power supply for my single ended amp

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I'm designing the PSU for my SE KT66 amp. 2 channels, approx. 150mA, Ht something between 440V (max. for fixed bias) and 480V (max. for cathode bias).

I want the PSU to have a low output impedance down to low frequencies, and I want superb smoothing.

I don't want a voltage regulator, though, because of this:
When the mains voltage gets a little low, the regulator might loose its "headroom" and lets a nasty ripple thru to its output,
or - if designed with lots of headroom - I'm wasting a lot of power under normal conditions.

So the idea is to use a source follower, with a passive 3-pole filter at its gate for good smoothing. This control voltage is derived from the reservoir cap with a voltage divider (trimpot integrated in the bleeder resistor string). So when the mains voltage gets low, the HT will also get low, but will still be smooth.

The first draft of the circuit is here:
http://jhaible.heim.at/se_kt66/se_kt66_psu_idea.pdf

Please let me know if this is a good idea, or what should better be different. It simulates good, but I'd love to get some feedback before I actually build it. I'm a little worried about the high impedance of the filter chain. In theory the MOSFET has a high enough gate impedance, of course, but maybe some parasitic effects will ruin its behaviour?
In simulation, I get the ripple down to a few millivolts, which seems too good to be true.

The lower part is a delay for the HT, derived from the heater voltage. I've breadboarded that part - works great. Gives a delay of 30 seconds. Is this long enough to prevent cathode stripping?

Comments welcome.

JH.
 
yagoolar said:
1. How does it respond to fast voltage spikes in the power line?

Ahh ... I never thought about that!
Thanks for pointing it out.

What exactly do I have to watch out for? The spikes bypassing the FET via parasitic capacitance?



2. How fast does the voltage on the output rises after switch-on?
--Yag

After a delay of 30 seconds (relay), it rises up in ca. 500ms (gate filter).

Is this too long? (overheating of the FET ?)
Or too short? (thumping in the speakers ?)

JH.
 
jaymanaa said:
Hi, Your last cap is labelled "motor start". I have never used one, but have heard much negative talk about them. Maybe a motor "run" oil capacitor would be better. Nice delay circuit, I think 30 seconds will be fine. Jay

I have browsed thru the forum, read about "start" and "run", but thought it was just different descriptions of the same thing - my bad.

I have no idea for which application it is - the description just read "Motor Kondensator".

Here is what I bought:

link

It's polypropylene - I thought this was good ...

JH.
 
jhaible said:


Ahh ... I never thought about that!
Thanks for pointing it out.

What exactly do I have to watch out for? The spikes bypassing the FET via parasitic capacitance?





After a delay of 30 seconds (relay), it rises up in ca. 500ms (gate filter).

Is this too long? (overheating of the FET ?)
Or too short? (thumping in the speakers ?)

JH.

AFA spikes are concerned watch for very narrow ones which appear on the output. And they may also origin from rectifiers while recovering.

Overheating is a matter of UxI on turn-on, so you should check both current and voltage characteristics in time domain - the most power is emitted at the intersection of U and I. I hope you know what I mean.
 
yagoolar said:


AFA spikes are concerned watch for very narrow ones which appear on the output. And they may also origin from rectifiers while recovering.

Overheating is a matter of UxI on turn-on, so you should check both current and voltage characteristics in time domain - the most power is emitted at the intersection of U and I. I hope you know what I mean.


I know what you mean in general.
And I know how to calculate a heatsink for permanent power dissipation. What I don't know is how much I can undersize the heatsink for a much higher power dissipation that only lasts for half a second.

Let me see:

I may have 50V * 150mA = 7.5 W permanently.

On power-on, very roughly, 300V * 150mA = 45 W, but only for
half a second. (45 W is the maximum power from spec - I guess it means maximum permanent power, but with an "ideal" heatsink.)

I intend to fix the MOSFET (Rth to case = 3K/Watt) directly to the enclosure, which will give a few more K/W in total. Should be enough for the permanent power. What I don't know is whether the short time of higher power will hurt or not.

JH.
 
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Klimon said:


Two words for the same thing; polypropylene (or paper) in oil = very good.

Simon

:hot::hot::hot::hot::hot:
No, not two words for the same thing!

There is a marked difference between a motor run and a motor start capacitor.

A motor start capacitor is intended for short term, intermittent duty providing starting current for an ac motor. They have a finite service life generally of a few years and are designed to handle large currents for a short period of time. They are not designed to dissipate significant power long term and will generally fail quite quickly in high ripple applications due to excessive internal heating.

A motor run capacitor on the other hand is designed to provide phase shift to one of the windings in a synchronous motor, and is physically larger and has lower internal losses to cope with continuous operation.

A motor run cap will typically have a film/foil or metalized film construction immersed in mineral oil in a large metal can. A motor start cap is generally dry, more compact, and may be contained in plastic or paper tubes - and is not designed to dissipate much heat.

Ripple current in a power supply will cause internal heating in any capacitor due to internal losses. It is important to choose a capacitor designed to handle that.

I recommend checking out the ASC Capacitors website for more information.

http://www.ascapacitor.com/technical_bulletins.htm
 
kevinkr said:


:hot::hot::hot::hot::hot:
No, not two words for the same thing!

There is a marked difference between a motor run and a motor start capacitor.

A motor start capacitor is intended for short term, intermittent duty providing starting current for an ac motor. They have a finite service life generally of a few years and are designed to handle large currents for a short period of time. They are not designed to dissipate significant power long term and will generally fail quite quickly in high ripple applications due to excessive internal heating.

A motor run capacitor on the other hand is designed to provide phase shift to one of the windings in a synchronous motor, and is physically larger and has lower internal losses to cope with continuous operation.

A motor run cap will typically have a film/foil or metalized film construction immersed in mineral oil in a large metal can. A motor start cap is generally dry, more compact, and may be contained in plastic or paper tubes - and is not designed to dissipate much heat.

Ripple current in a power supply will cause internal heating in any capacitor due to internal losses. It is important to choose a capacitor designed to handle that.

I recommend checking out the ASC Capacitors website for more information.

http://www.ascapacitor.com/technical_bulletins.htm


Ok - thanks for the warning!

Now I must find out which type I have. (It's already soldered in by now ...)
Too bad, the link I posted earlier to RS-Components says session expired.

I checked again: It says "Serie C878B" - does this ring any bells?

JH.
 
dhaen said:

Your power supply design is an interesting concept which I will watch with interest.:)

Ok - I don't know if anybody is still following this thread, but anyway, here's a warning:

*Don't* build the PSU circuit I posted!

I tried it on the weekend, and it was breaking into heavy oscillations, 100Hz distorted saw wave with tons of higher frequent bursts along the slopes, and the motor cap was singing this wave as if it were a loudspeaker.

I still have to analyse where this went wrong - I thought that Source Follower approach was pretty straight forward. Apparently not.
Guess I've built an oscillator, with 50 V amplitude.

JH.
 
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Joined 2004
Paid Member
Hi JH,
I'm still following the thread, just haven't had much time to reply lately. :D

The very first thing to do is to isolate the gate of your mosfet from everything else (probably including the protection zener) with a 1K - 10K resistor right at the gate. This should de-Q the resonant circuit formed by the gate capacitance and wiring inductance, not to mention the ESL of that last cap..

I assume those caps are 220nF not uF? Large caps have lots of ESL, not what you want right at the gate of a power mosfet.

I have had a lot of problems with oscillating mosfets at rf and vhf and this often manifests itself in huge amplitude low frequency stuff as well.

Some small ceramic caps right at the drain to chassis will often help as well. Something like a 0.01uF/1KV X7R ceramic disk may help.

Don't give up, the basic design looks sound. Adding rudimentary regulation to this design would be easy with a couple of zeners or gas tubes once you iron out the other bugs.

Fast rectifiers should not need the snubber caps, so what you have done so far is probably fine.
 
jhaible said:
I've made a screenshot of the product page.
It's in German, but maybe you'll recognize it nevertheless:
motor capacitor

Motor start or motor run ??

JH.


Klimon said:


Oooops:xeye: Luckily I had been charmed by the higher quality look of the motor-run caps, otherwise my basement might have been filled with unusable motor start caps.... Releived smilie :D

Simon


... but, when you look at the picture behind the link I posted: Is it motor start or motor run?

It only says "motor" (same word in German and English), neither start or run. But maybe someone recognizes the type??

JH.
 
JH,

I'ld say those are motor-runs; the case looks almost identical to a few caps I have here (working great in tube-amps) and capacitance / size ratio is about the same. Here's a great vendor with a stock of motor-run caps: http://www.die-wuestens.de/eindex.htm ; look under kondensator --> Öl- Ölpapier- und MP-Kondensatoren.

What surprises me is the 'lebensdauererwartung' on the datasheet you provide; I thought oil caps had almost infinite life (I've got lots of ex-industry caps, dirty and dented that work flawlessly) ---- 30000 hours isn't really impressive. I tend to think that the datasheet is rather pessimistic about the life expectancy, not that it's a bad cap. Maybe someone else can elaborate here.

Simon
 
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