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Old 4th June 2011, 10:58 AM   #1
Tyimo is offline Tyimo  Hungary
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Default NTC THERMISTOR as SOFT START for TUBES

Hi!
I would like to use NTC as soft start for my SS Tube PSU.
I have some questions:
Does anybody have experience with NTC in tube PSU circuit?
What is the best way to use a NTC Thermistor for Soft start?
1. NTC connected before the power transformer. On the primary side.
2. NTC connected after the power transformer. On the secondary side, which goes to the B+ supply.
Some people says soft start is needed only for B+,
another says for heather too,
or even for both (B+ and heather),
or even its not absolutely needed...
For me the most logical would be soft start for both supply, so the NTC should be in the input AC line.
Should I use in this case CL-60 or CL-150 NTC?
The transformer is 200VA and the AC line is 230Vac.
Greets:
Tyimo
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Old 4th June 2011, 12:07 PM   #2
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Tyimo View Post
Hi!
I would like to use NTC as soft start for my SS Tube PSU.
I have some questions:
Does anybody have experience with NTC in tube PSU circuit?
What is the best way to use a NTC Thermistor for Soft start?
1. NTC connected before the power transformer. On the primary side.
2. NTC connected after the power transformer. On the secondary side, which goes to the B+ supply.
Some people says soft start is needed only for B+,
another says for heather too,
or even for both (B+ and heather),
or even its not absolutely needed...
For me the most logical would be soft start for both supply, so the NTC should be in the input AC line.
Should I use in this case CL-60 or CL-150 NTC?
The transformer is 200VA and the AC line is 230Vac.
Greets:
Tyimo

Tyimo,

Some times you can get inrush current into the primary- this is more evident on some toroid Tx's. If you put an inrush current suppressor in the primary of the power Tx this will limit the inrush current. In turn this will help reduce the surge that happens with the heater low resistance at start.

The B+ can also have a surge of current if it’s a SS rectifier. Tube tends to give a soft start. This is because the capacitors in the PSU are not charged.

Sometimes you need a soft start on the heaters if you have a DC supply because the regulator can shut down "due to very low resistance" when cold.

Sometimes tube heaters can seem to "Flash" at start up due to current inrush the inrush suppressor can help to stop this effect.


Regards
M. Gregg
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Last edited by M Gregg; 4th June 2011 at 12:23 PM.
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Old 4th June 2011, 01:15 PM   #3
DF96 is offline DF96  England
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Yes, I would put it in the primary.
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Old 4th June 2011, 03:43 PM   #4
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When the amp. employs "fixed" bias, put the thermistor(s) in the rectifier circuitry. The idea is for the bias supply to turn on and protect against cathode stripping, before B+ is applied. I think it's OK to have thermistors on both the primary and the rectifier winding, when "fixed" bias is being used. The primary side part slows everything down, while the rectifier circuitry part(s) slows B+ down an additional amount.

BTW, the bright turn on flash associated with Philips, Mullard, etc., made tubes is intentional. Philips and the companies they owned (including Mullard) used such construction so small signal tubes would not cathode strip, when used in combination with fast starting, directly heated, vacuum rectifiers.
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Old 4th June 2011, 08:40 PM   #5
g(f(e)) is offline g(f(e))  United States
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Eli,

Is the flash you refer to only with rectifier tubes? Or do some of their signal tubes flash also?

Thanks

Gary
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Old 4th June 2011, 09:12 PM   #6
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If you put inrush limiter in the primary you take care of all rectifiers and capacitors together.
When the limiter did it's job it is better to short it by relay contacts. Relay can be powered from any of existing rectifiers, anyway speed of voltage raise on it's output will be similar to speeds of others, since they are in parallel to each other, and in series with inrush limiter. Since I rectify filament voltage for preamp tubes, I use it to power the relay.
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Old 4th June 2011, 09:59 PM   #7
Lo_Tse is offline Lo_Tse  Canada
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Join Date: Dec 2006
Quote:
Originally Posted by Tyimo View Post
Hi!
I would like to use NTC as soft start for my SS Tube PSU.
I have some questions:
Does anybody have experience with NTC in tube PSU circuit?
What is the best way to use a NTC Thermistor for Soft start?
1. NTC connected before the power transformer. On the primary side.
2. NTC connected after the power transformer. On the secondary side, which goes to the B+ supply.
Some people says soft start is needed only for B+,
another says for heather too,
or even for both (B+ and heather),
or even its not absolutely needed...
For me the most logical would be soft start for both supply, so the NTC should be in the input AC line.
Should I use in this case CL-60 or CL-150 NTC?
The transformer is 200VA and the AC line is 230Vac.
Greets:
Tyimo

Tyimo,

In the most recent issue of Elektor, there is an article on designing a PSU for a chip amp. The author mentioned that putting a NTC before the transformer on the primary side works fine to limit inrush current. The big caps will draw a lot of current rapidly during start up. Interestingly, according to the author, one does not want the NTC to be "in the circuit" after it does its job because its behaviour apparently will change (not just an innocent by-stander resistor of very low resistance) and will affect the performance of the amp (please read the article for details). The easiest solution is to place a switch in parallel with the NTC. This switch should be open when you start the amp. Shortly after start up (using the main power switch) and before you play music, turn this "NTC switch" on, thus bypassing the NTC. However, make sure that this by-pass switch is off before you start the amp next time. This is the simplest solution but you have to do more work (turn the switch).

Of course, a more elegant solution is, as Waveburn described, is to utilise a relay. In fact, in the Elecktor article, the authour opted for a very elaborate soft start circuit to take care of business.

Have fun!

Lo-_tse
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Old 4th June 2011, 10:10 PM   #8
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Quote:
Originally Posted by Lo_Tse View Post
Tyimo,

In the most recent issue of Elektor, there is an article on designing a PSU for a chip amp. The author mentioned that putting a NTC before the transformer on the primary side works fine to limit inrush current. The big caps will draw a lot of current rapidly during start up. Interestingly, according to the author, one does not want the NTC to be "in the circuit" after it does its job because its behaviour apparently will change (not just an innocent by-stander resistor of very low resistance) and will affect the performance of the amp (please read the article for details). The easiest solution is to place a switch in parallel with the NTC. This switch should be open when you start the amp. Shortly after start up (using the main power switch) and before you play music, turn this "NTC switch" on, thus bypassing the NTC. However, make sure that this by-pass switch is off before you start the amp next time. This is the simplest solution but you have to do more work (turn the switch).

Of course, a more elegant solution is, as Waveburn described, is to utilise a relay. In fact, in the Elecktor article, the authour opted for a very elaborate soft start circuit to take care of business.

Have fun!

Lo-_tse
Is this the soft-start circuit you are talking about?

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Old 5th June 2011, 12:16 AM   #9
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Quote:
Originally Posted by Wavebourn View Post
If you put inrush limiter in the primary you take care of all rectifiers and capacitors together.
When the limiter did it's job it is better to short it by relay contacts. Relay can be powered from any of existing rectifiers, anyway speed of voltage raise on it's output will be similar to speeds of others, since they are in parallel to each other, and in series with inrush limiter. Since I rectify filament voltage for preamp tubes, I use it to power the relay.
Which is very close to optimal, as amplifiers that do this seem to last much longer with less service issues.

The relay does not see much current so it does not need to be large but should have a flash suppressor capacitor across the contacts from the turn off surge as the inrush limiter will be cold at that point, so there can actually be more voltage across the contacts at turn off.

When using a relay you can also use a resistor, but high surge current rated resistors are not common and a bit pricey.

The downside of the less expensive inrush limiter is that they do fail and sometimes with a flame, so only put them in metal chassis away from other parts.
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Old 5th June 2011, 02:00 AM   #10
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Using a MOV across the primary winding is probably a better way to manage the turn-off voltage stress on primary switchgear. I doubt the NTC bypass relay contacts will see a stress (as it is delayed) - the main amp ac mains switch contact will take the stress.

A CL60 is not much series resistance (10 ohm cold) - I would have thought a CL-70 or even CL-80 would be more appropriate, as your average max input current is not going to get above an amp or so. Even then, you may still see heater 'flash', as the heater turn-on characteristic is relatively slow peak decay compared to capacitor charging and transformer in-rush.

Ciao, Tim
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