Do valves get “tired”

[HoverJohn]

I just read from another thread about valves getting weak/tired so what’s this all about?? So I pulled all the valves in the pre amp and amp and bunged in the cheap new set of spares and it sounds much better. I possibly should have done them one at a time but I suspect the main effect was the EL84 pair. The unit is notably louder and less distorted when cranked up.
Anyone else played with this slightly counter “original valves only” issue?
The old valves were all 1956 Gec Phillips and Mullard the new ones are new Mullard /Phillips and the el84s are brand new reissue sovtec.

The tone has improved and the tinny sound gone. The amp is doing more as there is now a slight hum of POWER.

Or have I randomly fixed something else eg how does one valve of a pair gone manifest??

[agardiner]

Valves lose emission over time as the cathode wears out. This gives the impression of them becoming 'tired' where they still work, but no longer to their optimal efficiency. Depending on the application will depend on how noticeable/important this is.

[G6Tanuki]

Yes, theydo get 'tired' - specially if worked hard

The usual thing is that the emission from the cathode falls away - meaning that the peak current the cathode can emit is reduced, causing the valve to behave less-linearly as the maximum output-power is approached - the tired valve will 'flat-top' earlier than a new one.

[Thisis particularly an issue in amateur radio transceivers/linear-amplifiers that use valves lije the 6146 ir TV line-output valves: SSB operation invilves a very peaky waveform and if the valves have been worked hard the falloff in emission can lead to a significant increase in intermodulation-distortion on the peaks].

[paulsherwin]

'Tired' is just casual slang for a valve that has seen a lot of use and isn't performing as well as it once did. Low emission is the usual problem. Tired valves can soldier on for years, depending on the circuit.

[HoverJohn]

Ok I better find a full set fit the radio module too then as they would have all been on for the same duration. It was the only gram they had from 1956 until it stopped playing about a decade ago!!

Is there any way to know which ones will wear out??

[G6Tanuki]

It was a well-known thing with the first AM/FM radios that the valve(s) in the FM front-end would get tired because the radio spent most of its life tuned to MW/LW and the FM valve(s) had their heaters powered but no HT so they developed a resistive layer in their emissive coating - "Cathode poisoning".

A similar effect troubled the first dual-standard TVs because they cut the HT to the UHF/625-line tuner when you were watching Band-I/III VHF BBC1/ITV. If you perhaps wanted to watch BBC2 on UHF some occasionally-worthwhile content you'd often find the result grainy and unimpressive because the poor PCC189 [or whatever] in the UHF tuner had spent hundreds of hours with the filaments on but no HT and had therefore decided to have a sulk.

[paulsherwin]

In addition, the more stressed valves will usually fail first - output and rectifier. Of course, these may have been replaced in service, so may have low hours.

Tuning indicators (EM34, EM80 etc.) had relatively short lives and were usually not replaced in service, as they are largely cosmetic. That's why they are hard to find today.

[PJL]

You can identify low emission valves by taking voltage readings. When they are 'tired' they will not conduct as readily.

[Lucien Nunes]

As mentioned above, the degree to which reduced emission affects performance depends greatly on the application of the valve and design of circuit. Audio output valves start to reveal their decline fairly early if pushed. Small signal valves often less so. The 174 oscillator valves in my Miller organ are all quite 'tired' with cathode poisoning, as they run most of the time without HT. But the organ works well and the difference between an existing valve and a new one is inaudible, until the valve gets so low that it fails to oscillate.

While most people today associate valves with radio / TV / audio and most forum dialogue concerns these fields, there were many applications of valves that had nothing to do with sound or picture. Industrial applications such as process controls, instrumentation, computers, variable speed drives, spot welders, lift controllers etc. Usually conservatively designed and often with the valves inside feedback loops, emission could drop away to quite low levels with minimal adverse effect on system operation.

[Ed_Dinning]

The Mullard HSVT was a good revenue source for radio/ tv shops where "valve testing 6p" was often advertised.
These would show low emission IF valves that would normally soldier on for much longer as they were in low stress parts of the circuit, but now would be replaced

Ed

[frankmcvey]

It's a well-enough known phenomenon in guitarist circles. I have a drawer-full of pulls of 6V6s and 6L6s from some of my amp customers who replace their output valves at least annually if they're gigging hard. There's little volume loss, but they complain that the amp isn't "singing" like it used to. I have to say that after nearly 40 years of working on jet aircraft, my old ears find it hard to tell the difference!

[HoverJohn]

Well I have a right ********** pair if you want a duff set of el84s to listen to!!

[Sundog-]

Another failure mode.

Cathode Interface.
From an article originally published in Electric Radio magazine, Issue 54, October 1993. The article, "Thermionic Mysteries", by Ray Osterwald, NØDMS:

Cathode interface resistance caused circuit problems for years, and was a direct result of the manufacturer's efforts to produce high-emission cathodes at low cost. This effect is caused when a thin resistive layer forms between the cathode's base metal and its barium-oxide coating after the tube has been in service for some time. When the resistive layer expands, a capacitor is formed. The base metal becomes one plate, and the coating the other. In a new tube, the interface resistance is nearly zero, but over time it can increase to as much as a few thousand ohms. The total capacity can approach .01uF. This capacity acts as a load on circuit constants, and partially explains why a particular tube might have differing performance in different pieces of equipment. The newly-developed resistance and capacitance has an associated R-C time constant, which will affect an amplifier's performance by changing its rise time and frequency response.
When a fast rising signal is applied to a stage having cathode interface resistance, the interface capacity acts as a direct short across the interface resistance for a short interval. The resultant RC time constant causes a loss of low frequency gain, while the high frequency gain is nearly unaffected. A fast-rise square wave will show cathode interface problems as overshoot on the leading edge. It is most noticeable in broad-band amplifiers and in circuits where plate current is cut off for long periods. A typical broad-band amplifier is found in the vertical channels of your trusty tube-type 'scope. A tube that spends long periods under cut-off might be found in transmitters, noise blanker pulse formers, or other equipment using gating and multivibrator circuitry.

[agardiner]

Interesting stuff. Perhaps this helps to explain the problems with the likes of ECC85's in VHF front ends. We have all had them where some work and some don't. Pushing the valve to the limits of capability and then adding in stray capacitance and additional resistance has the potential to play havoc with the oscillator.

[Glowing Bits!]

I have an old Pioneer tuner amp from 1967 with it's original valves, the AM one had to be replaced, the four EL84's are working in a fashion, they are badly mismatched, the volume now has to stay under half way or it hits distortion pretty damn quick!
One day I'll replace the outputs with a new matched quad set.

[Sideband]

Originally Posted by HoverJohn

Ok I better find a full set fit the radio module too then as they would have all been on for the same duration. It was the only gram they had from 1956 until it stopped playing about a decade ago!!

Is there any way to know which ones will wear out??

Don't get carried away changing valves just for the sake of it. As stated, most valves can soldier on for years before any noticeable degradation in performance. A lot of my radio's going back to 1934 still have some of their original valves fitted (some still have all of them). Rectifiers and output valves (the ones driven hardest) tend to fail before the others. Simple voltage checks usually reveal the dud ones.

I don't suspect valves initially when a radio doesn't work as well as it could. Make other checks first.

[Bazz4CQJ]

I remember when I first got my hands on a valve tester, taking valves out of working equipment to test, just out of curiosity. I was surprised to find quite a few valves that were very well down on their spec that seemed to be working quite adequately. A valve has to be very tired indeed before I dump it.

B

[emeritus]

I have mentioned here in the past how, when a young man with little practical experience with valves, I brought a relative's valve radio back from the dead by connecting a silicon diode in parallel with the rectifier valve. It was an AC/DC model where the 240V section of the dropper had gone o/c, so the set had been operated using the 200V setting. The diode was fitted after replacing the failed dropper section to no effect. Briefly switching on then resulted in audible crackling and visible sparking inside the valves, but this did result in the faint sound of music from a station from what had been a completely silent radio as the power drained away after switch-off. Successive brief switch-ons progressively increased the volume, and when no further increase happened, I removed the silicon rectifier and gave the now-working radio back. I don't know how much longer the radio lasted, but I guess the abuse of applying full HT before the heaters had warmed up must have blasted away the low emission surface layers.

[David Simpson]

Sundog & emeritus have given us great info regarding tired valves & particularly the physics & chemical composition of cathodes.
In general, all the thread replies have hit the nail on the head regarding the term "Pulls", and why boxes & boxes of pulls(particularly TV valves) keep getting discovered in lofts, sheds & cellars when sadly someone passes away or are "silent keys". A valve that might be responsible for poor TV picture quality &/or sound could sometimes have a second career in bog standard domestic radios, or homebrew AR gear.
Go to any junk sale, (& even, I suspect at times - internet sale sites), and you'll see a pristine"NOS-looking" valve box with a desirable ECC82 or E282CC valve inside, & even sometimes a KT or EL, etc. Only to get it home & find that its pretty duff ! This is because the old radio/TV repairmen of yesteryear put replaced valves back inside the box that the new valve came in. A "Steptoe" sort of attitude.

Regards, David