3. TIPS FOR THE SAVVY TUBE-MEISTER
3.1 What is 'tuberolling'?
'Tuberolling' refers to the process of swapping a tube, listening, and then swapping again in order to determine your favourite tube.
The moniker 'tuberolling' comes from the gentle circular rocking motion that is used to properly remove tubes from sockets. Remember - this is NOT a twisting motion (like turning a dial) but a gentle side-to-side rocking motion. If you yank a tube upwards or twist it like a dial, you could possibly break the glass envelope right off the base!
3.2 Why do different tubes of the same type sound different?
There are literally an infinite amount of factors that can affect the electron flow from cathode to the anode plate, which means that there are an infinite number of factors which directly affect how a tube `sounds'.
Here are some obvious factors: grid wire material, grid wire coil pitch, grid wire gauge, distance between cathode/plate, plate material, plate design, plate coating, glass coating, wiring, heater placement, triode separation distance, distances between tube parts which affects the capacitance between them, heat dissipation rate, etc etc...
To sum up, as the different tube manufacturers each had their own way of manufacturing the same tube, it can be expected that there would be variations in how tubes of the same type sound.
3.3 What are NOS tubes?
NOS stands for New Old Stock. NOS tubes are generally defined as tubes that were made before 1970, and more importantly, never used. They're old stock, but new - hence New Old Stock.
3.4 $3000 for a single tube??! Why are some NOS tubes so expensive?
engraved base WE 300Bs (circa 1930s) - people pay BIG $$$ for these
The short answer - supply and demand.
The long answer - Some tube amps are quite old, and their owners may prefer to maintain a completely `vintage' setup. Others believe NOS tubes sound better. Others are simply sheep with too much money to throw around who jack up the prices.
The NOS tube supply by its very nature finite - and dwindling each day. Some NOS tubes are very much sought after and are hence in particularly short supply. All these factors contribute to the much higher prices of NOS tubes when compared to plain new stock tubes.
You may want to consider what types of tube an amplifier uses before purchasing it - this WILL more or less determines the scale of your future expenditure when re-tubing the amplifier.
3.5 Do NOS tubes really sound better?
NOS tubes are reputed to sound much better than new stock tubes. The author of this guide believes, to a certain extent, that this is true. There are many theories floating around as to why NOS tubes sound better. I have cherry-picked a few that I am certain have at least a grain of truth in them.
1. The 1930s-1950s were the golden era of tubes. Equipment used to manufacture tubes and more importantly, skilled people to operate them were plentiful. All of these have been lost with the advent of the transistor and automation.
2. Tube R&D has come to a virtual standstill since the 1960s.
3. When compared to transistors, tubes are relatively expensive and time-consuming to manufacture (to achieve a good vacuum for a SINGLE tube, some vacuum pumps run for as long as 3 days!). In the highly competitive niche market for new stock tubes, quality may not be the top priority anymore. One amplifier manufacturer has been quoted as having to reject 1 in 4 6AS7 tubes for bundling with their amps due to unacceptable levels of microphonics/hum/channel imbalance!
Even then, it is strongly recommended that you try at least a few tubes of recent make before putting down some serious money for NOS tubes. Typically for the price of a single NOS tube you would be able to obtain at least 3-5 new stock tubes. You never know what you might stumble across...
3.6 Matched sections vs matched tubes
While purchasing tubes, you may come across the terms ‘matched sections’ and ‘matched pairs’. The meaning and implications of these two terms are covered in detail below.
Matched sections
There can sometimes be more than one active amplification element within a single tube. For example, there are triodes and then there are twin triodes - the 6J5 being an example of the former and the 6SN7 being an example of the latter. In other words, the 6J5 is virtually electrically identical to half a 6SN7.
With multiple sections (triodes, tetrodes, pentodes, etc) within a single tube comes the problem of variation. As a result of material and construction variation, there may be slight mismatches between the two sections of a twin triode like the 6SN7. Or the two sections of a twin beam tetrode like the 6DY7. Or the two sections of a twin pentode like the 6MK8.
Using a suitably equipped GM tester, the 2 sections within a tube can be tested and have their amplification ability measured. As a perfect match between 2 sections is exceedingly rare, a 5% match is the usual ‘gold’ standard with 10% and 15% being lesser grades of matching. This is the meaning of having a tube with ‘matched sections’.
Remember – knowing that a tube has ‘matched sections’ is useless without knowing how close percentage-wise they are matched! One retailer’s ‘matched’ may be 5% while another’s may be 10%!
Matched tubes
Matched tubes take the concept of matching a step further.
With matched tubes,
all the sections in
all the tubes to be matched must fall within a certain percentage of each other. For example, if we want to have a matched pair of twin triodes, it would mean that we would need to match 4 sections from 2 tubes. If we want a matched quartet of twin triodes, it would mean that we have to match 8 sections from 4 tubes.
Confused? Here’s an example to help you sort things out:
• Imagine we have 3 twin-triode tubes: A, B and C.
• Tube A has GM results 98/95, tube B = 96/97 and tube C = 62/60
• ALL 3 tubes have sections matched to 5%. Hence all 3, when sold
individually, can be advertised as having matched sections.
• But ONLY tubes A and B can be sold as a matched pair as all their sections match to within 5%.
As you can see, matching tubes can be a challenge - which is why vendors typically charge you a (sometimes hefty) premium for matched tubes. This premium increases sharply with the number of tubes that need to be matched. Ouch!
But wait - why do we even need matched tubes for amplifier use?
In general, only power tubes need to be matched. Rarely will you come across an amp that requires its preamp tubes be matched. Many amplifiers are designed to require power tubes that have similar cathode currents. Even tubes of the same type made at the same time will have variations in their cathode current, and this is why testing and matching is required.
While few amplifiers actually need matched tubes, it never hurts to have them. Please consult your manufacturer to determine whether your amp needs matched tubes.
3.7 Buying and selling tubes
Buying tubes
As with anything out there in the world, buying tubes involves 2 words – caveat emptor (‘buyer beware’). There are literally millions of tubes out there, and there may be hundreds of versions of a single type. Below I have attempted to summarize some things you should keep in mind while buying tubes.
Do ask for photos of the tube, paying special attention to:
1. Mica shape
2. Plate shape and color
3. Support rod pattern on mica (if applicable)
4. Glass color and proportion of color (if applicable)
5. Base material and color (if applicable)
6. Label or glass prints – size, font and color
7. Date codes (if applicable)
Do your homework. Know what the tube you want looks like. There can never be enough research with tubes. It's a cruel world out there and there are people who counterfeit tubes - however it is impossible for them to get everything identical. Once armed with prior knowledge, the battle is half won for you.
Once in a while (if you're lucky enough) you might stumble upon relabeled tubes. What are relabeled tubes? They are tubes made by a particular manufacturer but with another manufacturer’s labels printed on. Something like OEM tubes. Refer to section 3.8 Relabeled tubes for more on this.
Some more buying tips:
• If the tube does not have a top getter flash, but there are tiny getter 'splotches' on the top, this indicates that the tube is used. Some filament metal evaporates with use and usually condenses above the heater openings.
• Are black or ‘rainbow’ getter flashes bad? Most certainly not! In fact, black getter flashes are better than shiny silver getter flashes. Getter particles, being more finely dispersed, absorb light instead or reflecting it. This also means that black or ‘rainbow’ getter flashes are better at absorbing gas than shiny getter flashes. You can sometimes see a rim of ‘rainbow’ with shiny silver getters flashes.
• Even if a tube has black glass, it still is possible to observe the plate structure. Simply shine a powerful torchlight at the glass and you should be able to see right through it. This is useful if you are unsure whether 2 particular tubes with black glass are the same or if you have a couple of unlabeled black glass tubes and can’t tell them apart.
Reselling tubes
Anyone can get into the business of reselling tubes - especially NOS tubes. Here are some useful pointers on factors that increase or decrease the value of tubes.
The following INCREASE the value of a tube:
• Having closely matched sections - within 5% is usually the most desirable
• Having other tubes with similarly matched sections (to make matched pairs, trios, quads, etc)
• Being NOS
• Intact labels
• Intact bases (or having no rust in the case of metal bases)
• Having nice shiny opaque getters
• having military or original boxes - these vastly increase value. If you are taking a box from another source and packing your tube in it, it is
unethical to not mention so.
The following DECREASE the value of a tube:
• Sections that differ by >15%
• Previous usage or a hazy history (ie unsure whether tube is used/new) having no boxes or white boxes
• Having no labels
• Cracked or rusted bases
• Lacking a guide pin (when applicable)
• Faint or translucent getters (indicates getter is partially used up)
• Metallic splotches above the heater openings (indicate tube is used)
3.8 Relabelled tubes
Separated at birth? The story of a Ken-Rad JAN-CKR-6SN7GT and a Tung-So16SN7GT...
Do note that both tubes have...
• Black bases
• Copper grid posts
• Bottom getters in similar amounts
• Blackened glass in similar proportions
• Glass envelopes of the same height (not one tall-bottle and one short-bottle)
• And most incriminatingly, they both share the exact SAME 'staggered' grid post & cathode heater pattern on the mica plates!
They are the same tube!
3.9 Tube tweaks
Many people believe in getting the maximum from their tubes, and why shouldn’t they? The following tweaks are of the ‘last 5%’ variety, and should best be considered after your setup is complete. All the tweaks listed below are also linked in the Tube Links chapter.
Cryo-treating tubes
Tubes are immersed in liquid nitrogen for extended periods (up to 5 days) and then allowed to warm up gently. Proponents claim that this allows the metal atoms within the tube to realign themselves and stresses within the tube to relieve, extending tube life and helping the tube sound better.
Tube Dampers: Top Hats, Hal-Os, silicon and sorbothane rings
Tube damping is a tweak that definitely works.
Microphonics (the conversion of mechanical movement to measurable electrical phenomena) is a well known problem with tubes. All tubes are microphonic – it only becomes a problem if they are audibly so.
If you can ‘hear’ your tube without any music playing, it’s microphonic. Microphonics may take the form of rippling sounds, ringing noises or hum and can be set off by something as trivial as walking across the floor. Remember – never tap your tube to test for microphonics – if it’s really microphonic, you will know without needing to tap it.
Tube dampers help rectify this by either damping the mechanical movement (vibration) of tubes or by lowering its resonant frequency by adding mass. Either way, microphonics are lessened.
A good tube damper has excellent damping ability, has its physical properties remain constant even with the application of high heat for extended periods and does not cause the tube to retain excessive heat. There are dampers that offer various combinations of the above 3 criteria at many price points.
Tube Heatsinks
Tube heatsinks claim to extend the lifespan of your tubes by helping them dissipate heat. Tube heatsinks come in 2 forms -
1. Glue-on: a thermally conductive epoxy is used to literally glue heatsinks on. Rarely used as they
reduce the resale value of tubes - like diamonds, epoxy is forever.
2. 'Jacket' types: literally a slip-on heatsink for your tube. Think prophylactics.
A good tube heatsink is always matte black in color. Black surfaces radiate heat easily. Avoid heatsinks that are silver or shiny in texture – they may actually reflect heat back at your tube, making it hotter! Always locate your amplifier in a well ventilated place – otherwise your expensive heatsinks might just end up helping your tube retain heat.