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Old 1st March 2015, 08:51 PM   #1
Bwaze is offline Bwaze  Slovenia
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Default Soft start for tube amplifier

Does anyone know of a soft start board or kit for a low power amplifier (like headphone amplifier) or preamplifier (power requirements around 30W, solid state rectification)?

I know that they don't absolutely need soft start, but I would like to apply one for longevity of rectifiers and heating filaments - so I would like to apply it before transformer on mains. Easiest would be just a single NTC thermistor and a bypass switch to operate it manually. But I'd rather have automatic bypass with relay.

Would commonly used low power thermistors even work at such a low current? Inrush current is below 1A, steady current around 150 mA - would 1A rated thermistor even heat enough to drop resistance? I have 120 Ohm 1A and 33 Ohm 1.3A thermistors - would any of them be of any use in this case?
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Old 1st March 2015, 08:56 PM   #2
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When combining hollow state designs with solid state power, it's best to separate heater power from HV DC. That way, you can power up the heaters first, make sure everything's good and hot, before applying the HV. That's how I do it.
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Old 1st March 2015, 09:01 PM   #3
Bwaze is offline Bwaze  Slovenia
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In my case (Bottlehead Crack) HV is only 170V - well below the voltage that could cause any problems when applying it before heaters are hot. I'm just interested in slow capacitor charging and slow filament heating for longevity's sake.
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Old 1st March 2015, 09:07 PM   #4
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One soft start kit is here.
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Old 1st March 2015, 09:55 PM   #5
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Quote:
Originally Posted by Bwaze View Post
In my case (Bottlehead Crack) HV is only 170V - well below the voltage that could cause any problems when applying it before heaters are hot. I'm just interested in slow capacitor charging and slow filament heating for longevity's sake.
You don't have anything to worry about. The in rush current from heaters isn't all that much ~4X. Light bulbs have an inrush > 10X, and light bulbs last a good long while before the filament fails open. Even without any sort of soft start, your VTs will go low emission long before there's a heater failure. The only cases where I've seen heaters fail open is when connecting in a series string across the AC mains. In that case, some heaters can be over volted badly before they have a chance to really heat up. Once, this was with an old Admiral TV set where the CRT filament flared white hot on power up. That was the absolute worst VT to over volt.

It can't so any harm, but it isn't doing any good either, just adding complexity and cost to your project.
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Old 2nd March 2015, 04:05 AM   #6
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could just go for a constant current heater supply
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Old 2nd March 2015, 08:46 AM   #7
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Quote:
Originally Posted by Bwaze View Post
Easiest would be just a single NTC thermistor and a bypass switch to operate it manually. But I'd rather have automatic bypass with relay.

Would commonly used low power thermistors even work at such a low current? Inrush current is below 1A, steady current around 150 mA - would 1A rated thermistor even heat enough to drop resistance? I have 120 Ohm 1A and 33 Ohm 1.3A thermistors - would any of them be of any use in this case?
Mains side inrush current will likely be far greater than 1A - it's typically limited just by DC resistance of the primary, if the transformer saturates during application of initial mains depending on where on the cycle and what residual magnetism is in the core.

You've already identified the classic elements, such as relay bypassed primary series R or NTC thermistor. But IMO the only point to it is if the inrush current causes some problem externally, such as mains trip takeout or inducing spikes on the mains which might be damaging to other equipment. The transformer isn't 'transforming' when the inrush happens, by definition it's in saturation, I shouldn't worry about the secondary side for valve gear.

If you can be sure the transformer doesn't saturate then there are zero crossing point solid state relays which might provide some assurance if you really are concerned about the secondary rise time at switch on. Somewhere in the archive of my memory is that zero switching is the worst case for saturating the core if it is pre-magnetised, and peak switching (absurdly) is the best for avoiding core saturation - can't recall exactly why, but worth thinking about if you go down this route. Peak switching solid state relays minimise transformer inrush current, as I recall. HTH !
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Old 2nd March 2015, 09:16 AM   #8
Bwaze is offline Bwaze  Slovenia
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Bottlehead Crack had a 0.5A quick blow fuse, now they use 1A quick blow fuse - I think there can't be much more current during start except maybe for a first few cycles. I also read that low power EI transformers start quite softly, so there really shouldn't be any external problems. They really don't need soft start, this is more of an experiment than a necesity.
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Old 2nd March 2015, 10:51 AM   #9
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Originally Posted by Bwaze View Post
Bottlehead Crack had a 0.5A quick blow fuse, now they use 1A quick blow fuse - I think there can't be much more current during start except maybe for a first few cycles. I also read that low power EI transformers start quite softly, so there really shouldn't be any external problems. They really don't need soft start, this is more of an experiment than a necesity.
Wow, 0.5A quick blow suggests there really isn't any notable inrush current - presumably dc resistance of the primary is such that it inrush current self limits and/or the transformer doesn't saturate then, and so no need to be concerned about it I guess.

I spent a happy few months last year persuading a 1kW torroidal not to pop mains trips, the other end of the spectrum !
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Old 2nd March 2015, 11:14 AM   #10
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I'm using these from doukmall ebay.
They even have temperature sensors.
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