted to turn the negative side of the power pulse into the positive side. This is called a "full wave" rectifier. The resultant waveform still has the sine wave shape of pulses, just all positve, from zero to full and back to zero, (with no gaps) all in the same polarity, thus Direct current.(cycling from zero to full to zero constantly, but always positive)
For power supply that deliver a positive DC voltage from ground, AND an equal negative DC voltage (usually for balanced operation) the power supply is doubled up and one full wave rectifier does the positive, another the negative rail.
(the reason for DC smoothing capacitors is to smooth out these pulses and offer a steady direct current.)
So those giant caps in your amp serve more than just one purpose. They store energy yes, but they also smooth out the pulsing of the power supply.
The rectifier tube is not the direct cause of the quality of the sound in the power supply. The whole design of the power supply is what affects the sound quality. i personally would say as long as the rectifier tube is working properly, that is all it can do. A 'quality' tube may be better because it actually does the job better.
(this was cut and pasted from my response from same question by OP in AudiogoN thread.)

[woofersus]

Nice history lesson there.  Even though I'm a dealer I'm still relatively new to the world of tubes, so I've been gradually learning more and more, but I would welcome a single thread full of nitty gritty details about how these circuits work and what the various design compromises are.

[AVnerdguy]

How would this differ from the function of a typtical Driver tube (such as a 6SN7, 12AX7, 12AU7)? Also, would a driver tube typically have more affect on the overall sound (i.e. Which tubes would you roll out first if you wanted to change the sound of your preamp?)
A rectifier is used to change the AC current to DC to power the ckts in an amp, TV etc. A driver tube is used to preamplifier a small signal into a larger one. As a preamp it is a voltage gain device used to drive the power output devices. A power tube does the same thing but is more of a current device - used to drive speakers (hard load) hence it's physically larger and does the heavy lifting so to speak. As far as which one affects the sound more is a matter of opinion. I would say if the power tube doesn't have enough ooomph to drive the load properly it would affect the sound more (opinion)

Also, what is a "Getter" tube? A getter is not a tube - it's an element within the tube. A tube is a vacuum device and when they suck the air out of it they also want to remove any impurities in the tube that would sit on the plates or screen grids and reduce the life of the device. A getter is typically a ring shaped metal component that is used to attract the impurities in the final vacuum stage of the build. The plates are cooked at very high temps to boil off the impurities and the getter being at a different potential attracts the junk being burned off - hence the name getter.

[Ericus Rex]

The getter is what sprayed the inside of your tube with the silver stuff.  That stuff keeps on absorbing O2 since glass is not a perfect insulator.  When the getter (here I'm referring to the silver coating) is all used up it turns powder white.  This is an indicator that the tube has lost its vacuum.

[Steve]

To further explain differences, different triodes have different
characteristics, such as plate resistance, Miller effect, which affect the
high frequency (HF) response.

For instance a 12AX7 has very high plate resistance (say 60k to 100k ohms) which limits the HF response due to capacitance from both stray and Miller effects of the following stage (grid to plate capacitance times the gain +1 of the following stage), so another stage called a cathode follower is sometimes incorporated to eliminate the Miller capacitance, depending upon the application. Otherwise the HF response could easily be down 0,5db at 15khz.

The input capacitance of an amplifier is determined mainly by the interconnect cable (IC) capacitance, plus stray capacitance, plus Miller capacitance of the first stage (tube or transistor). This will cause the input impedance (Z) of the amplifier to vary with frequency.

A 12AT7, 12AU7, 6H30 all have less plate resistance, in descending order, that improves the bandwidth (each tube has different amounts of theoretical gain (Mu)). Gain Bandwidth product is often used to judge the quality of each tube listed. A 6H30 has a very low plate resistance, nice HF response, but needs a different power supply design than a 12AX7 tube, which can cause problems. Designing is dependent upon which tube is used, and even the same tube type from different manufacturers.

As posted earlier, output tubes work at higher voltages and currents, and usually with higher harmonic/intermodulation distortion than small signal tubes. Individual, even or odd order harmonics can be "manipulated", at least to some degree, by changing the slope of the loadline, idle current.

Almost all small signal stages are operated class A. Output tube circuits, either single ended or push pull, can be designed for class A operation, meaning the output tube(s) never shut off, but operate over the entire musical signal waveform. One can literally short the output with no damage as maximum plate dissipation occurs with no signal. Efficiency is lowest, usually 25% or so, although 50% is possible. Push pull operation lessens even order harmonics to some degree, typically around 40db if I remember correctly, using matched tubes.

Class AB1 push pull means each tube does shut off for some period of time, but the other tube is conducting during the shut off period. One should not run the amplifier with the output shorted as peak currents could overheat the output tubes. (Class AB2 means the grid is driven positive by the driving musical signal, thus drawing grid current. The input Z of the output tubes drops to a low level which the driver tube must be able to handle with low distortion.) Efficiency is greater but single ended operation is not desired.

Class B push pull operation means each tube is idled at near cutoff, and conducts slightly over 180 degrees, but distortion, including crossover distortion can easily become a "sonic factor". Again, one should not run the amplifier with the output shorted as peak currents could overheat the output tubes (unless designed otherwise). (Class B also means the grid is almost always driven positive by the driving musical signal, thus drawing grid current. The input Z of the output tubes drops to a low level which the driver tube must be able to handle with low distortion.) Efficiency is greatest, single ended is not desired. (Forget Class C operation for linear circuits.)

Getters come in different types, such as "flash silver on the glass", in holders, on the plate itself etc; each type differing in material and temperature of operation.

Cheers.
 

[rajacat]

Steve,

What is the Miller effect?

BTW thanks for using paragraphs in your explanation. It makes for easier reading.

Thanks,

Roy

ps. Well...not being too lazy, I went Wikipedia and answered my own question.
 It certainly is hard to explain electrical concepts without first defining basic terms  but its difficult without descending into complex math. It would be great if some of the terms used could be explained with metaphors and imagery instead of EE lingo.


form Wiki:
"In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the capacitance between the input and output terminals."
 

[Ericus Rex]

It would be great if some of the terms used could be explained with metaphors and imagery instead of EE lingo.

I'm with ya on that one Bro!

An oversimplified answer to what is Miller capacitance is that there can be some capacitance (storage and later release) of energy between the internal parts of a tube.  At least, I think that's what it is.  Can't always wrap my head around EE nomenclature.  If I'm TOTALLY wrong on that one I'm sure I'll be called on it...even if I'm only very slightly wrong   

[Steve]

(grid to plate capacitance times the gain +1 of the following stage)

To further expound; as an example, suppose stage X output feeds the following stage Y which has a gain of 50 with a grid to plate capacitance of 1.5pf.

The Miller capacitance is Grid to Plate capacitance (1.5pf) times 50 plus 1, or 51. The Miller capacitance is therefore 76.5pf that stage X output sees. Of course we need to add the "output capacitance" of stage X tube/SS section (usually quite small), plus stray capacitance of wiring, parts etc to figure the high frequency response. Of course it is advantageous to minimize the Miller capacitance.

If stage X output is the output of a preamplifier feeding an amplifier input (whether SS or tube) Y, we have to add interconnect cable (IC) capacitance as well. Generally the two largest capacitances are from IC and amplifier input capacitance (probably from Miller capacitance). It gets complicated with the following statement, but it is generally advantageous to use the lowest IC capacitance as possible since ICs vary from 20pf/meter to approx 500pf per meter. However, materials of the IC come into play which complicates things.

Anyway, the Miller capacitance tends to lower the frequency response (FR), so it is advantageous to have the lowest Miller capacitance as possible.

Cheers.