Gradual HT ramp up

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Tubetec

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I was wondering if there was a simple way to do this .
Essentially mimic the way a tube rectifier slowly ramps up the Ht at turn on ,
Could we dump current at switch on for maybe 30-60 seconds , meanwhile the power supply C|R|C networks take time to charge up .
If we had a resistor and thermistor( lo-hi resistance )PTC? in series across the Ht supply then another thermistor(NTC) in series with the smoothing filters would that work?
Lets assume a passively smoothed HT with CRCRCR filter , 220uf caps and resistances in the order of a few kilo-ohms .
Could we incorporate NTC thermistors into the HT bridge rectifier circuit in such a way that initially they drop a lot of volts and only allow the reservoir and filter caps fill slowly.


I know theres HT delay systems involving relays they still spike the HT on the caps and tubes although the heaters will have had time to warm , or maybe some kind of mosfet that soft starts things but I'd much rather a way to do it simply, passively , two legged components only .

'It should also be noted that valves actually last longer if they have some cathode current flowing while they are heated, which again is contrary to the theory of the standby switch. Source: Radio Designer's Handbook, 4th edition'
 

Tubetec

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Standby/relay delay is better than cold start with full HT applied but ramp up makes life much more gentle on the valve thermally and is idiot proof as far as user interaction is concerned as no front panel controls are needed .

If you have the chance use a laser thermometer to measure the cathode temp in a power tube after a suitable warming up period ,with heater voltage applied , when you apply HT the cathode temp rises sharply , meaning every other components within the glass bulb also has to get used to this 'new normal' , its obvious to me why both initial current limiting of the heaters and ramped application of Ht is a good thing and makes valves live longer ,standby puts an unquatifiable bump in the process of your tubes coming up to working temp ,so now they have to withstand cold(ambient), heaters on temp and with ht applied ,with simple ramp up strategies both in LT and HT your not forcing the structures inside the tube to bear undue temp rise/fall all of a sudden , a smooth curve , not hitting a brick wall like unloaded plate volts on cold cathodes or the thermal steping your tubes get with a standby switch . Effects on the tubes themselves aside your Ht caps wont complain about a gentle rise to volts , unlike the SMPS first filter cap which gets full mains ,rectified DC applied 'balls to the wall' ,without the loses of the transformer to buffer it every time its energised.

How long does it take a piece of tube gear like a mic or preamp to reach some kind of thermal stability where your happy to record with it ? whats a minute or two waiting for the HT volts to ramp up worth? could make the difference between 5 and 50 years or more in the life of the power supply components and save many trips to the repair man for new tubes because of thermal fatigue of one form or another .

Do you want your gear only to last till the next service interval or should we not be playing a longer game for the greater good?
 
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Winston OBoogie

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Easy to get ramping up H.T. - Use a MOSFET as a gyrator/cap multiplier. With a high value resistor into the cap on the gate.

I generally used a 10M R into a 10uF polyprop. C. * This ramped up at about the rate that felt right for a decent degree of the valve heaters warming up.

I had more stuff going on - current source into a zener or a resistor for voltage regulation. This was then fed to the 10M and 10uF. But you don't need this extra stuff.
You will need a protection diode for the MOSFET if there isn't one internal to the part. Put the end of the 10M right at the gate of the Fet, otherwise, put a small 100R right at the gate for parasitics etc.


*
time-constant-t.svg
= 100 seconds here of course so, adjust to taste :)
 
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rock soderstrom

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I am not sure if this is really necessary. Many old tube radios with sand rectifiers show that it works very well without.
The components needed for this are themselves at risk of becoming a problem, especially in long-term operation.
 

Winston OBoogie

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A very good point Rock. Truth is, I don't really know if it's better for the tubes or not.

As far as I'm aware, none of the higher end units I built have had issues with a failed MOSFET.

My thinking at the time was more like: I want the filtering and low impedance output of a MOSFET gyrator on the H.T. line anyway, whether regulated or not. And since the part is already there, why not have a longer time constant too by putting in a much larger value resistor.

A few units of mine still have the same tubes I installed over 25 years ago. These were nice NOS European types. I'm OK with that as far as value for dollars spent. If I'm still around in another 25 years I'll see where it stands ;).
 

MaxDM

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A rectifier tube tends to limit current in a similar way to a resistor

While it is true that it is not perfectly linear, like a resistor, it's close enough that you can just look at the charts and use voltage drop related to current and find a resistive value which is in the ballpark.

I did this to replace a rectifier tube in a 3W guitar amp and it worked surprisingly well, IMO
 

rmburrow

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I like indirectly heated rectifiers like the 5AR4, 5V4, 6X4, 6AX5, and some of the TV damper diodes. Directly heated rectifiers like the 5U4, 5R4, 5Y3, etc. bring up HT before the heater-cathode types conduct. There are some circuits using tubes like the 6AS7, 6L6, etc. as series pass voltage regulators that also bring up HT slowly. There are regulator circuits in the UTC transformer catalogs, etc.
 

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