The 85A2 valve

[Andrew.R]

Hi, has there ever been a "best" or very well regarded 85A2 valve? Sorry if this is an over simplistic answer but I'm just finding my way around! I'm looking for a new one to replace one in my Pre-Amp.

[paulsherwin]

It's just a voltage stabiliser. Is there something wrong with the one currently fitted?

[GrimJosef]

It depends what you mean by "best" of course. Things you might want include long life, stability of the running voltage, low noise and the absence of 'voltage jumps' (to which these voltage reference valves can be prone). Back in the day the big customers for these (typically military and industrial equipment manufacturers) also cared, among other things, about striking voltage and vibration/impact stability.

Philips/Mullard claimed that the 85A2 itself was a 'special quality' valve http://www.r-type.org/timeline/time-040.htm. They went on, however, to produce a presumably even more special variant, the M8090 http://www.r-type.org/exhib/abi0031.htm.

You may find that there is some variation in the parameters you care about between valves which carry identical labels. If so then all you can do, I'm afraid, is get hold of a bag full and check them out one by one.

Cheers,

GJ

[John M0GLN]

Some of these voltage stabilisers have an radioactive isotope added to help striking, the 0B2 I'm using has Cobalt 60, I don't know about the 85A2, perhaps they were available with or without.
The Valve Museum http://www.r-type.org/exhib/aai0248.htm does mention special quality versions.

John

[Boater Sam]

Got lots, NOS. boxed. Away till end March though.

[turretslug]

As mentioned, lots around and shouldn't be difficult to find respected brand ones, it was almost the go-to voltage reference for much 'fifties kit with regulated supplies. The professional use code was the CV449. One possible caveat is that many reference valves expect to be exposed to light (reasonable energy) photons to assist striking, as professional usage types were often in the depths of sealed equipment and moreover retained in place with locking cans, a trace of radioactivity was sometimes deliberately included to ensure striking. Sometimes the neon, argon, whatever was slightly radioactive, other times (as mentioned), a radio-active isotope of the electrode metal was used. Perfectly safe when contained by the glass envelope, but don't break 'em!

[TonyDuell]

I am not sure what the half-life of the radioactive isotope used is, but you would want it to be fairly short to get significant activity (to get the device to strike) with not too much material.

The problem comes, of course, that said isotope will decay naturally, and after some time the device will not strike at the data sheet voltage (it should still regulate correctly once it has started). There is nothing that could prevent this, a NOS valve will be the same as one that was in use in this respect.

Just seen that the half life of Cobalt 60 is around 5 years. So after 30 years (not a long time for the sort of things we consider) that's 6 half lives, or only about 1/64th of the activity remaining.

I believe I have seen this problem with gas dischage 7 segment displays (not Panaplex, but similar) which now need a much higher voltage to get them to turn on than the data sheet states.

I suspect the hazard from the radioactivity is close to non-existant!. If the gas is the radioactive isotope and the valve breaks it will disperse very quickly. If it's one of the solid parts, then provided you don't do something very silly like eat it, the main risk is worrying about it .

[FERNSEH]

Apart from high quality picture monitors it's unlikely an 85A2 stabiliser tube was ever employed in an UK made domestic TV receiver.
However, here's an extract from the circuit diagram of a high quality 625 line TV receiver developed in 1953 by the Philips Applications Laboratory, Eindhoven.
From the text about the operation of the sync separator and line phase discriminator: Finally it should be mentioned that the voltage stabiliser type 85A2, used for reducing the influence of main voltage fluctuations upon the multivibrator frequency, can be dispensed with when excessive mains voltage fluctuations are not likely to occur.
Due to the nature of the operation of the line flywheel discriminator slight frequency variations of the line oscillator can result in a certain amount of picture displacement.
Also, note the application of the EQ80 nonode as the sync separator and line flywheel discriminator.

DFWB.

[paulsherwin]

Regardless of any of the above, a different stabiliser is extremely unlikely to make an audible difference to your preamp. Most professional and broadcast kit didn't use voltage stabilisers in preamps. They were mostly used in lab equipment.

[turretslug]

Originally Posted by TonyDuell

I believe I have seen this problem with gas dischage 7 segment displays (not Panaplex, but similar) which now need a much higher voltage to get them to turn on than the data sheet states.

I've also heard that neon, being quite wee and monatomic, is good at escaping through glass, especially when stimulated by discharge (though not as slippery as that Houdini of gases, helium), so neon devices can go off after many years- certainly the HP180As here can take a minute or so to sort themselves out as the references sometimes tardily and differentially strike- I imagine that low mains periods don't help. When were hydrogen thyratrons first made, and was there an ageing symptom?

[frankmcvey]

They were mostly used in lab equipment.

Yup - they were used to stabilise the reference -85V in the Mullard High Speed Valve Tester series.

Cheers,

Frank

[Andrew.R]

Originally Posted by paulsherwin

It's just a voltage stabiliser. Is there something wrong with the one currently fitted?

No nothing wrong with it as such but I have no idea how old it is or any indication of it's strength. I would presume it would have problems doing it's job of voltage regulation if it was on it's way out. So, my thinking was to try and simply replace like with like but with a well respected alternative.

Originally Posted by GrimJosef

It depends what you mean by "best" of course. Things you might want include long life, stability of the running voltage, low noise and the absence of 'voltage jumps' (to which these voltage reference valves can be prone). Back in the day the big customers for these (typically military and industrial equipment manufacturers) also cared, among other things, about striking voltage and vibration/impact stability.

Philips/Mullard claimed that the 85A2 itself was a 'special quality' valve http://www.r-type.org/timeline/time-040.htm. They went on, however, to produce a presumably even more special variant, the M8090 http://www.r-type.org/exhib/abi0031.htm.

You may find that there is some variation in the parameters you care about between valves which carry identical labels. If so then all you can do, I'm afraid, is get hold of a bag full and check them out one by one.

Yes, I should have perhaps had a look into the specification data sheets to try and educate myself. I'm just after something that will best regulate the voltage into the pre-amp. At the moment the pre-amp has a valve regulated power supply but a popular modification to my pre-amp is to replace it with something solid state. I have seen the Mullard "special" quality variants over on r-type.org, interesting. I suppose it could be fun trying the various valves and seeing if anything happens.

Originally Posted by paulsherwin

Regardless of any of the above, a different stabiliser is extremely unlikely to make an audible difference to your preamp.

What makes you say this? It's just that many people on various Audiophile forums analyse things trying to find an explanation and usually end up generating a flame war even though the question was started was best intention.

[GrimJosef]

Originally Posted by paulsherwin

Regardless of any of the above, a different stabiliser is extremely unlikely to make an audible difference to your preamp ...

The only exception is if the reference is noisy, the regulator circuit doesn't allow that noise to be filtered out and the pre-amp hasn't got great PSRR. Then picking a quieter reference valve can help audibly.

You do have to ask a design question or two though if proper operation depends on selecting components.

Cheers,

GJ

[Graham G3ZVT]

Earlier today I mentioned in a different part of the Forum that a similar regulator tube used its noise to pick Premium Bond winners.

Originally Posted by RAMBO1152

Premium Bond winners were originally picked on the whim of nine QS92/10 neon regulators.

https://www.radiomuseum.org/forum/er..._computer.html

[Radio Wrangler]

Originally Posted by Andrew.R

I have seen the Mullard "special" quality variants over on r-type.org, interesting. I suppose it could be fun trying the various valves and seeing if anything happens.
/
/
What makes you say this? It's just that many people on various Audiophile forums analyse things trying to find an explanation and usually end up generating a flame war even though the question was started was best intention.

A gas discharge voltage stabiliser is a most unusual thing to find in a valve preamp. Most designers don't feel any need for them.

Such devices exhibit an abrupt negative resistance characteristic as they strike, and they also exhibit hysteresis. It is very easy to create crude oscilltors around them... it takes care to avoid inadvertently creating an oscillator whenever one is used. Gas discharge devices are also famous as creators of random noise and were for years used as the standards against which the noisiness of other components were measured. Consequently, any competent designer using one of these devices will filter its output very carefully before it gets used to influence any wanted signal. If any change in sound can be detected from changing such a device, Either the device is out of specification, or else the design of the amplifier is seriously flawed.

So long as your current regulator tube is steady at a voltage within its specified range, stick with it. Almost any of your other valves is far more likely to affect your sound.

The special quality ones may sound like a nice thing to have, but they were designed not to work better, but to survive more extreme environments and possibly to last longer. But the VR150/30 in my AR88 receiver is about 78 years old and still works completely satisfactorily. It isn't any sort of special quality. It was made in a hurry during the war.


This isn't an audiophile forum. The people on here may well be the diametric opposite! There is a large contingent of people with many years experience in the repair business and have seen just exactly what is inside the most mundane equipment as well as that laden with the largest amount of fairy dust factor. There are people who worked in the factories which produced some of the now most sought after gear. There is a good number of experienced design engineers who can and have designed things from the fundamental maths.

The amazing thing is how well we all tend to agree. The moderators would drop like a ton of AR88s on the beginnings of any flame, but they don't have to. People hereabouts get on rather well together, and though there are differences in viewpoints, differences are respected.

So you've landed amongst a well-tempered bunch of people who really can understand the maths and understand the design processes used in creating all sorts of equipment. If you state that something creates some particular audible effect, we're the guys (and gals) who then want to repeat it so that we can verify the effect, measure things and explore what mechanism is in-play in its creation. If it's a good effect, we'd like to explore more of it, if it's a bad effect, we want to find ways of dependably engineering it out. There are several people who, if you used the phrase "Fast Bass" to them would not only reply "Ah, you mean treble" but they could also prove it mathematically., deriving the Fourier transforms necessary.

You should find an enjoyable lack of contention and a general source of facts which have solid foundations and are repeatable for all people.

David

[Andrew.R]

David,

Thanks for the very informative post. I know that it's not an audiophile forum. I joined up so that I could learn a little more about valves and how they work. I'm just getting to grips with the nomenclature for all the various valves! I'm in the process of messing around with various 6SL7GT'S! Anyway, I'd like to say a big thanks to those that have helped so far.

[G8HQP Dave]

A voltage stabiliser would have a legitimate role in a phono preamp, as it would stop mains voltage variations getting in at the first anode and causing woofer flapping. No need for one in a line stage - in fact in most modern systems there is no need for a line stage at all.

Unless the original valve is faulty, or the circuit is poorly designed, there is no reason to swap it for an alternative. Note that 'special quality' valves (including some military ones) were usually intended for longer life under difficult conditions (e.g. high vibration); under normal domestic conditions they offer no advantage.

[Andrew.R]

Just how much of a stabiliser is the valve? Does it have a range of stabilisation, i.e. within a specific range of +/- volts? Some audiophiles will spend quite a bit of cash on trying to run their units within voltage range to supposedly get a "better" sound .

[G8HQP Dave]

Valve circuits are generally unfussy about supply rail voltage. 20% variation will do little harm. Some valve circuit users do not understand this. A phono preamp stage designed for 85V will be quite happy with 80V or 90V. The only problem is if the voltage keeps changing, as this will look like a subsonic signal.

You can judge how good the 85A2 is by looking at its datasheet. The voltage will vary a bit from sample to sample, and with current and temperature.

[Bazz4CQJ]

This seems like a new one. I have a fair stock of voltage regulator valves acquired over a long period of time and I've mostly associated them with roles such as supplying stabilised HT for RF oscillators. There are comprehensive datasheets for most/all of them and I thought their use was well-established and problem-free, subject to due consideration of effects like noise and possible oscillation.

Of course, with the availability of cheap, high-voltage zeners, the regulator valves can be replaced if you so wish. I've used zeners to clamp down the HT supply of a receiver which has silicon diodes in the PSU, and which would otherwise see the voltage go rather high during the period while the valves are warming up. Similarly, my M59 GDO Homage will see the original stabiliser 'toob' replaced with zeners, purely as a space-saving measure. No doubt, that for a very critical requirement, there will be pros and cons to consider.

B