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Old 6th November 2006, 02:16 PM   #1
jarthel is offline jarthel  Australia
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Default something is wrong with the shunt regulator

The ccs came from G pimm's latest self-biasing circuit (http://home.pacifier.com/~gpimm/self_bias.htm).

The shunt regulator is from raleighaudio (http://www.raleighaudio.com/ccs.htm)


I tested the CCS circuit first by placing a 2kohm load from CCS output to ground. Using the trimpot, I have set the output to 75mA. At this time, the shunt regulator is still not connected.


R1 and R2 of the regulator circuit is connected. but the TL431 and the tube are not yet connected in the circuit. I measured around 147V across R1 and around 2.5V across R2. This are the correct measurements.

I then inserted the TL431 and 6SL7GT to the shunt regulator circuit. Voltage across R1 is pretty much the same. But the voltage across R2 has been halved. It dropped from around 2.5V to 1.25V. I thought the reference pin of the TL431 is suppose to keep voltage across R2 at 2.5V. I also measured the voltage on the cathode pin of the TL431 and it is around 1.9V.

any ideas?
========

also when testing regulated B+, do I need a load? With both the CCS and shunt regulator circuits connected and no load, the voltage from triode plate to ground is around 340V. I thought that with regulator power supplies, no load is needed to set the required voltage/current.

My regulated PSU knowledge is limited to LM317 (and other similar chips) where no load is required to set current/voltage.

Thanks for the help.
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Old 6th November 2006, 08:39 PM   #2
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jarthel:
It sounds like your R1 and R2 are too low. When current is drawn by the tube and reference diode there is less for R1/R2, so voltage across R2 goes down.

Of the total 75mA provided by the source some must go to the load, some through the tube (shunt regulating) and some through the R1/R2 voltage divider. Plan on a few milliamps for R1 and R2, leaving the rest for the load and the shunt. I would try 500 ohms for R2 and 30K for R1. ( I am guessing based on the voltages you mention you are aiming for 150V output, about 70mA ?)

Make sure you do use a load when you set it up since the 6SL7 isnt rated for very much current, so the load must draw most of it. Either that or use a different tube.
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Old 6th November 2006, 09:44 PM   #3
jarthel is offline jarthel  Australia
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R1 = 1.5M
R2 = 24.9K

so I do not think that is low?
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Old 6th November 2006, 11:52 PM   #4
jarthel is offline jarthel  Australia
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here's the combined schematic:

http://img470.imageshack.us/my.php?i...titled3vd5.png

thanks for the help
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Old 7th November 2006, 12:39 AM   #5
kevinkr is offline kevinkr  United States
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Hi Jarthel,
I noted the TL431 reference input bias current is only about 4uA so 100uA current in the reference leg is not unreasonable, although I would probably go for 1mA or so..

Are you certain you have not swapped any of the TL431 pins (gnd and output?)

Also the current handling capability of the 6SL7 is extremely limited, a couple of say 6BX7/6V6/6CK4/12B4 might be a better choice. Under no load conditions all of the load current must flow through the shunt regulator tube. Any combination that can dissipate >12W ought to be ok.

Given that you used a 6SL7 it is highly likely that open circuit it would saturate and kill the TL431.
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Old 7th November 2006, 01:08 AM   #6
jarthel is offline jarthel  Australia
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Quote:
Originally posted by kevinkr

I noted the TL431 reference input bias current is only about 4uA so 100uA current in the reference leg is not unreasonable, although I would probably go for 1mA or so..
do you mean I need to decrease R1/R2 combo?
Quote:
Originally posted by kevinkr

Are you certain you have not swapped any of the TL431 pins (gnd and output?)
I've checked the proper pins several times by comparing the actual chip to the diagram in the datasheet. I do not think I made a mistake in this regard.

Quote:
Originally posted by kevinkr

Also the current handling capability of the 6SL7 is extremely limited, a couple of say 6BX7/6V6/6CK4/12B4 might be a better choice. Under no load conditions all of the load current must flow through the shunt regulator tube. Any combination that can dissipate >12W ought to be ok.
Where in the datasheet can I find the 12W dissipation info? I am looking at the 6BX7GT datasheet (http://www.mif.pg.gda.pl/homepages/f...3/6/6BX7GT.pdf) and cannot seem to find the relevant info.

when using a 6v6, I assumed you meant it's configured as a triode? if yes, any chance of explaining how this is done?

Quote:
Originally posted by kevinkr

Given that you used a 6SL7 it is highly likely that open circuit it would saturate and kill the TL431.
just wondering how do you know that a 6sl7 is unsuitable.

Thank you for the help.
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Old 7th November 2006, 02:01 AM   #7
jarthel is offline jarthel  Australia
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Quote:
Originally posted by jarthel
here's the combined schematic:

http://img470.imageshack.us/my.php?i...titled3vd5.png

thanks for the help
A mistake on the transformer label. it's suppose to be 520VCT.
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Old 7th November 2006, 03:38 AM   #8
jarthel is offline jarthel  Australia
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Quote:
Originally posted by kevinkr


Any combination that can dissipate >12W ought to be ok.

please ignore the reply above in regards to plate dissipation. I found it in the 6bx7 datasheet.
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Old 7th November 2006, 03:09 PM   #9
kevinkr is offline kevinkr  United States
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Hi Jarthel,
The 6SL7 is an inappropriate choice because:

a)If the supply is unloaded the tube will be called upon to sink the full 75mA, which is about 20 times higher than the tube design target value.

b.) Dissipation per section is 1W for a total of 2W which means that 150V the maximum current the tube can handle before the dissipation limit is exceeded is about 13mA.


Yes, reduce R1/R2 by a factor of 10 to get 1mA of current in the reference resistor string. Say 2.4K and 150K.

6V6 can easily be triode strapped, connect screen grid to plate using a 100 ohm 1/2W resistor.

What is the overall load current this supply is meant to deliver to the load? To assure good regulation under all conditions particularly if the load is not purely class A I would recommend at least 10 - 20 % of the load current in normal operation be diverted to the shunt regulator with the rest going to the load. This would mean for an external load of 75mA max you would want the current source to supply at least an additional 10mA which is "burned up" in the shunt regulator. Note that the tube must be able to dissipate the full power if the load is disconnected for some reason. (Wiring error, tube failure, failed component.)

I used lower power shunt regulators in some of my early pre-amp designs and found them particularly worthwhile for avoiding low frequency interactions through the supplies. (I used one for each gain block.)

Hope this all helps.
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Old 7th November 2006, 09:40 PM   #10
jarthel is offline jarthel  Australia
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Quote:
Originally posted by kevinkr
Hi Jarthel,
The 6SL7 is an inappropriate choice because:

a)If the supply is unloaded the tube will be called upon to sink the full 75mA, which is about 20 times higher than the tube design target value.

b.) Dissipation per section is 1W for a total of 2W which means that 150V the maximum current the tube can handle before the dissipation limit is exceeded is about 13mA.


Yes, reduce R1/R2 by a factor of 10 to get 1mA of current in the reference resistor string. Say 2.4K and 150K.

6V6 can easily be triode strapped, connect screen grid to plate using a 100 ohm 1/2W resistor.

What is the overall load current this supply is meant to deliver to the load? To assure good regulation under all conditions particularly if the load is not purely class A I would recommend at least 10 - 20 % of the load current in normal operation be diverted to the shunt regulator with the rest going to the load. This would mean for an external load of 75mA max you would want the current source to supply at least an additional 10mA which is "burned up" in the shunt regulator. Note that the tube must be able to dissipate the full power if the load is disconnected for some reason. (Wiring error, tube failure, failed component.)

I used lower power shunt regulators in some of my early pre-amp designs and found them particularly worthwhile for avoiding low frequency interactions through the supplies. (I used one for each gain block.)

Hope this all helps.
is dissipation the only factor for tube selection?
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