DC modules fot DHT amps....too expensive to buy pre-built, why not DIY them? - diyAudio
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Old 2nd December 2009, 07:23 PM   #1
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Default DC modules fot DHT amps....too expensive to buy pre-built, why not DIY them?

Hello,
I found these DC modules that retail for about $200USD a pair. I was looking at the simplified schematic they provided on the website and was wondering a few things about them.
Will you take a look at these for me and tell me if I can achieve the same thing with a self made module using a 78xx voltage regulator.

Here: Tube filament supply

As an experiment I was going to use the same thing I hand built from scratch for the KT88SE project I just finished, a simple rectifier bridge followed by a cap and voltage regulator and then another cap. Of course I provided DC to an indirectly heated filament tube (KT88) and I understand the DHT is entirely different.

This production module appears to have a few more IC's as a current source and something else. They are very expensive, about $200 a pair. I wanted to make my own modules that simply converted the AC to DC. Can one still install a hum pot with a DC source to the filaments if I chose to make a simple rectifier/filter module of my own? What is the purpose of the other stuff in their circuit and is it truly needed?

Best would be if somebody knew of a schematic that would provide me with the same results as these modules that I could just build myself for less $$$.

Thanks for your help.
Jeff
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Old 2nd December 2009, 07:52 PM   #2
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Quote:
Originally Posted by jmillerdoc View Post
tell me if I can achieve the same thing with a self made module using a 78xx voltage regulator.

With idhts i am quite happy using a 317 as a constant current source. Raw dc quality is still audible but it will probably remain so even with a high performance ccs. $200 for a ccs with automatic voltage control seems a bit silly.
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Old 2nd December 2009, 09:10 PM   #3
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Quote:
Originally Posted by analog_sa View Post
With idhts i am quite happy using a 317 as a constant current source....
I am not sure we are on the same page. I am planing to use the LM317 as a voltage regulator and smoother for the filament supply on a DHT, not the cathode current supply (which I have done elsewhere). Usually on a DHT the filament is supplied with an AC current/voltage with a hum pot. I want the filament to be heated with DC instead of AC.....so are we on the same page???
Jeff
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Old 2nd December 2009, 09:39 PM   #4
tomchr is offline tomchr  United States
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Assuming your DC supply can handle being floated at the cathode voltage you're running, I don't see any issue with running a regulated DC supply for DHT heaters. That's been done before.

The humdinger pot should not cause trouble when you convert to DC. It probably won't do much, but I don't think it'll cause trouble either.

With DC heaters, you have two options: Constant current or constant voltage. Both can be accomplished by an LM317. See Morgan Jones, "Valve Amplifiers" for more info.

~Tom
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Old 3rd December 2009, 03:42 AM   #5
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Ok, another question...the amp I am in the process of building the Hagtech Clarion 2A3
here: CLARION 2A3 Stereo SET Amp (follow link to article on that page)

He is using 2 separate filament supplies for a balanced cathode drive and no hum pots. Is this better than a filtered DC supply on the filaments....am I gaining or losing anything by rectifying the filament heater supply?

It just seems to me that a good filtered very low ripple heater supply would be better than anything AC, even this balanced cathode drive.....but I never fail to be suprised by what I learn. BTW this is my first DHT amp to build and I just want it to be one of the best I have built too.
Jeff
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Old 3rd December 2009, 03:59 AM   #6
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I saved this file that was posted a while ago...
Attached Files
File Type: pdf FilamentSupply.pdf (47.9 KB, 235 views)
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Old 3rd December 2009, 04:17 AM   #7
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astouffer,
Can this PCB be bought somewhere?
Jeff
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Old 3rd December 2009, 05:36 AM   #8
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Hi,

I'm in the process of making dedicated soft-start filament supplies and have boards on hand.

They use LM317T for up to 1.5A continuous and LM338T for 5A continuous.

I have a few blank PCB's ready and a lot more about three weeks mail away.

Cheers!


**edit**

Added PCB schematic
Attached Images
File Type: png CK-37_as_filament_regulator.png (24.1 KB, 396 views)

Last edited by Geek; 3rd December 2009 at 05:47 AM.
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Old 3rd December 2009, 06:51 AM   #9
Gordy is offline Gordy  United Kingdom
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Quote:
Originally Posted by jmillerdoc View Post
Hello,
I found these DC modules...

Here: Tube filament supply
Their module is specifically for DHT.

Their module is floating supply input and then they *ground* the 'positive' output to the audio ground end of the filament. Hence their module's 'negative' output is towards the more positive end of the filament. Look at the diagram and notes on page 6 of their application note:
http://www.tentlabs.com/Products/Tub...e_V01_AN02.pdf

I do not know why they do that but the designer is intelligent so there must be a reason. In other words it is significant, and it is best be understand it before you can hope to DIY copy it.

Anyone know why it is connected that way?
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Old 3rd December 2009, 07:37 AM   #10
Arnulf is offline Arnulf  Europe
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I know you Brits are 1 hour ahead of us so you must have still been very drowsy posting that

Quote:
Originally Posted by Gordy View Post
Their module is specifically for DHT.
What's good for the goose ...

Quote:
Their module is floating supply input and then they *ground* the 'positive' output to the audio ground end of the filament. Hence their module's 'negative' output is towards the more positive end of the filament. Look at the diagram and notes on page 6 of their application note:
http://www.tentlabs.com/Products/Tub...e_V01_AN02.pdf
I just did. What you said makes absolutely no sense If "-" terminal is at lower potential vis a vis "+" terminal and "+" terminal is attacked to amplifier's ground, the other end of the filament will be more negative. This is just how electricity works. It's cannot be "more positive".

Quote:
I do not know why they do that but the designer is intelligent so there must be a reason. In other words it is significant, and it is best be understand it before you can hope to DIY copy it.
Is he ? He forgot (or neglected to) balance heater supply in relation to ground. He's charging $200 for a board that is worth no more than $50 in components and time if custom made in miniscule quantities so he's definitely cunning businessman. This proves the old adage that there is a new sucker born every minute ... and it'd be a shame to let them keep their money.

Quote:
Anyone know why it is connected that way?
Because that's the way he drew his picture. It doesn't matter which way you turn the leads as long as you keep the same voltage differential from both ends of cathode to the rest of the tube (by tying negative lead to the ground he would have lost Vf volts).


Mmm, I think I should get into business of selling regulator boards at 5 times their actual value ...
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