The Phantom SR45 Build - Snow Creek Shuttle Episode 1-A Bottlehead Saga

ssssly · 10207

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Offline Paul Joppa

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The center tap is terminal 10 on the PT-7. The labeling is somewhat ambiguous.

Paul Joppa


Offline ssssly

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Mains and heaters wired. Only 1k resisters I had on hand were 5w. Overkill but it's putting out 2.6v.




Offline Paul Birkeland

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What's the 1K resistor doing there?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssssly

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First leg of a divider to drop the filament voltage to 2.5v.



Offline Paul Joppa

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You won't get 1.5 amps through a 1000-ohm resistor ...  :^D

Paul Joppa


Offline Paul Birkeland

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You can use two 1N5820 diodes to make a full wave rectified supply to feed the #45, then a small value resistor, then a 10,000uF cap.  The value of the resistor and the actual 10,000uF cap you use will interact together to result in an optimal value, but I think you could start with a 0.22 ohm/3+W resistor to see what happens. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssssly

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You can use two 1N5820 diodes to make a full wave rectified supply to feed the #45, then a small value resistor, then a 10,000uF cap.  The value of the resistor and the actual 10,000uF cap you use will interact together to result in an optimal value, but I think you could start with a 0.22 ohm/3+W resistor to see what happens.

Sounds like a wonderful idea. And I have everything but the resistors.

But i needed to order parts this week anyway.

What would you suggest as a reasonable range of resistors to buy to dial it in? Given the 10,000uf caps I have are the Xicons from a BeePre.




Offline Paul Birkeland

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I'd buy a 0.15, 0.2, and 0.3 or so ohms.  If you're ordering resistors, maybe look at getting a pair of 10,000uF 25V caps.  Those caps will live a pretty hard life in that position, so a physically larger cap with higher ripple current capabilities will live a lot longer.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssssly

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Dang it. I'm going to find something to do with those BeePre parts.

Though I suppose I should just knock it up and see what the real world voltage drop and ripple is with those diodes first anyway. Spec sheet says .475v peak drop which would only leave me with ~2.2v. So I'm guessing not much voltage left to play around with under load.  And fingers crossed that real world drop is under .6v combined.

And I should probably get the scope and boards out anyway. Need to design and test the voltage doubler for the HV side.

And now I'm sitting here thinking: I need to order stuff anyway, why not try to throw pi filter in there. Or model it up and see if I can do a constant current heater to prolong the life of the 45s. In other words why I always have 10 projects going at once.

Right. Order some good 10000uf filter caps and resistors. Build it and see what it sounds like.



Offline Paul Birkeland

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Which diodes did you select for the filament supply? 

I'm not entirely certain that constant current regulation for a filament actually prolongs the life of a tube, but YMMV.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssssly

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Which diodes did you select for the filament supply? 

I'm not entirely certain that constant current regulation for a filament actually prolongs the life of a tube, but YMMV.

1N5820s.

I'm not convinced either way either. And upon a minute of further thought, the circuit I was thinking of would require a second heater in series or a dummy load. So if I decide to try the current route, it would be in a different form.

I'm just going to build the full wave rectifier with over specd caps, see what I get for ripple and turn on voltage and go from there. I want to get them built and see what they sound like more than I want to fiddle with theoretical benefits at this point.

Oh, there is a connection labeled ground on the board you made me that is connected to the center tap in the hookup instructions. But that was for a PT-2. Would you still suggest connecting it to the center tap at terminal 10 on the PT-7?

From my incomplete understanding of the differences between the two transformers I was thinking ground.



Offline Paul Birkeland

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The PT-7 has only one secondary.  If you ground it, that will zero bias the #45 tube and things will go awry in a huge hurry.  That ground connection should connect to your audio ground and the heater winding floats (it will be biased up to the bias voltage of the #45 tube since it's serving as the filament winding).

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline ssssly

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Sweet. Thanks. That's what I thought.

I'm just happy I finally guessed correctly.




Offline ssssly

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When life gives you silly big capacitors, 3d print them a cap strap while you are waiting for your Mouser order.

« Last Edit: May 25, 2023, 07:32:58 AM by ssssly »



Offline 2wo

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Well, that's pretty cool...John

John S.