6AS7 OTL Headphone Amp - Removing the 5u4

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6AS7 OTL Headphone Amp - Removing the 5u4

Postby coop402 » Tue May 01, 2018 4:22 pm

I have built this headphone amp twice. It sounds awesome.
http://www.pmillett.com/wheatfield/manual.pdf


I would like to use a toroidal power supply and go to a solid state rectifier bridge.

If the tube gurus on this forum wanted to go from the 5u4 to solid state, what would the transformer voltage be, and what would you use to clean up the ripple, etc. That is my goal.

Ultimately, I guess I could wreck the sound. What I am trying to do is eliminate the 5u4 and the need for the 5 volt filament voltage. This would allow me to easily find a transformer that would meet my goal. I guess you never know unless you try. I just want to get my B+ just right the first time.
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby battradio » Tue May 01, 2018 5:54 pm

http://www.antekinc.com/as-1t250/ this and a full wave bridge will do the trick
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby mike567 » Tue May 01, 2018 6:20 pm

That transformer will give you about 265 volts unloaded 5U4 drops about 20-25 volts. Maybe the 230 volt version at 245 volts unloaded
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby battradio » Tue May 01, 2018 6:25 pm

mike567 wrote:That transformer will give you about 265 volts unloaded 5U4 drops about 20-25 volts. Maybe the 230 volt version at 245 volts unloaded


More like 353 volts DC
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby coop402 » Tue May 01, 2018 7:39 pm

I'm an idiot! OK, the original voltage was 540 ct. I thought it was going to be like 500 volts.
Can you tell me what the voltage will be once it comes out of the 5u4? I am so glad I asked this question.
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby coop402 » Tue May 01, 2018 7:51 pm

OHHH. Wait. That transformer has 2 250 volt taps. You can put them in serial for 500v. but through a bridge that would be..... 720? So, 160 or 175 would be better x 2?
This is why I asked!
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby battradio » Tue May 01, 2018 11:35 pm

The 5U4 drops about 40 volts , so if he parallels the 250 volt windings and uses a bridge rectifier the voltages will come out right .

If he uses one winding for each channel and builds two seperate supplies, it will even be better
Last edited by battradio on Wed May 02, 2018 3:41 pm, edited 1 time in total.
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby coop402 » Wed May 02, 2018 7:27 am

So the increase in voltage is times 1.414 for a bridge rectifier.
250 volts x 1.414 is 353.5. I see a reference past the first parallel resistor of 300 volts. Is 353 too much?

Secondy. I was looking at the caps and choke for filtering the B+ power.
I was thinking since I am moving to solid state that I might have to change that.
When I look at the Darling amp schematics (solid state half wave) I see that the filtering is near identical.

Sounds like I can get 4 diodes and build a bridge rectifier, get an Antec power supply, and that is that.

Only thing is the voltage......
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby battradio » Wed May 02, 2018 3:53 pm

The 5 henry filter choke and the series resistor R2 & R3 will drop the voltage from 353 to about 300 volts , giving you the correct voltage . If the voltage is still too high the value of R2 & R3 can be increase to 25 ohms 2 watts .
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby mike567 » Wed May 02, 2018 4:27 pm

Battradio:
I have one of the 230 volt versions
original transformer is 540 ct = 270 volts
270-40 volt drop before first cap= 230. Maybe better to use the 230 volt version?
Use only one high voltage winding as there will be some sag.
240 volts * 1.414=339 - (about 5% for sag)17=322. Less voltage to drop.
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby battradio » Wed May 02, 2018 4:43 pm

AS long as it has enough filament current for the 6AS7's and enough voltage drop to get rid of all the ripple , this being a headphone amp .
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby mike567 » Wed May 02, 2018 5:05 pm

antek transformer has two 3 amp 6 volt windings
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby coop402 » Thu May 03, 2018 7:32 am

That makes total sense. I really appreciate all of your help, folks.
I will post the amp when finished.

I sure am glad that Antec is around. This is the first time I have bought one of their transformers, and they are priced to sell. Super cheap compared to the comparable Hammond.
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby mike567 » Thu May 03, 2018 3:51 pm

At almost 6" in diameter they take up alot of realestate. :'(
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Re: 6AS7 OTL Headphone Amp - Removing the 5u4

Postby battradio » Fri May 04, 2018 3:12 pm

mike567 wrote:At almost 6" in diameter they take up alot of real estate. :'(


Mount it vertically with a L or u bracket , Under the chassis works also .
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