Tubes/Thermionic Valves.. discuss them here

I may be over-analyzing things but is there a preferred method for changing valves regarding fingerprints? Do they damage the glass and would it perhaps be wise to wear cotton gloves?

Gordon.

Fingerprints on valves isn't a problem, unlike with Halogen lamps.
Halogen lamps heat up very quickly and extremely hot and fingerprints can prevent the glass bulb to 'expand' in an equal amount and the glass may break.
Not so with tubes, they don't get that hot nor quickly so the glass is 'safe'.

You may want to be carefull with the printing on some tubes.
In some cases the lettering comes off very easily.

I need to add some notes about additional letters meaning -

6n2p       - basic model of tube, expose more parameters instability and lesser lifespan;
6n2p-e    - tube with improved electronic characteristics , i.e. high(er) grade and quality, more stable parameters, longer lifespan ~5000h at least for this valve ;
6n2p-ev  -  same as "e"  but added increased tolerance to vibrations  ( meaning of letter V  or  B - in cyrillic approx. same pronunciation in russian  ), proposed to use in aviation or somewhere in military;
6n2p-ep  -  same as "e" but added increased resistance  to high density outer radiation ( R - radiation or  P - in cyrillic, approx. same pronunciation ) , proposed to use in military industry ( concerned with                 nuclear equipment ), i don't suppose these valves are using or have been used in russian nuclear bombs or have been removed from these ones for later trading ; 
                I think you'd better to try old tubes dated 60- 70  as they have better quality, or have been tested and approved by special  military quality control  and marked with special stamps ( really hard to                 find now );
                According opinions of some persons tackled with 6n2p even 6n2p-ev valves differ in terms of quality of sound depending on batch......

For example: some valves with higher heater voltage - 7V PCC88 are compatible with ECC88 tubes and as they say being used in conditions where equipment suppose lower filament voltage ( 6.3V ) these ones live longer. I use these type of old tubes in my MFXC amp.

a bit about tubes in hybrids.

There is two kinds of power (voltage x current) when we are taking about tubes.
The plate current & Voltage and the heater current & Voltage.
There is 3 voltages on a triode tube, grid voltage (very low, just a few Volts), plate Voltage (high Voltage) and heater Voltage.
The plate (or anode) current is determined by the plate voltage, the bias voltage on the grid (is NOT the same as the bias voltage of these amps) and the load provided by the CCS (constant current source)


The heater needs to heat the cathode so electrons can fly away from it easily.
In all the tubes used in most hybrids the filaments inside the tube sections are all 6V.
Also the 12V tubes (at least the 9-pin Noval tubes) all operate on 6V per heater.
In 6V tubes the heaters are in parallel (between pin 4 and 5) and in 12V tubes they are in series (between pin 4 and 5) making them 12V.
The fun thing about it is that for 12V tubes the heaters on one tube section is connected between 4 and 9 and from the other section between 5 and 9. So when 4 and 5 are connected together and 6V is connected between 4-5 and 9 that 12V tube runs on 6V.
In G1217 amps you can switch between these heater arrangements using a jumper. The Project Ember does it automatically.

Usually we speak of 6V tubes but in reality it is 6.3V (or 12.6V) . The currents at that voltage can vary depending on the type of tube between say 300mA and 1A in general.

The Pxxx and 7xxx tubes run on 300mA currents and are designed to be set in series with each other.
When there is 300mA flowing through it there should be around 7V over it (7.6V actually).

Light bulbs work longer when used on a dimmer but these things need to emit light and to get as much as possible light from it they sort of design the filament a bit too thin. More light (and heat) but shorter life span.
In electron tubes there are similar filaments but all they need to do is heat the cathode and don't have to be stressed nearly as much as light bulbs. As a side effect they also glow. This can range from orange glow to almost white light.

There are basically 2 types of filaments, those that take a relative long time to heat up and thus also to start 'working'.
Those are the tubes that last longest (filament wise).
When switched on it takes a while before you see them glow.

Then there are tubes that, when switched on, give a bright 'flash' and then glow softer quickly.
Because of this (intentional) behaviour the tube is much faster at its operating temperature.
The filament, however, draws much higher currents on start up.
This sometimes causes problems with the heater supply in some G1217 amps.
The heater voltage regulator goes in 'protection' and switches off and on again after a short moment to try again.
Some tubes keep 'flashing' because the flash is too short to really heat up (when the heater gets warmer the current draw gets less) and some heaters do get a chance to warm up and 'flash' a few times and then stay on.

These 'quick heater' tubes are more likely to blow a heater than the slow-on types, certainly when switched on and off very often during the day.

The output relay (that mutes the output during start-up) timing in G1217 amps is set slightly shorter than the time it takes for normal heaters to reach their operating temperature.
'Fast heater' tubes thus are of little benefit, when a quick 'on' action is required as the amp may work already but the sound is muted anyway.

You can run these tubes at a lower voltage and expect a longer life time (of the filament) BUT in reality it is never the heater the goes dead as it isn't designed like a light bulb. Tube filaments are basically under-dimensioned on purpose for longevity.
Tube filaments are thus designed to have a long life (longer than the cathode itself).
For this reason lowering heater voltages doesn't do anything for the lifespan of the tube itself as it is NOT determined by how long the heater will operate during its life span.

In all G1217 amps without the supercharger you will find the heater voltage is always slightly below 6.3V (somewhere between 6.1 and 6.2V depending on the heater current) but this is not because of longevity but has to do with the used heater voltage regulator which puts out 6.5V. This is lowered with a Schottky diode to the mentioned voltages.
7V tubes will thus always run on undervoltage (on their heaters) BUT the tubes itself will nNOT last longer.

6.3V tubes should not be run at voltages below 6V and above 6.6V. The tolerance of heater voltages is about +/- 0.3Vfor most tubes so with 6.1 to 6.2 volt one is already on the low side of voltages ensuring longevity of the heater.
But as mentioned a slightly lower heater Voltage says NOTHING about the longevity of the tube itself which, in my experience, is never determined by the heater giving up its ghost.
In all the years I have worked with tubes I never had to replace a tube because the heater was dead.
This doesn't mean it cannot happen and some people may have needed to replace a tube because of a broken heater though.
In G1217 amps (without Supercharger) all 6V and 12V tubes thus run at around 95% of their nominal voltage.

7xxx and Pxxx tubes have a nominal voltage of 7.6V actually tolerance of around +/- 0.6V so these run optimally between 7V and 8.2V.
Running of 6.2V is thus too low (runs on 80% in that case) and less electrons will leave the cathode.
Not a big issue for low voltage hybrids though as the tubes have very low plate currents anyway BUT it won't live longer.

Longevity of a tube is predominantly determined by the ability of the cathodes to emit electrons. This becomes slowly less over time.
Those remembering old CRT TV's and perhaps the 'green indicators' on tube radios know that the amount of light coming from them slowly dies with age. This is because less electrons leave the cathode and this is what determines the life span of the tube.
It is impossible to predict how much 'life' there still is in a tube, vintage, NOS (New Old Stock) or used nor for new tubes.
Even a measurement is still a point in time and ONLY says something about the emission at the point of measurement.
It could be the same after a year or usage or be almost dead.
When running tube heaters on too low Voltages the tubes actually are emitting less electrons to start with and also gets less over time.
Running heaters on under-voltages thus doesn't prolong longevity but actually shortens it as the point where too little electrons are generated is coming sooner !
This is why tube manufacturers have specified a minimal voltage (to ensure the cathode is hot enough) and a maximum (to avoid the heater from burning out).



First is the grid voltage. This is really only important for all tube amps running on plate voltages > 80V and on higher plate currents.
In low voltage hybrid amps the grid voltage is very low (between 0V and 1V depending on tube type) and is 'self regulating' in all these types of amps and cannot be set manually.
I just mentioned it because grid voltages in all tube amps determine the bias point and are of importance there.
In hybrids the bias voltage point is determined by the current source setting in the anode (plate) circuit and needs to be around half the power supply voltage to ensure maximum output voltage swing.
In high voltage designs the voltage swing is not (less of) a problem.

Then there is the plate voltage and its maximum plate voltage.
Most people think tubes are supposed to work on voltage well above 100V and wouldn't even operate on voltages below that.
All nonsense... they do work more linear over a larger voltage range though and thus has less distortion.
That distortion, however, is just what these hybrid amps (and their sound) is all about unless they have lots of overall feedback (think Musical Fidelity amps)
It is true that higher anode voltages make the tube perform better as they can attract the emitted electrons from the cathode a lot faster and thus higher bandwidths can be reached. For audio the bandwidth doesn't have to be much higher than 50kHz though.

Some tubes are even specifically designed to run of car battery voltages, like the ECC86/6GM8.
These have max plate voltages of 30V and thus cannot be used in the Ember and Horizon for instance (having a48V power supply).

The G1217 amps all work on either 24V (Starlight, Solstice, Sunrise) or 48V (Ember, Horizon).
Higher voltages aren't really needed aand are safe for open frame designs as you can touch voltages/components.
For indoor situations (not bathroom) is 60Vdc the maximum allowed voltage.
These hybrid amps are designed around are the 6DJ8/ECC88 type (and its many equivalents).
Their plate voltages are rated at 90V max (and run at 22V or 46V max depending on power supply voltages).
This certainly isn't too low as some think.
These tubes can still even amplify on 12V power supplies (5V plate voltage nominal) though not really optimally.
ECC88/6DJ8 types of tubes thus run comfortably below their maximum ratings and still work very well, even on 24V power supply voltages

Tubes like the ECC83/12AX7 are intended for 250V power supply volatges and indeed perform far from optimal operating points and may be quite noisy, but to some, the sound (being rolled-off well within the audible band in this case) is pleasant.

6SN7 tubes are designed for plate voltages of 450V and thus not really suited.
Given the fact that its sound is liked by many and even amps running on 24V do sound excellent shows that even 450V tubes can still do their job at 24Vpower supply voltages.
Not only the plate voltages is just 3% of its maximum voltage also the plate current is about 7% of its maximum so the total plate 'power' is even below 1% of what it is supposed to work on.

Very well written Frans . A + ! Word to the wise if I understand correctly. Spending an insane amount of money on even a NOS tube is not any guarantee of its longevity. It is really a toss of the dice, so to speak. Or did I misread this?

So if the tube is being operated correctly, then how long it lasts is mostly a function of the condition of the cathode and its ability to produce sufficient electrons to operate the tube. And there is no way to test the condition of the cathode. Seems from what you have said that an audible imbalance between sections may give a hint if the tube is going out, but an imbalance in Gm or mu between sections could have been there since the tube was manufactured and may say nothing about how much life a tube has left. So even a tube that has a high Gm or mu may not last long and a lower testing tube could last years. So in conclusion, a "strong testing" tube only has a high Gm or mu, and may or may not have a long number of hours left on it.
Professor Frans please correct my paper for inaccuracies.