• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Extending Life Expectancy of Power Amp Tubes like EL34 by which kind of Switch-On Variants - which one is most useful ?

Until now I have found the following switch-on variants in commercial tube pre-/power-amplifier devices:

1) Separate switching on of heating and anode voltage (in case of present electronic control units, the anode voltage is switched on approximately 5-10 minutes later than the heating voltage - this is from my view no real soft start)

2) Regulated DC voltage for the heating rail with integrated soft start function - go to
Gerhard Haas (experience electronic) basically implemented it in various of his models

3) Simple series resistor in the heating rail, which is shorted manually by switch or electronically after a certain time (but the question arises here as to which voltage on the heating filaments is ideal for warming up)

4) Soft start in the 230VAC supply (this I know only in amplifier devices like e.g. Krell KSA250 as inrush current limiter, where the toroidal transformer has an extremely small DC resistance on the primary winding and blow away the house fuse when switched on without an additional soft start unit - thus I doubt the benefit in terms of extending the lifespan for audio tubes).

Some guys claim, that even the versions from 1) to 3) has no real influence on the life expectancy of any tube and that such steps can be completely dispensed with (but this confuses me, because the cold (on-) resistance of the heating coil is significantly smaller than the resistance at operating temperature in the glowing state and therefore also the inrush current is correspondingly high).

Thank you very much for information and calling URL's to specialist articles or corresponding specialist essays (e.g. from Tube CAD).

P.S.: I started this thread after a friend who uses the Compact 100 from VTL (at whole 8x EL34 for power stages) was recommended by a modification/refurbishing center to retrofit a warm-up device to significantly extend the lifespan of these power amplifier tubes.

PS-II: Here I compile the URL's of the most interesting articles and descriptions posted during this thread:

Extend the useful life of those precious tubes: use a Tube Amplifier High-Voltage Delay
Techniques To Maximize Power Tube Life
The Internal Life of Vacuum Tubes
High Voltage DC Time Delay Copyright © 2015 - Rod Elliott
Consequences of heater standby voltage of 4V5
Getting the most out of VACUUM TUBES, by Robert B.Tomer, published in 1960
http://www.tubebooks.org/Books/Atwood/Tomer 1960 Getting the Most Out of Vacuum Tubes.pdf

Threads here on diyaudio:
1) Anode voltage (higher, lower and value for time delay after switch-on - maybe with inrush current limiting)
2) Heater Voltage (higher, lower, inrush current limiter)
3) Loss Power
4) Lifetime of Tubes/Valves
5) Tube Design Idea (Lifespan Extension by reducing heater voltage without anode voltage)
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Amplifier tubes with plate voltages < 2000Volt has no need for delay or soft-start.
Delayed B+ where voltage is turned on when filaments are hot will affect tubes negatively and should be
Filament failures in general is very scarce, mostly occurs at arrival from factory. Note that in a typical amp
filements has low resistance when cold, thus draws a lot more current initially which in a typical
power supply results in a ramping up voltage during < 1s

House fuses might need some current limiting at power on for huge amps.

There exists a lot of misconceptions in this area.
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From my personal experience, tube failure in amps are more often due to a bad design of the circuits or too severe working conditions.

For example, on a Blackstar guitar amp using 2xEL34 at the output stage, these tubes have their filament connected in serie. Regularly, one of the filament cuts at the start, when voltage is unbalanced between them. The EL34 has not been designed for serial filament operation : it is a problem of design.

Similarly, many EL84-duet powered guitar amps claim to put out 25W or even 30WRMS output. When you check their idle setup, you immediately understand why these poor tube are unable to last very long, and be prone to failure : too severe working conditions.

Assuming the circuit/components are stock, then I would be prioritising other areas with long term reliability in mind. Bias failure comes to mind, so I'd be rebuilding that with an improved circuit (see below) and better components (multi turn trim pots). Perhaps also adding fuses should failure occur, to prevent things going into melt down.

I used to have an vintage AU-111 which had the caps replaced and a new better bias circuit. It used solid state diodes for the 500v HT, no delay, no soft start etc. After 1000's of hours and daily on off cycles the JJ 6L6GC'S all still tested like brand new; that said their dissipated was limited to around 15watts.

Your mileage may vary; I could have been lucky.

I still haven't located that article, but I found another one that may actually be more applicable:

K. Rodenhuis, H. Santing and H. J. M. van Tol, "The life and reliability of valves", Philips Technical Review, vol. 18, 1956/1957, no. 7, pages 181...216, 15 December 1956

I think it is on the Internet somewhere without paywall.
Extend the life of output tubes?

Stay within the data sheet limits.
Some data sheets specify that you only use one limit at a time, not 2 or more limits.
Design a Hi Fi amp/Stereo amp, do not design a guitar amp that has to fill a stadium (using output tubes that were not made for that much power).

Just in case, I use both a fast blow fuse that can take the power-on inrush current,
and a slow blow fuse that can take the quiescent, medium output power, and maximum output power of the amp.
Power Mains is connected this way: Hot line, fast blow, slow blow, power switch, power transformer primary, Neutral line.
Per the different currents of power-up inrush versus warmed up operation, the fast blow is rated for more current than the slow blow.
I hope you get the idea.

The fast blow fuse may pop open if you have a Hot Start Event. That is OK. Better that, than risking your tubes.
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What I remember from the article I can't find is that the main thing was derating the valves, that is, stay well below the ratings. Also design the circuit such that small parametric shifts don't upset the circuit much. Slowly turning on the heaters was done as a common sense measure, but there wasn't much (if any) evidence that it actually helped.
Nowadays, no one can confirm whether a particular 'switch-on variant' has an improved service life than any other, as no one can or would perform statistically significant testing. One off examples, or even small batch testing is not statistically valid.

The only valid testing has been presented for devices like the ENIAC, and they used special tubes and special powering systems. Similarly, MIT tried to minimise valve stress and failures in early valve computers such as the 'Whirlwind 1'. "Thermistors for the gradual application of heater voltage to thermionic tubes" by Gano & Sandy, 1958. The only known reference to valve heater filament failure statistics related to turn-on stress. Gano did his Master's working on Project Whirlwind 1, and then continued to work on power coordination with Lincoln Labs. Thermistors for filament cycling was a project by Sandy starting in 1955 (9 and 23 Sept and 4 Nov 1955 biweekly reports).

Rodenhuis paper link: https://dalmura.com.au/static/The Life and Reliability of Valves-1956.pdf
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There is quite a lot of good stuff in this Ericsson document describing how to eke the last 10.000hours out of their long life tubes ...

It can be seen that heater overvoltage substantially reduces lifetime while deficient heater voltage leads to unsatisfactory operation during a relatively long initial operating period. As shown in curves C and D, behaviour from tube to tube varies considerably even when operating at equally deficient voltages.

Skärmbild 2023-09-19 222242.png

Operation with two or more heaters in series is not recommended. Deviation in heater current characteristics from tube to tube causes some tubes to consume too much power and others too little. Dissimilar warm-up times can also cause series connected tubes to be exposed to damaging overloads. These premonitions do not, of course, apply to tubes especially designed for operation in series.
Operation with two or more heaters in series is not recommended. Deviation in heater current characteristics from tube to tube causes some tubes to consume too much power and others too little. Dissimilar warm-up times can also cause series connected tubes to be exposed to damaging overloads. These premonitions do not, of course, apply to tubes especially designed for operation in series.

Yes. That's exactly what occurs in those Blackstar guitar amps with EL34 serial heaters. The EL34 have not been designed for serial heater operation : it is a design flaw from the circuit itself, done by guys who think they know about tubes...

How many times I see this ? Another example is on a MESA amp where a mere 5Y3GT fronts directly a 110µF entry filtering capacitance, where regular blowoff of that tube occurs : unadequate working conditions...

There is also the issue of the positive temperature coefficient of the resistance of the heater. The hottest heater has a larger resistance than its colleague. With a parallel connection, this leads to less dissipation, with a series connection, to more dissipation in the hottest heater. The loop gain is well below unity so you don't get thermal runaway, but still, it aggravates temperature differences to some extent.

K. Rodenhuis, H. Santing and H. J. M. van Tol also recommend parallel connections and a well-regulated heater voltage; either nominal voltage +/- 5 % or nominal voltage - 5 % regulated to within +/- 1 %. You can use a regulator with a current limiter to ensure gradual heater warm-up.
I have some real world datapoints that may be of interest to someone:
  • I bought a McIntosh MC240 in 1978, used but fresh out of the service clinic, and with a brand new set of tubes
  • The power tubes are RCA 6L6GC, nominally Ip = 72mA and Gm = 6mA/V
  • The power supply has a CL80 thermistor
  • I didn't like the sound, so I changed a few things; among others, I set the standing current to 66mA, for a dissipation of 28W/tube
  • The amp has a "normal" use
  • In 2012, 34 years later, I finally decided to replace the tubes
  • The original tubes look terrible, but they're still alive, sort of: Ip = 77, 76, 66, 50 mA, and Gm = 6.5, 6.5, 5.5, 4.5
I replaced them with NOS Philips ECG. Sadly, one arc'ed.
Now I'm running NOS Winged-C/Svetlana from St Pete.

I am not so sure that 'quality died' rather than the customer has changed their expectations. The DIY Audio tubes in the main are all the usual suspects, used in guitar amps. More often in discussions about a new amp I see people refer to 'current production' as being a criteria for valve choice, so the NOS continues to gather dust as a very few are prepared to exploit them.

The guitar fraternity have a different perspective on quality, and see tubes as consumer items that are shredded and replaced at service intervals.

Using robotic manufacturing methods, companies should be able to produce the most consistent, highest quality tubes ever. That is visible, I think, when buying matched quads of tubes that all lie much closer in tolerance than the best of the old days.

The unsung heroes are the Chinese manufacturers who have enabled the masses to experience tube amplification, and do seem to be prepared to promote different topologies. It feels to me that there is an ever increasing number of people who take a basic, cheap amp and 'pimp' it to extremes, getting hooked on the technology along the way.

There is something called the Great Britsh Valve Project, and they are currently refurbishing old Mullard machinery with the goal to revert to the glory days. Not sure if the Americans would agree that the spiritual home of tubes is the UK ;-/

Not all things built today are low quality, but across various industries, and many different kinds of products there has been a lowering of quality (many things, not even having to do with tubes).
One example is the Food industry.

I try and design all my tube amplifiers to use JJ tubes, from Eurotubes.com
I know the rigorous re-testing they do, and I worked with one of the designers of at least a couple of testers that Eurotubes uses (that worker and I used to work together at a major T&M company).

Years ago, when I was doing 2A3 and 300B amplifiers, those came either from China, Europe, or Russia . . . most of them were good.
One particular 300B manufacturer had problems with filament to grid shorting at power-on.
I also used NOS tubes back then.

I got a couple of NOS Philco 45 tubes, both of them played great for several hours; but when I turned one on the next morning,
It went very gassy, glowed purple, and oscillated (that was the only pair I had then, so the amplifier I built just before VSAC had to be rebuilt in 2 days into a 4-65A amplifier so I could at least show something.
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