managing inrush current advice

[black_labb]

hello again everyone
i've been working on the bass amp and am wondering if inrush current should be managed, or if it is not an issue. the amp is running 6 6550's/kt88's (decided 8 is unneccesary).
the power supply is 2 torroidial transformers ( 450vac for the plates, 250ac for the screens and the preamp) the plates should be around 630vdc with 350uf (2x470uf in series and 2x220uf in series, may add an extra pair of 220uf's if i feel neccesary as i have them around).
the screen/preamp supply is running ~350vdc with 220uf just after the rectification.

would it be beneficial to use some inrush current management? i've seen inrush current thermistors reccomended, but they seem to be only rated at 265v for the ones that are available. obviously it wouldnt be a good idea to run that on the 630vdc supply. are these meant to be run on the input supply to the power transformers? (ie connected to the wall voltage after the fuse).
are there any other methods that appear to be good? i was considering puting a resistor on the standby switch to the amp so that all current from the trannies would run through it, but this didnt seem like a great solution, and its hard finding switches 3pdt switches rated at the current (i'm sure they can be found in a more specialised sources.

thanks everyone

[tubeswell]

Another way might be hefty power resistors on either side of the HT winding before the rectifier. Merlin b has an article on limiters (which altho' it applies to tube rectifiers, would also apply to SS AFAICT):

The Valve Wizard

[Chuck H]

At 630V plates I can't be too sure. Voltage spikes and all. I've had some trouble with B+ spikes on less sensitive projects than this. But it wasn't the tubes that had a problem. Rectifier tubes seem to be especially sensitive to inrush current. I haven't had a problem with power tubes...Yet. Maybe put it on a scope and watch what happens switching off standby. That should tell much of the story.If it seems to be a problem you could use a Pi filter type arrangement AFTER your initial B+ smoothing with a smaller cap on the B+ side. I think the smaller cap would slow down the inrush current but you would still have the benefits of the large filters eliminating ripple and hum.

Chuck

P.S. Test the idea on Duncan PSU. It's a good tool, and very accurate IMHE.

[black_labb]

thanks guys
i think i may bypass the standby switch with a resistor. i'm not sure what the best method of applying a standby to the plates. 630v is alot of voltage for most switches available. would the resistor across the standby make it safe enough to use a 250vac switch for 450vac? seems to be pushing things...

i have always thought of just leaving the B+ on the plates like in my holden/wasps, but i'd like to keep it a bit safer inside for working on when standby is on.

[Steve Conner]

Hey Black labb

If the fuses don't blow at turn-on, no inrush current management is needed.

Yes, those NTC thermistor thingies are meant to be used in the mains line. But several in series might work for your HT.

The classic method is "step-start" where you have a big-ass resistor in the line that gets shorted out by a relay after a second or two.

No equipment is safe to work inside while it's on, standby or not. The only truly "safe" condition is with the plug pulled and the capacitors discharged.

[Merlinb]

If the fuses don't blow at turn-on, no inrush current management is needed.

The classic method is "step-start" where you have a big-ass resistor in the line that gets shorted out by a relay after a second or two.

+1

A step-start is dead easy, and can prevent speaker thump. Here's an age-old way of doing it: you just adjust the limiting resistor to give the desired warm-up time until the amp's own current pulls the relay in.

Also, an NTC thermistor on the primary also limits inrush current on the secondary of course- the transformer reflects impedances both ways after all.

[Steve Conner]

FWIW, here's the step-start circuit I used on a solid-state hi-fi amp I built years ago. It uses the "amp's own current" method as recommended by Merlin. The amp had a huge bank of smoothing capacitors, and would blow fuses at turn-on without the step-start.

http://scopeboy.com/psu.gif

Instead of a switch, it has start and stop buttons like a motor starter. The stop button is also hooked up to the protection circuitry, so the amp turns itself off on serious faults.

[black_labb]

thanks steve and merlin, seem like a good idea. i think i will explore it a bit more

[R.G.]

If you use step-start, be very, very sure that the resistor is inside some kind of flame-proof enclosure, or pick a resistor with a power rating that will let it stay engaged forever without overheating.

If the relay ever fails to pull in, the resistor stays engaged, and the value usually works out to be such that the resistor goes up in smoke and flames. The one sure thing I know about relays is that they will fail to operate someday. When that day comes, you might not be there to recognize the disaster as it happens and stop the fire from spreading.

I believe it was exactly this which caused the popularity of NTC inrush devices.

Well, OK, also that they're much smaller and cheaper than relays and power resistors...

[Steve Conner]

This is one advantage of my circuit: If the relay fails to pull in, the circuit won't latch on, and the resistor won't catch fire unless you hold the start button in while pondering where all the smoke might be coming from.

The second pole of the Off switch is to stop you from burning out the resistor by holding both buttons in at once.

Nevertheless, I blew one of those gold Dale resistors during testing, shooting smoke clouds out of the ends. It just couldn't take the dissipation of repeated starts.

Resistors with built-in thermal fuses are available, they were popular in old TVs.

[defaced]

What would 130 volts do? I've run my setup at 500v into 440uf and not had an issue (50 or so starts before I disassembled it to build the next prototype). I'll shortly know what 560v into 340uf does. Does anyone know if Duncan's PSU designer is accurate with it's simulation of inrush current (I think that's one of the options for observation)?

[Diablo]

While on the subject of inrush current limiters, can someone explain the meaning of the codes on them? I'm using one that reads "NTC 100D-11", and I'm curious how many amps it's rated for. I've googled until I have a headache. The room temperature resistance is 107 ohms, according to my measurement. I'm using this on the standby switch to protect the single rectifier tube (5U4GB) from the surge when charging up the filter capacitor. Good or bad idea? This is a Tweed Twin 5E8A.

[black_labb]

i'm not sure diablo, why not check the datasheets?

[Enzo]

100 is the cold resistance (room temperature) in ohms. D is the type, 11 is the diameter

[black_labb]

maybe i will use a thermistor on the plate transformer and have the B+ on the plates in standby, and have a resistor across the standby switch so that the grids and preamp have a slow start as well.

the slow start circuit looks really useful. dont think i'll use it though, doesnt seem completely neccesary and i'd rather not use a relay if i can avoid it. seems to be a complicated way of doing what the thermistor will do. if the plates werent on at startup then it would probably be a good alternative.