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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Fri Mar 11, 2011 2:15 pm 
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Шаман, типа...

Joined: Tue Feb 10, 2004 12:30 pm
Posts: 7312
Location: Pleasant Hill, California
Knarf wrote:
wavebourn wrote:
Hey, what news? Shock
I live in San Francisco Bay Area. It's quiet and sunny here.

Glad to hear it, the news companies made the situation sound pretty grim for a while.


That's why we don't watch TV at all. I have a cable for phone and internet only, no TV at all. Well, I have Netflix online, we watch movies we choose.
I don't know what people who run TV are thinking about themselves, but all what they actually do, they get people scared and angry, like compete who can scary more...

Quote:
Auto-biasing the MOSFET off via a source resistor is a neat trick, except it ruins the regulator output impedance somewhat. Will have to think about that one a bit.


Don't worry: just shunt output by something like 200 uF film capacitor. It is loaded on a tube stage that drives speakers through 5K impedance. What's 22 Ohm against 5K?

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Sat Mar 12, 2011 7:52 am 
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Posts: 41
Location: Denmark
wavebourn wrote:
That's why we don't watch TV at all. I have a cable for phone and internet only, no TV at all. Well, I have Netflix online, we watch movies we choose.

There is something to be said for that approach. TV rots your brain, but have yet to be able to give it up completely myself.

wavebourn wrote:
I don't know what people who run TV are thinking about themselves, but all what they actually do, they get people scared and angry, like compete who can scary more...

They are simply trying to keep people glued to the screen. More eyeballs equals more ad revenue. And the rest of the effects you describe are just 'unfortunate side effects'. The worst offense is when people are taught that complex scientific problems can be explained in 15 seconds worth of talking head talk...

wavebourn wrote:
Don't worry: just shunt output by something like 200 uF film capacitor. It is loaded on a tube stage that drives speakers through 5K impedance. What's 22 Ohm against 5K?

I already have 110uF/700VW worth of electrolytics there, was hoping that would be sufficient? Pray

Something like a 200uF film cap rated at at least 700VW would be quite expensive, as it is a bit beyond the specs of normal motor run caps. I *can* get a 150uF 'DC link' PP cap of sufficient voltage rating, but they cost around $100. Each.

As for the short circuit protection of the MOSFETs, then I have been running a bunch of simulations.

The executive summary is that protecting the MOSFET against a dead short of the output isn't trivial. The power dissipation would be much too great, even if we limit current to, say, around 250mA via using the 22ohm source resistor. Trouble is the 500V Vds encountered during a short, and the associated power dissipation. It seems virtually impossible to cool the MOSFET sufficiently, so that it survives more than a few hundred ms. It is Rt,jc which sets the limit here, and not the general 'size' of the part.

There was only one halfway reasonable solution I could find to the short circuit problem. It is to limit current to around 6-700mA during a short, and then use the common 440mA quick blow fuses rated at breaking 1KVDC, available as spares for quality test equipment like Fluke DMMs. A TO-220 MOSFET *may* survive long enough for the fuse to blow, but then we have spent $10 on protecting a $1 part. Laughing


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Sat Mar 12, 2011 8:21 am 
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Posts: 41
Location: Denmark
Quikie,

Forgot to mention this, but one potential cause for randomly blowing power semiconductors could be fake parts. They are unfortunately so common in the retail channel in many cases, that you can all but guarantee you will run across some of these. The only way to avoid this pitfall would be if you make 100% certain all your semiconductors are sourced from reliable suppliers.

Here is a fun example, though not a semiconductor (one would hope...)


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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Sun Mar 13, 2011 6:35 pm 
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Шаман, типа...

Joined: Tue Feb 10, 2004 12:30 pm
Posts: 7312
Location: Pleasant Hill, California
I've found that 2 microfarads of film is sufficient in parallel with 470 microfarad electrolytic for 2x80W P-P amp with 800V B+ (actually, I used 4x2,000x200V and 2x4uFx450V).
Speaking of 250 uF films, I bought very cheap box of 245 uF x 250V made for electricians, and used one in my 4P1L amp.

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 12:42 pm 
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Joined: Wed Oct 01, 2008 9:14 am
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Location: Denmark
Sounds great Anatoliy, as my 110uF is much more than that, considering the difference in power output. Of course my amp is SE, but that is why I add the regulator. A few 2-5uF film capacitors are much more manageable.

Contacted Edcor today, will be interesting to see what they think of my mains transformer specs.

Also, here is my current revision of the schematic. Think I just have to go into waiting mode until I have some components to work on. This design looks good on paper, so I need to get my hands dirty to have a chance of improving on this. The transformers will probably take about 6 weeks from the time I give the go-ahead. Confused

- Frank.

Image


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 8:30 pm 
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Joined: Mon Mar 22, 2010 1:10 am
Posts: 238
Knarf wrote:
Quikie,

Forgot to mention this, but one potential cause for randomly blowing power semiconductors could be fake parts. They are unfortunately so common in the retail channel in many cases, that you can all but guarantee you will run across some of these. The only way to avoid this pitfall would be if you make 100% certain all your semiconductors are sourced from reliable suppliers.

Here is a fun example, though not a semiconductor (one would hope...)


I have built the circuit with Zener diodes across G-S. Diode across D-S to prevent back voltage damage to Mosfet. The safety features most will build into a MOSFET regulator. Except the resistor after the source recommended by Anatoliy.

Yes. A big possibility would be fake parts, very common in this part of the world. Even the 2N3055 is faked to glory here.

Another possibility would be mains voltages that swing > 20% causing overating the current and voltage specifications of the device (but come on, an IRF840 is supposed to be almost indestructible at my circuit specs, DC not exceeding 400V, and fairly conservative current demands, < 1A., this is for a device supposed to be able to take 500V, 8A continuous current, 32A pulsed current, and 125W SOA dissipation) So now I tend towards high rating parts just to have that extra leeway in cased of that.

Managed to get some SEC SSH5N90 (renamed FQPF5N90 after taken over by Fairchild) parts at USD3 ea. Non insulated parts but what the heck. Just won't be screwing them anywhere on the chassis. Seller claims they are legit. Got 4 pieces and will give them a spin to see how they perform. Don't order any parts for me just yet.


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 8:50 pm 
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Posts: 238
Knarf wrote:
Sounds great Anatoliy, as my 110uF is much more than that, considering the difference in power output. Of course my amp is SE, but that is why I add the regulator. A few 2-5uF film capacitors are much more manageable.

Contacted Edcor today, will be interesting to see what they think of my mains transformer specs.

Also, here is my current revision of the schematic. Think I just have to go into waiting mode until I have some components to work on. This design looks good on paper, so I need to get my hands dirty to have a chance of improving on this. The transformers will probably take about 6 weeks from the time I give the go-ahead. Confused

- Frank.

Image



For your gridstoppers, why don't you just use some ferrite beads as the values you are using would probably be too low to prevent oscillation.


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 8:59 pm 
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Joined: Wed Oct 01, 2008 9:14 am
Posts: 41
Location: Denmark
quikie22 wrote:
For your gridstoppers, why don't you just use some ferrite beads as the values you are using would probably be too low to prevent oscillation.

They are not grid stoppers, schematic doesn't include all constructional details yet. The silly values in the schematic are current measuring resistors for the simulation. The 1 ohm resistor at the input was a test resistor used to judge the influence of a large value grid stopper on the amp's frequency response.

OK about the IRF840s, was just curious if you had put up a complete schematic somewhere for future reference. I will let you know ahead of time, when I do get around to ordering stuff for this project. Have no intention of buying anything for you, unless I know for certain you are interested. Wink


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 9:13 pm 
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Joined: Wed Oct 01, 2008 9:14 am
Posts: 41
Location: Denmark
quikie22 wrote:
Another possibility would be mains voltages that swing > 20% causing overating the current and voltage specifications of the device (but come on, an IRF840 is supposed to be almost indestructible at my circuit specs, DC not exceeding 400V, and fairly conservative current demands, < 1A., this is for a device supposed to be able to take 500V, 8A continuous current, 32A pulsed current, and 125W SOA dissipation) So now I tend towards high rating parts just to have that extra leeway in cased of that.

Actually...

This doesn't tell me much. Wink

That is why I asked for an accurate schematic. If your mains can vary up to 20%, and you run the devices at low currents with upwards of 400V across D-S, for instance during startup, then you may run the device into avalanche breakdown on mains voltage peaks. MOSFETs are severely limited in how much energy they can take during an avalanche breakdown event, so you might kill them pretty quickly that way.

Originally I used a 600V part in my PSU, yet the simulations showed my device would go into avalanche during startup, and for so long that it would almost certainly die. This even though Vds never go above 550V or so. During normal operation I only have about 30V Vds. Avalanche breakdown is accurately simulated in Spice, as this is an important feature in some schematics, similarly to the body diode.


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 10:35 pm 
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Шаман, типа...

Joined: Tue Feb 10, 2004 12:30 pm
Posts: 7312
Location: Pleasant Hill, California
Frank;

I personally would prefer to stabilize voltage for CV83's screen grid.

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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Mon Mar 14, 2011 11:02 pm 
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Joined: Wed Oct 01, 2008 9:14 am
Posts: 41
Location: Denmark
Do you prefer it enough that you feel I should build it in from the start? Was thinking of experimenting with one of three configurations: VR tube stabilization, source follower with either zener or VR tube reference, and the simple voltage dropper shown above.

The stabilized screen supplies would of course be much more impervious to mains voltage fluctuations. Hmm...?


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Tue Mar 15, 2011 7:29 am 
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Joined: Mon Mar 22, 2010 1:10 am
Posts: 238
Knarf wrote:
quikie22 wrote:
Another possibility would be mains voltages that swing > 20% causing overating the current and voltage specifications of the device (but come on, an IRF840 is supposed to be almost indestructible at my circuit specs, DC not exceeding 400V, and fairly conservative current demands, < 1A., this is for a device supposed to be able to take 500V, 8A continuous current, 32A pulsed current, and 125W SOA dissipation) So now I tend towards high rating parts just to have that extra leeway in cased of that.

Actually...

This doesn't tell me much. Wink

That is why I asked for an accurate schematic. If your mains can vary up to 20%, and you run the devices at low currents with upwards of 400V across D-S, for instance during startup, then you may run the device into avalanche breakdown on mains voltage peaks. MOSFETs are severely limited in how much energy they can take during an avalanche breakdown event, so you might kill them pretty quickly that way.

Originally I used a 600V part in my PSU, yet the simulations showed my device would go into avalanche during startup, and for so long that it would almost certainly die. This even though Vds never go above 550V or so. During normal operation I only have about 30V Vds. Avalanche breakdown is accurately simulated in Spice, as this is an important feature in some schematics, similarly to the body diode.


So how can we overcome this? Maybe you have a circuit I can look at? The part may not cost very much to you, but it is equivalent to the price of a simple meal to us, not to mention a big bother to change out.


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Tue Mar 15, 2011 8:14 am 
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Location: Denmark
quikie22 wrote:
So how can we overcome this?

We can overcome this if you point me to any place, where you have posted the *exact* schematic of what you have built, including actual component values used, like capacitor capacitances, and any modifications made. I believe I have asked before. Only this way will I stand at least a theoretical chance to stuff your design into Spice and try to figure out what might be killing your MOSFETs. And/or just try to analyze things the old fashioned way. This may tell us how to proceed, no matter what the outcome.

quikie22 wrote:
Maybe you have a circuit I can look at?

What do you want a circuit off? You still haven't *shown* me what you are trying to do, and in which situation you encounter dead MOSFETs. I am unfortunately not very good at mind reading, particularly not at this distance.

Pen. Paper, digicam. I know you have these. Draw, photograph, post somewhere, send me PM with link if you don't want the schematic out in public.

quikie22 wrote:
The part may not cost very much to you, but it is equivalent to the price of a simple meal to us, not to mention a big bother to change out.

And why exactly is this relevant to the present discussion?


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Tue Mar 15, 2011 8:33 am 
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Шаман, типа...

Joined: Tue Feb 10, 2004 12:30 pm
Posts: 7312
Location: Pleasant Hill, California
Knarf wrote:
Do you prefer it enough that you feel I should build it in from the start?


No, I feel I may let you know my personal preferences so you one day may try and check for yourself if you like it or not. Screen grid current is non-linear and depends on signal level. Looking from another side, screen grid is one more control grid, and anode current depends on voltage on it. R-C network integrating current consumption peaks varies regime of the tube, depending on signal level. On higher loudness idle anode current will go down, anode voltage will go up. And vice verse. If you like it, or don't care, it is fine, of course.

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А женщина даже в мужской рубашке выглядит гораздо приличнее, чем мужчина в женских колготках! Be-be-be!


 
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 Post subject: Re: GU-50 Single Ended Amplifier Design
PostPosted: Tue Mar 15, 2011 1:03 pm 
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Joined: Mon Mar 22, 2010 1:10 am
Posts: 238
hey frank

please don't get upset. just wanted to let you know that the parts do cost quite a bit over in this part of the world and put things in perspective. not in any means a direct reference to you. Smile

the circuit is a simple one that is posted all over the web. http://fhs-consulting.com/ind/HV_REG_cropped.png


 
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