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Old 12th March 2012, 07:53 PM   #1761
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by Rod Coleman View Post
Hi Ale, Yes, it is short of voltage. C2 should have about 10V across it to set a stable current in the P-FET, and The FET needs about 20-30V of Vds, minimum. When the drain-source voltage is too low, the current source shows a lowered dynamic impedance, which makes a bad current source. The LED string needs to increase to at least 25V for good CCS behaviour.

The 6Ж51П circuit looks really good!
Yes, I thought you were going to say that You did tell me last time as well when designing the 45 SE driver stage!
Perhaps I can try the cascoded MOSFET gyrator instead given that my HT is limited to 200V?

Cheers,
Ale
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Old 12th March 2012, 08:31 PM   #1762
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Originally Posted by revintage View Post
. Have not checked for a while. Have they gotten too popular?
Hmmm... They went down!
Looks like some military warehouse barn gates had been opened.

Yes, 6Э5П is a very good candidate for cathode follower here.
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Old 13th March 2012, 06:18 PM   #1763
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What is an ideal source impedance to drive the 26? Is it similar to other DHTs where it needs to be quite low? I am trying to plan the summing stage to my mixer, originally I was going to use 6SN7s for the output stage which can take a higher source impedance, but now this thread has taken control of my life I need to plan for using the 26

I really love the free heat idea, im going to look into using such valves in the design. It would be also be great to not have so much heat in the unit when there won't be much space around.

... Still playing with ideas, thanks for you help

Charlie
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Old 13th March 2012, 06:25 PM   #1764
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Just a quick update:

Over the weekend I wired up my preamp without the 26 circuitry; basically making a passive preamp to test that all of my autoformer wiring was correct. I had one mistake (forgot to reference the secondary to ground). This got me thinking....
Im using Kevin's design, which uses a .1uF input cap. Being that Im using an autoformer - do I really need this input cap ? I would assume it is in place to block DC from the previous stage...

Thanks !

EDIT - on second look - doesn't seem like it will work with the schematic as-is. I'll be starting off with Grid Bias, so with the secondary grounded it'll short out the bias circuit.

Last edited by Steven-H; 13th March 2012 at 06:37 PM.
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Old 13th March 2012, 07:16 PM   #1765
Magz is offline Magz  United States
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Autoformers don't block DC. They only have one winding.
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Old 13th March 2012, 07:19 PM   #1766
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Question, if using filament bias on a c-LC-RC-L filament supply that can reduce ripple to 2mV using two 25mH DC filament chokes. Is a 25mH choke sufficient as the second L to isolate the filament supply from the signal, since the "LCL" configuration has been promoted?
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Old 13th March 2012, 08:08 PM   #1767
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Quote:
Originally Posted by jamesdb View Post
Question, if using filament bias on a c-LC-RC-L filament supply that can reduce ripple to 2mV using two 25mH DC filament chokes. Is a 25mH choke sufficient as the second L to isolate the filament supply from the signal, since the "LCL" configuration has been promoted?
25mH is only about 16 ohms, even at 100Hz - so it would be some way short. It's usually worth checking the leakage capacitance of these, too - to see that there is some HF noise blocking.
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Old 13th March 2012, 08:26 PM   #1768
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Quote:
Originally Posted by CSlee View Post
What is an ideal source impedance to drive the 26? Is it similar to other DHTs where it needs to be quite low? I am trying to plan the summing stage to my mixer, originally I was going to use 6SN7s for the output stage which can take a higher source impedance, but now this thread has taken control of my life I need to plan for using the 26

I really love the free heat idea, im going to look into using such valves in the design. It would be also be great to not have so much heat in the unit when there won't be much space around.

... Still playing with ideas, thanks for you help

Charlie
It's not quite the brute that a 300B is - where the grid resistor is only 50K for fixed bias, and the Cin is up to 100pF.

The corresponding values are usually 220K and about 40pF for the 26, and the grid drive voltage is usually below 5V peak.

For those conditions, the drive current is about 70uA peak for 5V peak, even at 50kHz.

This is a lot easier than a 10 or 20K pot, as may be seen on many power amps. You should be able to drive the 26 with almost any kind of valve or solid state source - well, a ECC83/12AX7 stage might be pushing it.... but please remember to calculate the cable current, as well - if you are running any.
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Old 14th March 2012, 03:15 AM   #1769
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Hi!

Quote:
Originally Posted by CSlee View Post
What is an ideal source impedance to drive the 26? Is it similar to other DHTs where it needs to be quite low?

The 26 does not require a low source impedance to be driven. It is very easy to drive. Not much different from a 6SN7

Best regards

Thomas
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Old 14th March 2012, 03:36 AM   #1770
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Hi!

Quote:
Originally Posted by jamesdb View Post
Question, if using filament bias on a c-LC-RC-L filament supply that can reduce ripple to 2mV using two 25mH DC filament chokes. Is a 25mH choke sufficient as the second L to isolate the filament supply from the signal, since the "LCL" configuration has been promoted?
I agree with Rod that 25mHy would be too low. I'm using Ls in the range of several 100mHy. Lundahls LL2733 can be gapped for 1A and yields 700mH like this.

Also 2mV ripple on the filament of a DHT in a linestage is quite high. This will be amplified by the 26.

If you want to use LCL, use some serious inductance. If budget or space does not allow for bigger coils, I'd rather use Rod's regulator

Best regards

Thomas
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