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Can someone explain tubes to me?

post #1 of 7
Thread Starter 

I have no idea where to start with these. I know that they work via a vacuum, but that's all I know. What is tube rolling? Where can I find tubes? I'm planning on buying a Schiit Lyr at some point along with a Bifrost DAC and Audez'e LCD-2. 

 

Sorry for this kind of post, I just couldn't find a general overview of how they work and the different kinds etc.

 

Thanks in advance,

Sevin

post #2 of 7
Quote:
Originally Posted by 7Sevin View Post
 

I have no idea where to start with these. I know that they work via a vacuum, but that's all I know. What is tube rolling? Where can I find tubes? I'm planning on buying a Schiit Lyr at some point along with a Bifrost DAC and Audez'e LCD-2.

 

Sorry for this kind of post, I just couldn't find a general overview of how they work and the different kinds etc.

 

Thanks in advance,

Sevin

 

How Tubes Work

Tubes work by applying a differential voltage between two metal plates.  One metal plate (cathode) is set by the accompanying circuit at a much lower voltage than the other metal plate (anode).  It is also heated by a separate heater circuit.  The heater causes electrons to be emitted from the cathode metal plate.  The other plate - at a much higher voltage (+ positive) means the electrons flow from the cathode metal plate to the anode metal plate.  The flow of electrons causes and electrical connection and current flows between the two plates.

 

A grid, made of a type of metal mesh, is placed in the middle of the two plates.  The grid is connected to the music signal that you wish to be amplified.  The fact that the grid is in the middle of the two plates where electrical current is flowing, causes the signal to be superimposed on the receiving, or anode metal plate.  That means the music signal, going through the grid, is "completely" replicated onto the metal anode plate, with the addition of whatever power/voltage was being produced by the cathode metal plate.  That results in amplification of the music signal.

 

In the early days of tubes, many thought that a gas needed to be present to "convey" the electrons between the plates.  As they got more experience with tubes, however, they found that the opposite was true - the current flowing between the plates was actually best when there was as near to a perfect vacuum as the equipment on hand could produce.

 

There's a little more to it than that, really - for instance, the cathode actually has a separate voltage applied to it in addition to being heated.  What I described is known as a tetrode (3 points of electrical charge - cathode, grid, and anode).  However, they added grids and other components to make many types of combinations - like a pentode: two plates and three grids (5 points of electrical charge).

 

Anyway, to summarize: the two plates sitting at different voltage values causes an electrical connection and current flows.  Placing a grid between those two plates means the output plate's response is "imprinted" with the signal (music) applied to the grid, resulting in amplification.
 

Tube Rolling

Tubes were manufactured using mid-20th-century production techniques.  In many cases, hand labor was significantly involved.  Production processes also varied widely from manufacturer to manufacturer: the shape of the plates, the type of grid, the glass envelope, etc.  The result is that most tubes of the same tube type might vary noticeably in sound signature.  Further, so many tubes were made that many types are sometimes sufficiently similar to other types so that they can be substituted in the same [amplifier] circuit.  The resulting difference in sound signature may be even more distinct than just manufacturer differences.

 

Over time, swapping out different brands of tubes - or further - different types of equivalent tubes, resulted in sonic changes that were somewhat unpredictable.  Putting a positive spin on it - factors of different manufacture, different production dates, and different tube designs all meant that "improvements" in sonic performance might be possible.  Thus, tube "rolling" was born - meaning the practice of swapping in and out different manufacture tubes or different equivalent tubes to see if the sound performance [of an amplifier] could be improved.

 

Tube Dealers

Google "vacuum tube dealers" or look on ebay.  Just an FYI - ebay is uncharted territory.  You are just as likely to find a $5 tube selling for $50 as you are a $10 tube selling for $5.  One way to protect yourself on ebay is to reference the price for a particular tube type with the prices offered by the tube dealers that you found from Googling.

post #3 of 7
Thread Starter 
Thanks for the response tomb! That clears up everything I asked for, but is there anything else I should know regarding tubes? Any references of manufacturers or something? It all sounds kind of random to me.

EDIT: Are the "6922"s and "6DJ8"s etc. class distinctions? A lot of brands have the same name, just different brand names.
EDIT 2: What are "d-getters" and "halo getters"?
Edited by 7Sevin - 11/6/13 at 6:41pm
post #4 of 7

To add to that, in the headphone culture, there are basically three types of tube amplifiers:

  1. OTL
  2. Tube-Transformer-Coupled
  3. Hybrid

 

In OTL, the amplifier has been designed to block the high voltage needed for the tubes (100-300V, typically) by using coupling capacitors.  I can't go into electronic theory here, but suffice to say that the combination of the coupling capacitors (they block the high DC voltage and only allow a music signal to go through) and the headphone impedance results in what's known as an RC circuit.  It means that the bass frequencies will be filtered out if the impedance of the headphone is low or if the coupling capacitors are too small.  Further, only the output of the tubes is available to power the load.  Since tubes need high voltage compared to headphones, the pre-disposition of the amplifier is to supply voltage to a high-impedance headphone.  This type of amplifier is limited in supplying current to a low-impedance headphone.

 

In Tube-Transformer-Coupled, you have transformers on the output of the amplifier.  The transformers take the high voltage from the tubes and "transform" it down into an ideal combination of voltage and current that the headphone needs.  The drawback is that the transformers are sometimes very expensive for high-performance and it means that they must be made with many different impedance taps if you are to able to use all kinds of headphones with the amp.  Typically, a transformer-coupled amplifier allows for a High-Z and Low-Z (impedance).  This is usually optimum for 32 ohm Grados vs. 300 ohm Sennheisers (or some other brands at those impedances).  Headphones with impedances in-between those values or outside of them, the amplifier is performing in a compromise, at best.

 

In Hybrids, tubes are used for the signal amplification, but a solid-state output "buffer" (transistors) is employed to provide the necessary current for low-impedance headphones.  Ultimately, the solid-state buffer is the last item in the signal line, so the sound signature often ends up sounding solid-state with perhaps a "tad" of tube "flavoring."  Also, sometimes the power scheme for a hybrid can be quite complicated.  It's easy to see this by virtue of the high voltages needed for the tubes versus the very-low voltage and high-current needed for the solid-state output.  The power supply complexity will dictate whether the tubes have enough voltage to run at their optimum (100-300V), while at the same time powering the solid-state devices with enough high-current, low-voltage (12-30VDC) power to make the most of the solid-state devices.  It's not a trivial problem to provide a hybrid design that can do both well.  Many hybrids take a drastic shortcut and just supply the tube(s) with the same voltage that the solid-state buffer needs.  The result is that the tubes may be running 100+V or more below "what they want."

post #5 of 7
Quote:
Originally Posted by 7Sevin View Post

Thanks for the response tomb! That clears up everything I asked for, but is there anything else I should know regarding tubes? Any references of manufacturers or something? It all sounds kind of random to me.

EDIT: Are the "6922"s and "6DJ8"s etc. class distinctions? A lot of brands have the same name, just different brand names.
EDIT 2: What are "d-getters" and "halo getters"?

6922's vs. 6DJ8 are those functional equivalents that I mentioned.  The ratings and operating parameters for those two tube types are so close that they are regarded as the same.  Sometimes, the numbered differences may just be the fact that one number was manufactured in Europe and the other number was manufactured in the United States.  Or, numbered differences may mean a legitimately different tube.  Only a close inspection of the tube's data sheet will tell you for sure.*

 

I use the distinction of "getters" myself to distinguish tubes.  Even in the same manufacturer, there may be differences in plate construction and getters.  (Getters are the square or round metal pieces suspended by a wire up near the top of the tube (in most cases).  They have a chemical imbedded that when heated to very high temperatures, "flashes" over onto the glass as a silver coating.  That coating is a catalyst for gas molecules that may have been left in the tube when the vacuum was pulled.  Anyway, getters can be a square-shaped wire, a ring, a circular loop (rings or loops are often viewed as "halos"), or even a small metal plate that might look like a flying saucer.  (There are still other types, too.)  They're pretty much the most visibly unique characteristic of a tube.  So, it helps to identify different tubes by their "getter."  Note that the "getter" has no effect on sound signature AFAIK, it's just a distinguishing characteristic that helps people know that the rest of the tube structure may also be different (and that may very well mean a change in sound signature).

 

* However, there are plenty of cross references for tube numbers on the Internet that quickly tell you which tubes are all equivalent for the circuit.


Edited by tomb - 11/6/13 at 7:04pm
post #6 of 7
Thread Starter 

OK, thanks again tomb!

 

What about voltage of tube amps? What if I were to pair the Schiit Lyr with the Audez'e LCD-2. What voltage would be appropriate for the Lyr +LCD-2 combo? The LCD-2 is a 60 ohm headphone and the voltages available for the Lyr are 115 and 230.

post #7 of 7
Quote:
Originally Posted by 7Sevin View Post
 

OK, thanks again tomb!

 

What about voltage of tube amps? What if I were to pair the Schiit Lyr with the Audez'e LCD-2. What voltage would be appropriate for the Lyr +LCD-2 combo? The LCD-2 is a 60 ohm headphone and the voltages available for the Lyr are 115 and 230.

Sorry, I guess I didn't explain this well.  115VAC and 230VAC are the input voltages to the amp from the wall supply.  115V is normal for house power in the US and Canada, whereas 220/230VAC seems to be in use internationally.  What you're seeing on Schiit is only that their amps are made to take both US and International voltages.

 

The voltage that runs within the amplifier can be anything, almost literally - it depends on the design.  When I refer to voltage within the amp, it's usually meant to be the "voltage swing" available in the amp.  A music signal resembles a sine wave, although with many more peaks and valleys.  To re-produce or amplify a sine wave (the music signal), you need to be able to supply a + and - voltage.  The difference between those + and - points defines the amplitude of the wave, thus the sound level.  Go beyond that voltage difference and the amplifier will clip, with huge distortion as a result.  Once you are out of the power supply and into the amplifier circuit itself, the voltage difference output from the amp's power supply will define the limits of the amplifier circuit before it clips.

 

Just an FYI, but the reason the CMoy (world's first portable headphone amp?) was created because for many years, the Sennheiser HD580 and HD600 were the best headphones in the world.  Yet because of their high impedance (300 ohms), the new wave of Walkman portables and later, the iPod, had insufficient voltage swings to fully power those headphones.  Chu Moy came up with a simple headphone amplifier circuit that was powered by a 9V battery.  The 9V battery offered +or- 4.5V available to the amplifier circuit, whereas most portables at the time were limited to 2xAA batteries, providing a voltage swing of only +or-1.5V - or worse, a single AA battery with only +or- 0.75V.

 

Voltage swing is more important as the impedance of a headphone increases.  Current impulse capability is more important as the impedance of a headphone decreases.  At 60 ohms, the LCD-2 will tend toward wanting more current than voltage, which is really what the Lyr is designed to produce.

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