Thermistors in valve gear

[Sinewave]

Has anyone else here gone to the bother of selecting a thermistor for their valve gear to lessen the inrush on the transformers and valves?

I'm considering selecting a few items to do this to. I know the Mi TF144 has one from factory, but wondering if some of the other gear could benfit from an NTC thermistor.

What's the general thought? Thanks.

[Leon Crampin]

How many failures of thermionic equipment are caused by current inrush? In my experience, these things just die gracefully.

Leon.

[Sinewave]

Well, I'm thinking that it's accumalative, it doesn't just die from inrush, it's one of the stresses every time it's switched on and with some valves and transformers being either expensive or both expensive and rare I wonder if it's worth considering.

[Radio Wrangler]

Once you've found ways to defeat most of the obvious stresses and causes of failure, it's remarkable what you have to do next to beat failure rates far into the undergrowth.

Making a factory safe in terms of static electricity was a massive undertaking but it was one of the key moves in reducing early failures by a factor of 20. They were good by industry standards to start with, but the world was changing and expectations were increasing.

Look at what the Colossus people had to say then and now about valves. Running continuously resulted in far better reliability than turning on and off. And they had a good statistical population to get data from.... think of the number of valves times the number of colossi built. There wasn't just one.

For the rebuild, Claude Lyons very kindly made and donated a motorised ganged variac for the filament supply. This brings up the heaters in a gentle, slow and smooth manner.

So part of me says it's a good move, and also helps with the inrush if applied to HT supplies and is kinder to capacitors as well as rectifier cathodes. the other part of me says that for building new things, there's nothing wrong with transistors, and transistor stuff too benefits from slow-start operation.

David

[G6Tanuki]

I've always seen 'inrush protection' as a thing intended to protect against nuisance-trips on the supply-side, rather than protecting the equipment. (lots of the stuff I designed was located in remote unmanned locations, where replacing a fuse could involve six man-hours of billable time, 5.5 of which were the driving-there-and-back).

Personally I don't like thermistors in such applications - the problem is their thermal-inertia: they stay low-resistance for quite a few seconds after they cease having current passing through them, meaning they pffer no protection against the summer-lightning-induced problem where the power goes off, comes back on for a few seconds, goes-off-again several times in a 15-second period before either failing 'hard' or remaining on.
My preferred inrush-protection was a N/O relay with surge-limiting-resistors across the contacts, the relay's coil being energised from the HT. The relay would drop-out within 300 milliseconds of loss-of-supply.

But beware -if you get a short loss-of-power - enough to drop the relay out but the heaters remain hot - then you apply power again after half a second, the load on the HT provided by the already-hot valves can be enough to prevent the HT rising to the relay's pull-in voltage - so the surge-limiting resistors across the contacts are never switched-out!

It's easy to engineer for a steady-state, but forseeing and engineering to *safely* handle such odd transitional states is difficult (and expensive!).

[Radio Wrangler]

Understood!

Back in the nineties, the big monitors for serious graphics on engineering workstations from HP were re-badged Sonys. Needless to say, in an HP plant with R&D, marketing , production etc for a few divisions, there was rather a lot of them.

They had a thick film hybrid whose function was to determine whether it was in a 120v mains region, or 230v and switched the mains rectifier of the SMPS between bridge and doubler modes.

You can see where this is going....

Now some neds threw a lump of wire rope over an 11kV rural feeder across the other side of the village.

The local mains dipped, went off, and then did a few abortive attempts to come up, before it finally did.

The auto voltage selector thought power had gone off and then decided the retries looked like the USA and switched in the doubler. Unfortunately, determination was a one-shot job on power-up. The doubler was locked in so long as the voltage didn't go down to nowt for a while. It didn't, it wemt up, to 240 eventually.

Kapow

Hundreds of multi-kiloquid monitors went bang.

Official word from the computer divisions was "Not serviced below unit level on internal transfer price units" Meaning: "Hard cheese, you got a discount so buy a new one!"

I was a bit miffed at the inconvenience, and if both my monitors had just become landfill, there was nothing to lose by taking a shufti.

1) a fuse had popped
2) there was a big zener across each of the reservoir capacitors. They did not seem to have enjoyed the experience. A look inside a unit that had been off power during the event showed the devices with markings intact.

So I worked out what they were and got some bits in from Farnell, for the following day.

Success and I was back at work. I'd got enough bits in to fix a few more monitors so I was unusually popular for a while.

Officialdom reared its head, learned that the damage was fixable, and started leaning on HP and Sony.

Sony sent someone round to start working through the monitors.

Some moonths later, there was a repeat performance. Oddly, mine survived. Might be something to do with disabling the voltage doubler mode and labelling the outside perhaps?

It is very difficult to second-guess what power line voltages might do. It seemed the local power recovery profile just hit the vulnerability of a bit of circuitry that was trying to be too smart.

David

[Refugee]

You could just remove those voltage switches.
Open circuit was 240V so that was what we did when screens were worth fixing for sale second hand.
We never got a single one back for not working on 110V.

[Diabolical Artificer]

I put a thermistor in all my valve amps and slow/soft start mainly cos I so far have used biggish toroidal tfmrs and quite large caps. The delayed start means I can use caps with a wkg V near or lower than HT. So I think it's a good idea if you have a gurt big lump of iron, a lot of amp makers skimp on caps too, have worked on an amount of amps with rails or HT over V max. it also might be a bit kinder to ECCXX type heaters some of which light up like fairy lights at sw on.

Incidentally I watched a video yesterday on Angus Young's guitar rig, they used some Japanese power doodads so his vintage Marshall's saw 236v RMS. the tech bloke said Marshall advise 234v for their amps but they found it a bit squashy.

Rod Elliote wrote an excellent article on this subject, can't find article at present, BB playing up.

Andy.

[trobbins]

If you were keen to look at NTC's, or sequencing, there is some design detail in the linked doc.

It was all a bit easier with just valve diodes, as there was inherent inrush reduction, and not a toroidal PT to be seen.

I'd strongly suggest you firstly assess the basic protection (fuse values, or even fusing if there is none) in case the manufacturer value was really just a burn your house down mitigator, rather than a save-as-much-collateral-damage-as-practical fuse value. And then assess the more likely parts to fail/wear-out, and what can be done to protect against collateral damage.

https://www.dalmura.com.au/static/Va...p%20fusing.pdf

[vinrads]

Question! would it be better to limit the ht to say half with a series resistor, then switch to full ht after 12 sec via a delay on timer ,at the moment I have the HT switched on after 12 sec ,I am using a full wave bridge rectified ht . Mick.

[trobbins]

Why have you got the HT switched on after 12 secs?

[vinrads]

Hi I am using a solid state rect therefor the HT rises to 30 volts above the working voltage of the smoothing until the valves have warmed up , also less stress for other components . Mick.

[trobbins]

If the first filter cap is a modern electrolytic, then it is rated to 110% for intermittent use (ie. power turn-on events). What is often seen is that downstream power rail filter caps, and coupling caps, have their max voltage ratings exceeded at turn-on as there may be no loads throughout a valve amp.

[Diabolical Artificer]

On my delayed/soft start a relay shunts out a big 50w 1k2 resistor that comes after the tfmr, before the bridge rectifier, IE, in the AC part of the circuit. I use a resistor and cap to time the relay sw on to come on after 10 seconds, this goes though a 12v zener into the base of a tranny that switches the relay on. In my case normal HT - 420v is reduced to around 250v. The relay is a big Omron 3 x SPST ganged together.

In my case main smoothing/reservoir caps are rated for 500v, so they're fine, but de-coupling caps are only rated for 385v, so this was a trade off between size and wkg V.

Again theres a lot of useful info over on Rod Elliots site, he really goes into the tradeoffs, + & -'s involved.

Andy.

[vinrads]

Thank's Andy I have the relay switching one leg of the AC to the bridge rect , I think I need to experiment switching a resistor ,so as to allow a lower HT to start with , Mick.

[William78]

Not exactly protecting a valve, I fitted a thermistor in series with the lamp (A1/207, 1000 Watts ) of my Bell and Howell filmosound projector.
According to the datasheet the thermistor is 10 ohms and drops to between 0.11 - 0.05 ohms depending on the load.
It seems to work, those projector lamps are pretty expensive to buy.

William