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Old 6th March 2006, 07:32 PM   #1
sgerus is offline sgerus  United States
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Default Will ccs work with a DHT?

Will ccs work with a DHT?

Please forgive me if this is a stupid question.

I was wondering if a constant current sink could be used in place of the cathode resistor with a DHT (2A3)?

I just made a cc source for my 6sl7 input and it made a huge improvement over the standard plate loaded setup.

The chip I have in mind (10M45) can handle the current and power.
My thinking is to just replace the cathode resistor with a ccs.
Right not the cathode resistor is off the heater transformer center tap.

What do you think?
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Old 6th March 2006, 07:40 PM   #2
isaacc7 is offline isaacc7  United States
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I think I read in Morgan Jones' book that you can't do it in a SE topology but that it can work very well in PP. Something about 100% feedback with SE. I'll have to go look it up...

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Old 6th March 2006, 08:14 PM   #3
timpert is offline timpert  Netherlands
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Hi Isaac,

No, it is not a stupid question. It will work, if you replace the cathode resistor by a CCS set at the normal standing current of a 2A3, then the tube should bias normally. You may see a somewhat larger spread between tubes from the nominal bias point than with a cathode resistor, but I wouldn't worry about that. Of course, you shouldn't omit the cathode decoupling capacitor or otherwise you'll hear nothing.

I understand from the IXYS datasheet that the xx in xxM45 indicate the nominal current, so you'll have to use the 50M45 or 60M45.

I cannot say anything about the sound, other than that the cathode decoupling capacitor carries the vast majority of the AC current in the cathode resistor case, and all of the AC current in the cathode current-source case. There seems to be a consensus about this cap being much more important to the sound of the amplifier than the cathode resistor. But if you're willing to experiment, by all means do so and please share the results!

Regards,
Jurgen
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Old 6th March 2006, 08:30 PM   #4
sgerus is offline sgerus  United States
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Jurgen,

I a bit confused by 10M45 datasheet. The top of the sheet shows a 2-100mA range
But lower in the datasheet they show a 7-15 mA range!

My chip seems to be working fine at 3mA for the 6sl7

Anyway, would I want to run the 2A3 at a lower current, say around 35mA, to keep the power under the 15W level when the voltage swings way to the right (on the load line chart)?
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Old 6th March 2006, 08:31 PM   #5
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I have used the IXYX 10M45 as the plate load for a 45 in two different circuits. One is used as the driver for an 845 and the other was used in a chokeless parafeed. Both of these worked quite well. I have not tried it in the cathode circuit, but it should work well.

The IXCP10M45S is an adjustable version that can be set for any current from 2 to 100 mA. They are available at DigiKey.
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Old 6th March 2006, 08:37 PM   #6
sgerus is offline sgerus  United States
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Hello Mr Tubelab,

I got the ccs idea from YOUR SITE.... thank you for providing all that info for the rest of us!

I will let you know if / how it works on a DHT
(in a few days)

-Thanks, Scott
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Old 6th March 2006, 08:49 PM   #7
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Quote:
I a bit confused by 10M45 datasheet. The top of the sheet shows a 2-100mA range But lower in the datasheet they show a 7-15 mA range!
The chip will adjust from 2 to 100 mA. The 7 to 15 spec shows the chip to chip variation with a 300 ohm resistor. This illustrates what several of us have found out, these chips are all different, and you have to adjust each one individually.

I have found that each chip in the batch that I got from DigiKey were reasonably similar, but each batch was different from the batch before it. I have used them at 30 mA with a 45 but the resistor value was different each time.
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Old 6th March 2006, 11:18 PM   #8
isaacc7 is offline isaacc7  United States
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Quote:
Originally posted by timpert
[B]Hi Isaac,

No, it is not a stupid question. It will work, if you replace the cathode resistor by a CCS set at the normal standing current of a 2A3, then the tube should bias normally. You may see a somewhat larger spread between tubes from the nominal bias point than with a cathode resistor, but I wouldn't worry about that. Of course, you shouldn't omit the cathode decoupling capacitor or otherwise you'll hear nothing.
Aha! I know I remembered something like that:-) So I'm not completely insane after all...


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Old 7th March 2006, 07:47 AM   #9
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Quote:
Originally posted by timpert
Of course, you shouldn't omit the cathode decoupling capacitor or otherwise you'll hear nothing.

Regards,
Jurgen
Jurgen,

Are you saying that the unbypassed cathode ccs will cause the circuit to operate under 100% negative feedback if not bypassed?

I thought Allen Wright used unbypassed ccs in some preamp circuits without cathode decoupling capacitors?

Thanks
Dan
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Old 7th March 2006, 09:47 AM   #10
Giaime is offline Giaime  Italy
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It seems so strange to me. I'm using a CCS-loaded cathode follower in my preamp, unbypassed. It works, so? You don't need to bypass the cathodes of SE output tubes if you use a CCS.

Imagine of the tube being a resistor, the value controlled by the grid. The CCS sets a current throught this chain (OPT+tube+CCS), but single values of "resistors" (aka. the voltages at the cathode or the anode) can change, keeping the sum constant. If you inject a positive signal at the grid, the current try to increase, the CCS keeps it constant but you have shifted down the voltage at the anode (because "CCS" seen as a resistor has it's value increased).

Or am I wrong?
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