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Old 26th Dec 2019, 9:11 pm   #41
Bazz4CQJ
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Default Re: "Burning - in" Double Triodes

Many thanks for that pointer Ed, it's a site which is new to me. I'll take a look through his list and see if I can find a promising title. He clearly has a lot of basic valve data material, and a lot of material on circuit design, so I may have go through the list carefully to find something on "the functioning of valves".

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Old 27th Dec 2019, 9:59 am   #42
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Default Re: "Burning - in" Double Triodes

Hi Bazz, Mullard in their day invested a huge amount of time and money in statistical quality control of the production process and knew exactly what any change in material or process would do to valve characteristics. This was lost over the years (unless anyone knows better); but there may be data in the patent office in the patents that were granted to them.

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Old 27th Dec 2019, 11:21 am   #43
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Default Re: "Burning - in" Double Triodes

I haven’t read the whole thread so this might have been covered but Tektronix burned in valves in vast numbers before fitting to equipment. This was to remove early failures and to make sure that selected examples and differential amplifiers had consistent characteristics at calibration time. If you had a new instrument with new valves and a differential stage then the burn in stage would age the valve to the point of least calibration drift and offset over time. If you didn’t do this your brand new tek scope would need recalibration after a week.

Heathkit also recommended it for the same reasons in the VTVM products. The differential amplifier would lose zero pretty quickly otherwise. I found this to this day fitting a cheap 12AU7 from RS in one.

Audio, probably doesn’t matter much. It’s all AC coupled and offsets are unimportant. All in the head not the ears.

Semiconductors are aged too. Particularly references! A good aged LM399 or LTZ1000 is worth a few quid!
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Old 27th Dec 2019, 11:31 am   #44
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Default Re: "Burning - in" Double Triodes

I to remember a thread some years ago mentioning burning in. From memory it was only about power output valves. It seems that after years, decades, of none use the nickel structure in a valve will absorb gas molecules. If the valve is then powered up then the metal outgasses and this can cause a runaway with the grid losing control and the valve possibly damaged. So the valve had to be run hot to get rid of this outgassing, for the getter to absorb. Only then was it safe to run the valve at full rated dissipation.

I did do some looking in books but so far not found any supporting evidence, too many books.

There was also some comment I remember about the Chinese etc power valves, why they can't cope with the original volts and amps and spit, they were not burnt it during manufacture and the cathode material would flake off. Reading original Mullard etc details of how valves were manufactured and they were seriously heated before and after after sealing to get rid of as much outgassed molecules as possible.

This was of importance to me because I had just acquired some Mullard EL34 xf1 valves, and didn't want to destroy them inadvertantly.
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Old 27th Dec 2019, 2:01 pm   #45
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Default Re: "Burning - in" Double Triodes

There's a paper here from CPI [who are the company which now owns Eimac] that talks about what needs to be done with RF power-tubes before first putting them in service:

"Conditioning of Large Radio Frequency Power Tubes" - https://www.cpii.com/docs/related/22/AB21.pdf

Along with "Extending Transmitter Tube Life" - https://www.cpii.com/docs/related/22/AB18.pdf

The whole series of papers published in their library is worthwhile reading for any power-valve fans! - see https://www.cpii.com/library.cfm/9
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Old 27th Dec 2019, 2:37 pm   #46
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Default Re: "Burning - in" Double Triodes

Sorry for the 'noob' question but are there any issues with power sequencing with new (or old) valves? The reason I ask is that I'd like to have a tinker with a 12AU7 after looking at the datasheet. I'm only going to run it at 60V HT but are there any issues with power sequencing? Is it OK to turn the heater on first and then the HT a few seconds later? What about turning it all off?

As these valves are expensive nowadays I'd like to minimise any stress on any valves I buy. I assume they will survive any sequencing method but if there is a preferred sequence for power on/off I'd like to try to adhere to it...

I remember buying valves like this for an old Yaesu FT200 back in the 1980s and they cost less than £1 each back then. I think for about a fiver I managed to buy quite a few (small) valves to swap out in the radio. I didn't really know how to test the valves back then so just swapped them out. The same valves today cost anything from £20 to £70 each!
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Old 27th Dec 2019, 3:22 pm   #47
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Default Re: "Burning - in" Double Triodes

Quote:
Originally Posted by G0HZU_JMR View Post

As these valves are expensive nowadays I'd like to minimise any stress on any valves I buy. I assume they will survive any sequencing method but if there is a preferred sequence for power on/off I'd like to try to adhere to it...

At your lower voltage it is likely less of an issue but it is better for the heater to be warm before there is a likely voltage surge between cathode and anode.

Personally I prefer to have a slow start heater circuit which builds up heater voltage over a few seconds to reduce surge current in the heater, then turn on the HT.

It is best to leave the HT on when in standby to reduce the possiblility of cathode poisoning. With a low HT, unless you are running at a high current long term degredation should be small
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Old 27th Dec 2019, 3:45 pm   #48
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Default Re: "Burning - in" Double Triodes

Thanks! Good point about the slow startup for the heater. I'm going to try to have a current limited startup on the heater to replicate your setup. I plan to use a 6.3V DC PSU (Thurlby PL320) but with the current limit set maybe 200mA higher than the steady state current on the datasheet. I assume it will then take a short while for the heaters to come out of current limit and settle. The PSU can otherwise deliver 2A which seems a little excessive.

Then I'll turn on the HT (do some testing) and then turn off the HT before I turn off the heater PSU.
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Old 27th Dec 2019, 3:59 pm   #49
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Default Re: "Burning - in" Double Triodes

Running heater without HT is considered to risk cathode poisoning if carried out long term.

On the other hand some comms radios killed the RX HT when transmitting, but that was short term. Some VHF transceivers kept TX heaters hot but HT off between transmissions.

In battery valve receivers, it's usual for heater power to be switched on when the set is on, but for HT to be unswitched and on 24/7.

Whatever belief you choose, there are plenty of contrary examples! I think the biggie is to avoid heaters on with no HT at all, but it's a slow degradation.

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Old 27th Dec 2019, 3:59 pm   #50
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Default Re: "Burning - in" Double Triodes

Just switch it on!

I've never heard of there being a need for heater/HT-sequencing in 'consumer' valves - the only thing I can remember is the use of a Brimistor in series-HT strings but that was more about limiting inrush-current when energising a string of cold heaters directly from mains.

Mercury-vapour rectifiers need a warmup-interlock too but I doubt you'll come across one of them in a domestic environment.
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Old 27th Dec 2019, 5:04 pm   #51
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Default Re: "Burning - in" Double Triodes

There is a balancing act with valves for consumer devices. You don't want them to last so long you won't sell any more of them -- but neither do you want them so short-lived you get a reputation for unreliability.

Indirectly-heated rectifiers give a natural soft-start action unless the power is interrupted long enough for the reservoir capacitor to discharge, but not long enough for the heaters to cool.
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Old 27th Dec 2019, 6:29 pm   #52
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Default Re: "Burning - in" Double Triodes

You don't need a "soft-start" for normal audio valves, only for high power transmitting ones.
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Old 27th Dec 2019, 6:51 pm   #53
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Default Re: "Burning - in" Double Triodes

If you want a good example of what valves can handle:

The UK "WS88" military transceiver - which uses 1.4V-filament battery-valves - does transmit/receive switching by turning-off the filaments of the receiver-valves and energising the filaments of the transmitting-valves every time you press the 'transmit' switch.

HT stays on all the time. The valves didn't seem to suffer any sorts of issues from such treatment.

Later on, there were VHF power-amp valves like the QQZ06/40 used in two-way vehicle radios, where the filaments were switched with the press-to-talk button on the microphone.

http://www.r-type.org/exhib/aad0308.htm

Again, they didn't seem to have issues.
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Old 27th Dec 2019, 7:33 pm   #54
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Default Re: "Burning - in" Double Triodes

You can bet the people who made valves for a living would have tested them as hard as they could, and known exactly how much abuse they could withstand. They would have had labs with whole rows of valves -- both new designs and examples of current production, for purposes of ongoing improvements -- on continuous test, probably configured as simple oscillators, and subjected to every conceivable misuse: sudden application of HT, loss of heater supply, overvoltage, undervoltage. Samples would then be individually tested for performance, then cut open and inspected internally after set numbers of hours,with a similar autopsy awaiting any premature failures.

I've worked in a lab where the mantra was if it ain't broke, try harder .....
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Old 27th Dec 2019, 7:54 pm   #55
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Default Re: "Burning - in" Double Triodes

OK thanks. It looks like I don't have to be that careful. I may still have the valves from the old FT200 in the loft but I suspect they were binned a few years ago when I had a big clearout.

I looked in the service manual for my ancient Tek 585 scope and it uses a couple of 12AU7 valves. It's very tempting to nab one of these as I haven't used this scope for decades and it has been left sulking in a corner of the garage. However, I'd expect the valves to be very tired and probably not worth the effort to find them inside this huge old scope. There are some NOS 12AU7 valves on ebay for about £15 and I think I'll be patient and buy one in the new year. I'm sure these little valves only cost about 70p 35 years ago!
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Old 27th Dec 2019, 8:01 pm   #56
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Default Re: "Burning - in" Double Triodes

12AU7 valves are still readily available - see

https://export.rsdelivers.com/produc...a-base/6784105

for £7.59 +VAT.

So no need to go 'gralloching' old equipment to find them, or worrying about inadvertently killing a few during your experiments!
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Old 27th Dec 2019, 8:04 pm   #57
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Default Re: "Burning - in" Double Triodes

What a player, thanks!
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Old 27th Dec 2019, 8:12 pm   #58
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Default Re: "Burning - in" Double Triodes

Some jukeboxes (Wurlitzer?) used a relay with its coil in series with the output valve cathodes, to switch heater tappings on the power transformer. The heaters would receive 12.6V when first powered up, in order to get the cathodes up to temperature quickly; dropping to 6.3V once they were warm enough for anode current to start flowing and change over the relay contacts.

As brutal as it sounds, this can't have been all that bad for the valves, or they wouldn't have bothered (the amusement hire company have to pay for new valves, but the proprietor of the establishment is paying for the electricity the jukebox uses!)
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Old 31st Dec 2019, 4:35 pm   #59
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Default Re: "Burning - in" Double Triodes

My 12AU7 arrived from RS today. It was a timely order as I had already planned to buy several reels of tinned copper wire from them. See below for the packaging that the 12AU7 lives in. I was half expecting a classic cardboard sleeve box with 12AU7 written on one end. But it was always going to be something in plastic

I was planning to take a Vg=0V curve across anode voltages of 5 to 62V but I might also look at Vg=-1V and -2V and Vg= +1V and +2V. I might also stack a 32V PSU in series and explore up to 94V DC at the anode.

Running at Vg=+1V and +2V takes me outside the datasheet curves. Is this wise? Will I risk stressing or damaging the valve like this?
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Old 31st Dec 2019, 5:05 pm   #60
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Default Re: "Burning - in" Double Triodes

If you take the grid positive of the cathode you run the risk of passing some grid current. In principle this might heat the grid, which would be A Bad Thing (the wires are physically fragile and not designed to disspiate much heat). I'd be surprised if you did any damage but you might want to put a current-limiting resistor in series with the grid supply.

Cheers,

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