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Old 11th July 2008, 05:07 AM   #11
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Quote:
Originally posted by Poindexter

A caution; there's some example-to-example variation, about 10% or so, in Id/Rg-k, so you need to put a little pot in there as well. I use a 250Ω metal film and a 100Ω 12-turn little pot.  It's fun to adjust your OP with a screwdriver and hear what you can hear; very instructive.

Aloha,

Poinz [/B]

Hi, is the pot placed in series with the resistor that sets the CCS's current? So you are using the pot to match plate current between channels?

Thanks for bringing it down a level for me

Dave
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Old 11th July 2008, 06:01 AM   #12
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Yes, that's exactly right.  You series the fixed resistor and the pot to get exactly the current draw that you want.  If 10% is close enough, and you don't have two channels to match, you don't need to do it.  You can read the current from any resistance in the power supply current path, even the fixed resistor in the resistor-pot series.  If you have a bigger resistor, you'll get a more precise reading.

My application is in the 'tail' of a singly-driven diff-amp.  This is a pair of 6GK5s, each loaded by a 43K resistive load, and this load is where I take the reading.  A 190 volt drop across this resistance indicates a 4.4mA current draw.  There are two of these, so (approximately, within tube matching tolerance) 8.8mA.  If this were the 249Ω fixed resistor in the control circuit, I would be looking for a 2.2V reading.

I didn't have to drop it a level for you; you're right on the cam.

Aloha,

Poinz
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Old 12th July 2008, 04:42 AM   #13
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OK, thanks... I'll be interested to see how close they are.
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Old 12th July 2008, 06:05 AM   #14
kmtang is offline kmtang  Canada
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Default CCS - 7A4 to 300B

Hi,

I bought a few of IXYS10M45 from Digikey and have them implemented in my 300B monos. The IC is used as the plate load for the 7A4 driver. The 7A4 employs RC in the cathode and it takes approx 5mA plate current.

I am rather impressed with the sound after changing from 27K resistor load to CCS. I read another article saying that the high frequency isn't that good with CCS because the high grid capacitance at the 300B. My plan is to change the 7A4 to 7N7 so that I can add a buffer stage between the CCS driveer and the 300B.

According to the datasheet, this IC works starting from 10Vdc to 450Vdc. Therefore, it is best for use as plate load.


Johnny
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Old 12th July 2008, 06:32 AM   #15
rdf is offline rdf  Canada
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Default Re: CCS - 7A4 to 300B

Quote:
I read another article saying that the high frequency isn't that good with CCS because the high grid capacitance at the 300B. [/B]
Hmmmm, the typical data for a 7A4 shows roughly 7.2 kohm plate impedance. In parallel with 27K that's 5.7K. A 20% difference? I wouldn't worry.
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Old 12th July 2008, 02:49 PM   #16
kmtang is offline kmtang  Canada
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I haven't measured the frequency response with use of CCS. According to the information from http://www.pmillett.com/current_source.htm, the difference in -3dB point at high frequency between resistor load and CCS is 79kHz to 53kHz.

CCS is just little weaker at extreme high frequency. In this case, it is better for me to live with the pure CCS drive to the 300B.

May be I can use paralleled 7N7 with CCS plate load to drive the 300B which should get more better high frequency response. The CCS chip is mounted to the chassis plate which forms excellent heatsink. I could run it at around 15mA total.


Johnny
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Old 12th July 2008, 04:10 PM   #17
zobsky is offline zobsky  India
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Well, I got the amp working with load resistors on the 12AT7. Need to get the CCS in (luckily, I still have a fair bit of space left in the chassis).

Regardless, this amp is something special, .. though just 1.5 WPC running into some old 5K console OPTs and off-the-shelf components, and adhering to the rats-net school of amp. wiring . Makes one reallywonder about the law of diminishing returns.

Just a wee bit of hum through around 100dB speakers at less than 1 meter, so I'm not going to worry about it. Power supply is minimal (5uF film - 10H - 47uF electrolytic). Haven't bothered with decoupling driver and power stages.

I'll post schematics and better pics in a separate thread when done.
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Old 26th July 2008, 07:21 PM   #18
zobsky is offline zobsky  India
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What kind of power does the current adjustment potentiometer between the G and K terminals on the IXYS CS need to be able handle?

I'm currently building the CCS . I have a couple of tiny 1K clarostat trim pots . Will they suffice? I don't expect to pass more than 10mA (probably more like 3 - 5mA) through the CCS .

Also, I'm assuming I use just 2 of the pot terminals (the wiper and one end) and ignore the other.

Thanks and apologies for the solid state ignorance.
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Old 26th July 2008, 07:35 PM   #19
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If you draw 10mA in a 100Ω pot, power dissipated will be .01W (one one hundredth of a watt).  In this case, wattage rating of the pot is not a critical parameter.  I use Digi-Key part CT94P101-ND.

Edit:  Parmy, parmy; the current (ROHS compliant) part number is CT94EP101-ND.  They're a few cents cheaper, too.

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Old 26th July 2008, 08:15 PM   #20
zobsky is offline zobsky  India
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Thanks, ... these are really tiny (a little bigger than a screwdriver head). I'm not convinced they'll run even 0.5 watt . I can't find any info based on their part number 9105ja (pretty sure they were clarostats, or maybe bourns). That's what you get for buying from a surplus shop.

I'll pick up something suitable soon.

Thanks
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