Author Topic: It looks for me as stupid schematic (Oldtimers welcome!)  (Read 6002 times)

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Offline 001Topic starter

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It looks for me as stupid schematic (Oldtimers welcome!)
« on: September 29, 2019, 04:37:36 pm »
Check ebay toy here https://www.ebay.com.au/itm/100-300V-DC-High-Voltage-Power-Supply-PSU-for-6P6-6N6-EL34-Tube-Preamp-Amplifier-/332777814374
It looks stupid for me
Why they bias transistor this way?  :-// Is it good schematic?

 

Offline Yansi

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #1 on: September 29, 2019, 04:58:35 pm »
Why wouldn't they bias it this way? I do not see anything wrong there. That's how simple regulated linear PSUs've been made last decades. The schematic is not stupid, but could be made certainly way better.

There is certainly missing a grid-stopper resistor. (gate series resistor would be likely required even when mosfet used instead of tube). Can oscillate, and probably will without it, due to the stupidly placed capacitor from G pin. (making the power tube/mosfet work as a grounded grid/common-gate RF amplifier/oscillator)

The NPN needs to be a high voltage type, but, more intelligent designer would use a cascode configuration with a high gain low voltage one, or stick in there even a TL431 to get very good results.

To get better voltage regulation, one could supply the reference (75V zener in this case) from the output of the supply itself. (stick a resistor from the zener cathode to the output of the PSU) - that way it self-regulates the zener current. The more current from the output to the zener compared to the current through the NPN, the better the result.

The circuit is lacking any over-current and short protection.  Implementing fold-back current limiter may also be handy.

 

Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #2 on: September 29, 2019, 05:35:52 pm »
Why wouldn't they bias it this way? I do not see anything wrong there. That's how simple regulated linear PSUs've been made last decades. 

The problem is about maximum voltage between base and emiter (Typicaly it is lower than 5V). It is 300V PS
« Last Edit: September 29, 2019, 05:50:04 pm by 001 »
 

Offline Yansi

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #3 on: September 29, 2019, 05:51:21 pm »
And why do you think the reverse Vbe is violated in the circuit?

Here you have the approximate steady stage voltages when the output will be set for 300V.

(plus a note how to ad the grid-stopper resistor)

 
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Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #4 on: September 29, 2019, 06:18:42 pm »
And why do you think the reverse Vbe is violated in the circuit?

Here you have the approximate steady stage voltages when the output will be set for 300V.

(plus a note how to ad the grid-stopper resistor)

(Attachment Link)

Well
Turn pot to HV bus
 

Offline Yansi

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #5 on: September 29, 2019, 06:29:00 pm »
Then this will happen.

So what now?  :)



//EDIT: Note the output voltage is a function of the potentiometer tap position. Output voltage will be equal to about
Vout = 75*(1+Ru/Rb)
where Ru is the upper part of the resistive divider and Rb is the bottom part.

(Also note the range of the pot will be something like 90 to 330V as the output setting - depending on the exact zener voltage, transistor gain, resistor and pot. tolerances, etc.)
« Last Edit: September 29, 2019, 06:33:59 pm by Yansi »
 

Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #6 on: September 29, 2019, 06:34:51 pm »
Then this will happen.

So what now?  :)

(Attachment Link)

//EDIT: Note the output voltage is a function of the potentiometer tap position. Output voltage will be equal to about
Vout = 75*(1+Ru/Rb)
where Ru is the upper part of the resistive divider and Rb is the bottom part.

(Also note the range of the pot will be something like 90 to 330V as the output setting.)

Why Yours pics sign constant pot voltage for different output voltage?
 

Offline Yansi

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #7 on: September 29, 2019, 06:38:13 pm »
Because that's how the circuit works :)

The zener diode voltage is (kind-of) constant. So is the voltage drop across the B-E junction. Hence voltage from the base to ground will be constant.  But output voltage will change with the change of the potentiometer tap position.

You can analyze the circuit knowing the voltage from base to ground is about 75V. That voltage has to be present on the voltage divider tap to ground. That tells something about the current flowing through that divider (you can assume for simplicity current gain of the NPN is high, the base of the transistor therefore consumes negligible current).

You know the good ole TL431?  Think of this has the same behavior - just the reference voltage will be about 75V (given by the Vzd + Vbe).

//EDIT: This is the simplified circuit, same working principle:
« Last Edit: September 29, 2019, 06:43:07 pm by Yansi »
 

Offline Yansi

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #8 on: September 29, 2019, 07:19:46 pm »
With TL431 it would look like this:



However this would only work up to the voltage limit of the 431, which would be about 25-30V. If further voltage is required, then cascode configuration may be used: you add a common base amplifier on top of it. Such amplifier has none current gain, however large voltage gain.



In this case TR2 is what takes most of the voltage. It is a good practice to bypass R5 with a capacitor, and/or replace it with a zener diode. The R4/R5 divider shall be designed so about 10-20V is present at the base of TR2.
 
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Offline ArthurDent

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #9 on: September 29, 2019, 09:24:41 pm »
Check ebay toy here https://www.ebay.com.au/itm/100-300V-DC-High-Voltage-Power-Supply-PSU-for-6P6-6N6-EL34-Tube-Preamp-Amplifier-/332777814374
It looks stupid for me
Why they bias transistor this way?  :-// Is it good schematic?

The basics of the supply above are very similar to a supply I made decades ago using all tubes.
 
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Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #10 on: September 30, 2019, 02:56:19 pm »
Check ebay toy here https://www.ebay.com.au/itm/100-300V-DC-High-Voltage-Power-Supply-PSU-for-6P6-6N6-EL34-Tube-Preamp-Amplifier-/332777814374
It looks stupid for me
Why they bias transistor this way?  :-// Is it good schematic?

The basics of the supply above are very similar to a supply I made decades ago using all tubes.

Have You some links to this project?
 

Offline ArthurDent

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #11 on: September 30, 2019, 03:31:43 pm »
oo1 - "Have You some links to this project?"

Here's a post I made in another thread a while ago and that's basically all there is. Keep in mind I made this supply about 40 years ago and had to redraw the schematic recently. Like I said, it is very similar to the transistor version shown above

"When I made my 150-250VDC 100ma power supply I was trying to use parts I had in my junk box while still making a good regulated power supply. The case was from a military surplus magnetic amplifier and the case was the only part of that unit that was useful to me. I didn’t have one transformer that would work but I did have 2 identical smaller transformers that would fit perfectly in the back part of the case and give me the voltages and current I was looking for if I wired the HV windings in parallel. I did buy new meters and to give you an idea how old this is, I bought them from Lafayette Radio Electronics (kind of like Radio Shack) who went out of business in 1981! The circuit I used was one I modified from one I saw in an HP manual from about that time period.

The voltmeter was modified to have an expanded range by using a 150V Zener and using a series resistor to make the meter really read 0-100 volts and then marking the dial to read 150-250 volts. If you’re wondering why there is a 1N4007 between the output and the meter, that is to prevent meter damage if the output somehow dropped to zero, effectively putting 150 volts on the meter in the reverse direction. The milliamp meter is the same basic movement as the voltmeter with the series resistor removed and a shunt resistor in parallel and the scale marked in Ma.

One of the transformers 6.3 volt windings is used for the heater supply for the tubes and not referenced to ground to eliminate any heater to cathode voltage problems. The other 6.3 volt winding is connected to terminals on the back of the case to power tube heaters in devices I might be constructing.

The capacitor from the supply output to the pot wiper eliminates the D.C. component but sends the outputs a.c. ripple voltage unattenuated to the error amplifier. Instead of using a 5651 gas reference tube like the original design I used an 82 volt Zener that gets its supply voltage from the regulated output. It feeds the reference voltage to the error amplifier through the triode section of the 6U8A used as a cathode follower. Where I didn’t use the 5651, the triode might not be needed and the Zener could probably supply enough current to be connected between the error amplifier cathode and ground, then I could replace the 6U8A with just a 6AU6 pentode. The 6BX7 has a maximum plate voltage rating of 600VDC so it is used within its specs.

The HV rectified output from the two 1N4007 diodes is connected to the two 40 mfd electrolytics through a 10 ohm resistor that lessens the surge current and is also an emergency fuse. The two deck power switch is three position non-shorting and in the off and standby positions there is a 6.8K resistor connected from the two 40 mfd electrolytics to ground to safely discharge them. This capacitor has a 500 VDC rating because with no load there is about 450 volts on this capacitor. When the switch is in the operate position the 6.8K drain resistor is switched off and the HV center tap is connected to ground and the output is present."
 
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Offline T3sl4co1l

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #12 on: September 30, 2019, 03:52:22 pm »
No current limiting (aside from the impedance of the poor tube itself), and it'll explode if ever you should accidentally short the output.  But yeah, that's about all you need for a basic dumb regulator.

Tim
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Bringing a project to life?  Send me a message!
 

Offline Audioguru again

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #13 on: September 30, 2019, 04:13:29 pm »
I am also an oldtimer (74). Why are you playing with antique tooobs that soon wear out? I sold my tooobs tuner and amplifier about 55 years ago and the solid state replacements still work fine today.
 
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Offline ArthurDent

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #14 on: September 30, 2019, 04:36:25 pm »
T3sl4co1l - "No current limiting (aside from the impedance of the poor tube itself), and it'll explode if ever you should accidentally short the output."

As I said, I made that supply 40 years ago and I'm still waiting for the 6BX7 tube to explode, maybe in 50 years.:-DD I've never ever seen a tube explode but I have seen some tube plates glow bright red.

There is both a primary fuse and a 10 ohm resistor as protection in case you missed that. I also stated I wanted to build a supply out of parts I had in my junk box and I didn't intend to make a complicated supply that would have all the bells and whistles that my programmable precision bench supplies have today. You should be pleased I even put meters on the supply!  8)
 
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Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #15 on: September 30, 2019, 04:59:12 pm »
great project
thanx a lot for charing  :-+
 

Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #16 on: September 30, 2019, 05:46:16 pm »
With TL431 it would look like this:

(Attachment Link)

However this would only work up to the voltage limit of the 431, which would be about 25-30V. If further voltage is required, then cascode configuration may be used: you add a common base amplifier on top of it. Such amplifier has none current gain, however large voltage gain.

(Attachment Link)

In this case TR2 is what takes most of the voltage. It is a good practice to bypass R5 with a capacitor, and/or replace it with a zener diode. The R4/R5 divider shall be designed so about 10-20V is present at the base of TR2.

Is it good or bad example? http://www.next-tube.com/articles/hvs/hvrEn.pdf
 

Offline T3sl4co1l

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #17 on: September 30, 2019, 05:57:01 pm »
As I said, I made that supply 40 years ago and I'm still waiting for the 6BX7 tube to explode, maybe in 50 years.:-DD I've never ever seen a tube explode but I have seen some tube plates glow bright red.

I was referring to the OP schematic, perhaps I should've been more specific.

The same hazards do apply in your case, but with much less severity thanks to the low current capacity, huge energy capacity, and high voltage ratings (especially K-G) of the tubes in question. :)

Tim
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Bringing a project to life?  Send me a message!
 

Offline SiliconWizard

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #18 on: September 30, 2019, 06:16:17 pm »
Indeed. The eBay kit should work fine, but is it designed to last or be safe? Not quite.
Can be an interesting starting point for those willing to experiment/learn about tubes though. Proof is that this eBay sale triggered a series of interesting points made in this thread. ;D

Oh, and while we're at it, just be careful. High voltages can be a hazard. So this is not the kind of kit for your kid's birthday, or if you're currently drunk.
 :popcorn:
« Last Edit: September 30, 2019, 06:18:28 pm by SiliconWizard »
 
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Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #19 on: September 30, 2019, 06:36:30 pm »
Proof is that this eBay sale triggered a series of interesting points made in this thread. ;D

Do You have some ideas about possible improovments? Is hfe critical here?

So this is not the kind of kit for your kid's birthday, or if you're currently drunk.

I`m drunk for at least last 56yrs so I can
 

Offline SiliconWizard

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #20 on: September 30, 2019, 10:28:33 pm »
Do You have some ideas about possible improovments? Is hfe critical here?

Well, re-read the thread. Others have suggested a couple things. Add a series resistor to the grid. Add a current limiter. Add a fuse at the input.
You could also add some kind of physical protection (like a plastic shell or something) around the high-voltage part, so there's less chance you touch it when you're experimenting with the board.

Is hfe critical here?

Probably not that much.

I`m drunk for at least last 56yrs so I can

No comment. But still be careful, I don't think it gets any better with age.  ;D
 
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Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #21 on: October 01, 2019, 01:35:32 pm »
Thanx for Yours comments

But I still misunderstood why transistor didn`t blow when base-emitter  voltage  rises to 300-75=225V instead 5V  :-//
 

Offline SiliconWizard

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #22 on: October 01, 2019, 06:04:40 pm »
But I still misunderstood why transistor didn`t blow when base-emitter  voltage  rises to 300-75=225V instead 5V  :-//

Because it doesn't?
 
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Offline 001Topic starter

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #23 on: October 03, 2019, 12:41:26 pm »
OLALA! I GET SOMETHING SIMILAR!

Can U tell me is it good idea to use  MOSFETs? Why author use neon bulb instead zeners?
Check this:

 

Offline ArthurDent

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Re: It looks for me as stupid schematic (Oldtimers welcome!)
« Reply #24 on: October 03, 2019, 02:18:06 pm »
001 - "Why author use neon bulb instead zeners?"

The 0A2 is not a neon bulb as you imagine it but a 7-pin miniature gas filled cold cathode tube designed to be a voltage regulator with fairly tight regulation, unlike a neon bulb designed to be a really cheap indicator lamp.

http://www.mif.pg.gda.pl/homepages/frank/sheets/035/0/0A2.pdf
 


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