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 Post subject: High Voltage CCS ?Posted: 18 May 2010, 15:59

Joined: 05 Jan 2010, 21:16
Posts: 680
I need a CCS that can operate at voltages above the rating of the LM317HV.

It seems to me if the current is known and fixed, a simple voltage dropping resistor could be used to increase the voltage allowable on the input to the LM317HV CCS as shown below. In this example, if I(ccs) is chosen as 250 mA, a 200 ohm resistor would drop the input voltage by 50 volts, allowing an extra 50 volts of headroom before the LM317HV maxes out at 60 volts (total output 110 volts). A larger value of resistor would accomodate even higher voltages. This would allow biasing push pull tube pairs with cathodes that far exceed the voltage rating of the LM317HV. It looks to me like the series resistor can go on either side of the CCS.

Will this work? If not, why not?

BTW, the unlabled resistor is 5 ohms to give 250 mA output (I=1.25 / R).

Thanks.

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Stephen

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 Post subject: Re: High Voltage CCS ?Posted: 21 May 2010, 05:13

Joined: 23 Jan 2010, 09:17
Posts: 3
You could just add a transistor cascode ( see attached ). Then your LM317H would only see 5V but with a 200V transistor you could have 195V of swing. Or just use the transistor with a resistor instead of the LM317..

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 Post subject: Re: High Voltage CCS ?Posted: 21 May 2010, 18:44
 Editor

Joined: 28 May 2008, 21:53
Posts: 4586
There are some ideas in here:
http://classicvalve.ca/docs/CVD_CCS_pre ... y_docs.pdf

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 Post subject: Re: High Voltage CCS ?Posted: 23 May 2010, 23:15

Joined: 05 Jan 2010, 21:16
Posts: 680
RJM wrote:
You could just add a transistor cascode ( see attached ). Then your LM317H would only see 5V but with a 200V transistor you could have 195V of swing. Or just use the transistor with a resistor instead of the LM317..

This looks like a great idea. Thanks.

There are also some nice alternatives in the link to the CCS page, but I like the simplicity of this.

This is the last piece of the puzzle for biasing my GM70 amp with the GM70 tubes in a SIPP configuration using a CCS.

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Stephen

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 Post subject: Re: High Voltage CCS ?Posted: 30 May 2010, 00:43

Joined: 05 Jan 2010, 21:16
Posts: 680
RJM wrote:
You could just add a transistor cascode ( see attached ). Then your LM317H would only see 5V but with a 200V transistor you could have 195V of swing. Or just use the transistor with a resistor instead of the LM317.

This looks like a good candidate for the CCS transistor:

It can dissipate 50 watts with the proper heatsink so it should handle 200 mA at 120 volts average output voltage with no problems. Vceo is 450 volts and it's a fast device. hFE is only about 50 which is why I was considering a Darlington pair.

This in "cascode" with an LM317 seems like it would provide a great high voltage CCS for anyone desiring to use high grid bias voltage tubes in the SIPP output configuration of the oddwatt. Any opinions on this transistor or the Darlington configuration?

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Stephen

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 Post subject: Re: High Voltage CCS ?Posted: 31 May 2010, 00:55

Joined: 23 Jan 2010, 09:17
Posts: 3
A HFE of 50 is fine, with 200ma of load current you will have 4ma of base current ((1/50)*200). Just set your load current on the LM317 to (load current) + (1/50*(load current)) to compensate for the base current. A Darlington will just reduce the base current error.

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 Post subject: Re: High Voltage CCS ?Posted: 01 Jun 2010, 10:12

Joined: 05 Jan 2010, 21:16
Posts: 680
RJM wrote:
A HFE of 50 is fine, with 200ma of load current you will have 4ma of base current ((1/50)*200). Just set your load current on the LM317 to (load current) + (1/50*(load current)) to compensate for the base current. A Darlington will just reduce the base current error.

I struggled with this awhile. I came the the conclusion you're probably right. As long as hFE is relatively constant over the operating range and the base current is directly proportional to the collector current then one transistor would be OK. A Darlington pair has other problems I won't go into.

Thanks.

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Stephen

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 Post subject: Re: High Voltage CCS ?Posted: 01 Jul 2010, 05:00

Joined: 05 Jan 2010, 21:16
Posts: 680
RJM wrote:
You could just add a transistor cascode ( see attached ). Then your LM317H would only see 5V but with a 200V transistor you could have 195V of swing. Or just use the transistor with a resistor instead of the LM317..

Just FYI, I used your circuit suggestion with a BUX85 high voltage NPN transistor and it works like a charm. It's on one channel of my oddwatt amp and sounds the same as the other channel with only the LM317.

Thanks.

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Stephen

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