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PT Wiring
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Cygnus X1



Joined: 22 Apr 2008
Posts: 915
Location: Midlands South Carolina

PostPosted: Sun Jun 29, 2008 2:21 pm    Post subject: PT Wiring Reply with quote

Just to make sure I'm not messing up before I apply juice...
Which wires am I supposed to use to get 300V to each side of the diodes?

Looks like red-yellow, red-white, and then red to common?

(370,70,0).

It's a weber WPTGP universal PT.

How do I past an image directly, anyways?
The link to the layout of the PT is here:
https://taweber.powweb.com/store/PTGPsch.jpg
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Phil_S



Joined: 23 Oct 2007
Posts: 3441
Location: Baltimore, MD

PostPosted: Sun Jun 29, 2008 3:11 pm    Post subject: Reply with quote

You can use 620-0 or 690-70 to deliver 310v on each leg. I don't see an appropriate CT, as they will be uneven. There doesn't appear to be a 300-300 combination. Build the type of rectifier that doesn't use a CT.

What you suggested won't work.
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Cygnus X1



Joined: 22 Apr 2008
Posts: 915
Location: Midlands South Carolina

PostPosted: Sun Jun 29, 2008 4:05 pm    Post subject: Reply with quote

Thank you, Phil.
The fullwave rectifier I am using doesn't use a center tap.(Two diodes).

But which wire do I use for the negative//ground?
The red 0V one?
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Luthierwnc



Joined: 18 Feb 2006
Posts: 856
Location: Asheville, NC

PostPosted: Sun Jun 29, 2008 4:44 pm    Post subject: Reply with quote

Interesting design. You connect the leads that give you the correct voltage to either side. In Weber's example for 300 volts you would use the 370 and 70 and tape off the rest. Keep in mind you are only getting 300 volts total -- not 300 per side. Run it through Duncan's power calculator to get your volts and amps right.

Do you need fixed bias? You might take a look at an Ampeg Gemini schematic for pulling off a bridge rectifier. It is quite different than the usual Fender style.

Skip
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Phil_S



Joined: 23 Oct 2007
Posts: 3441
Location: Baltimore, MD

PostPosted: Sun Jun 29, 2008 8:02 pm    Post subject: Reply with quote

Cygnus X1 wrote:
Thank you, Phil.
The fullwave rectifier I am using doesn't use a center tap.(Two diodes).

But which wire do I use for the negative//ground?
The red 0V one?

It probably doesn't matter, but I'd use the 0 wire.
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Ears



Joined: 09 May 2007
Posts: 292
Location: New Zealand

PostPosted: Mon Jun 30, 2008 7:36 am    Post subject: Reply with quote

Phil_S wrote:
Cygnus X1 wrote:
Thank you, Phil.
The fullwave rectifier I am using doesn't use a center tap.(Two diodes).

But which wire do I use for the negative//ground?
The red 0V one?

It probably doesn't matter, but I'd use the 0 wire.


I'd tape the red wire off. But until you supply a pic the rectfier you're using all replies are shooting in the dark and should be ignored. I think being explicit with diagrams schematic etc is VERY IMPORTANT if asking for advice about high voltages. Otherwise someone will be responsible for killing you.
Put safety first.

Edit: I'm serious, very often written explanations or vague assurances as to cct topography are inadequate - how does anyone know what you might understand or might not understand? Schematics clear things up and eliminate the possibility of fatal errors or misunderstandings.
The label 0V on the red wire means that it is at zero potential in reference to itself only, the reference is is only a label. You could call it 750v if you choose to, only then you'd have to re-label the other secondaries as well. So, why choose the (AC) "0" as your ground potential? one of the (AC) 600Vor whatever taps would do as good a job.
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Phil_S



Joined: 23 Oct 2007
Posts: 3441
Location: Baltimore, MD

PostPosted: Mon Jun 30, 2008 1:13 pm    Post subject: Reply with quote

Ears: you are right about this. I did not properly qualify my advice. I started, but wasn't thorough. I said to pick a rectifier with no center tap, but that was less than half a loaf. Thank you for your comment.
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Ears



Joined: 09 May 2007
Posts: 292
Location: New Zealand

PostPosted: Mon Jun 30, 2008 8:15 pm    Post subject: Reply with quote

Thanks Phil, I wasn't necessarily meaning to denigrate your reply.

Cygnus, your question suggests you might benefit by reading the following:

http://www.play-hookey.com/ac_theory/ps_rectifiers.html
or the Valve wizard site in the UK.

Summary: Full wave (two diode) rectifiers require a centre tap. Two diodes without centre tap can supply two half wave rectified outputs or can function as multipliers. Otherwise non-centre tapped windings can be used as a half wave rectifier or require a bridge rectifier. Look at the ground connections in schematics in the websites provided.
Your transformer is not centre tapped.
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joebob



Joined: 18 May 2008
Posts: 82
Location: Georgia

PostPosted: Mon Jun 30, 2008 8:42 pm    Post subject: Reply with quote

Ted says that PT is for 'Bridge Rectifier', with no CT that would mean a Full Wave Bridge Rectifier. Using either 4, 8 or 16 diodes to form one. Or use one of these 'Rectron Bridge Rectifiers' or similar rated bridge. I am sure some one here would know approximately how much voltage you would get from a Full Wave Bridge rectifier.

http://www.mouser.com/catalog/634/460.pdf

Quote:
General purpose multi-tap transformer for bridge rectified high voltage supplies. 120, 220, and 240 primary.

Just my 2 cents.
Dave

_________________
Instructions...I don't need no stinkin instructions
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Cygnus X1



Joined: 22 Apr 2008
Posts: 915
Location: Midlands South Carolina

PostPosted: Mon Jun 30, 2008 10:20 pm    Post subject: Reply with quote

Thanks all...
I neglected to post the schematic.
It's the AX84 project SEL.
It calls for a Hammond 270EX, but I have the Weber, and it looks like it will work.
I take it I will have to build a Wheatstone Bridge style full wave rectifier.
It calls for 365 Volts on the B+.
So given that...what wires do I use on the transformer to do it?

I have several N4007 and N5408 rectifier diodes...will either of these work?

The schem and layout are here:
http://www.ax84.com/static/sel/AX84_SEL_080522.pdf
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Phil_S



Joined: 23 Oct 2007
Posts: 3441
Location: Baltimore, MD

PostPosted: Mon Jun 30, 2008 10:37 pm    Post subject: Reply with quote

Ears: I was not offended. Don't think about it.
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Cygnus X1



Joined: 22 Apr 2008
Posts: 915
Location: Midlands South Carolina

PostPosted: Mon Jun 30, 2008 10:46 pm    Post subject: Reply with quote

Well, I'm counting on you (all)!

I haven't done a thing until my confusion is cleared.
I usually only build to exact schematics..except for mixing and matching components to get values that I don't have on hand.

Smile
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Luthierwnc



Joined: 18 Feb 2006
Posts: 856
Location: Asheville, NC

PostPosted: Mon Jun 30, 2008 11:51 pm    Post subject: Reply with quote

I ran a quick Duncan power supply designer on you schematic. Assumptions are that the power tube will pull 50 ma and the preamp tubes will add another 10. At a target voltage of 350 on the plates and 60 milliamps of draw, you get a load approximation of 5k8 or 350/.06 (it's sloppy since it doesn't really figure the reduced volts on the preamps but it works here).

That means you need around 260 VAC of differential between the two corners of the bridge. Whether it is 260 and 0 or 500 and 240 doesn't matter.

To test the numbers yourself, download the PSUD from this site. It is free and allows for a lot of variables.

http://www.duncanamps.com/psud2/index.html

The voltage calculation for a bridge is: (vdc + 1.4)/square root of 2 In this case 250ish. Allowing for some load loss you are back around 260. Rule of thumb is you get 1.41 times the volts from a bridge (which is also the square root of 2) no load. Pull off some load loss and you get the 1.35 ratio by dividing 350 by 260.

Back to the trannie leads for a minute. You are looking for the two that give you closest to 260 volts in differential. Double heatshrink the tips of the leads you don't use. That stuff isn't rated for old school amp voltages. I also wouldn't cut them short until you do your spark test. One, you may come up long or short on the calculation and two, you might decide you want a different voltage than the schematic. With a solid state rectifier and a target of 350 volts, you will be on the hot side of that tube's sweet spot. Food for thought.

Good luck, Skip
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Cygnus X1



Joined: 22 Apr 2008
Posts: 915
Location: Midlands South Carolina

PostPosted: Tue Jul 01, 2008 12:05 am    Post subject: Reply with quote

OK..so the 620 minus 370 is 250.
I can play with the numbers to get closer, perhaps.

The tranny is listed at 150 ma, so that should be plenty (at least that's part of the reason I figured I could go ahead and use this tranny for this project).

So...build the bridge out of the N4007 diodes? (1 amp, 1000 volts).

I am so completely lost around the power supply, I just never "play" with them. Not very many problems elsewhere in the amps, though.
I've seen this all in theory many times (I have old college books), but rarely utilize it.

Thanks again, Skip and everyone!
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Luthierwnc



Joined: 18 Feb 2006
Posts: 856
Location: Asheville, NC

PostPosted: Tue Jul 01, 2008 1:07 am    Post subject: Reply with quote

I think the sweet spot for that amp would be about 335 on the plates. Running single-ended with a small cathode resistor will wear out most EL84's pretty fast.

As far as diodes go, 1N4007's will work fine. I usually use UF4007's. They are only a couple pennies more and might give you a little more speed. I'd also be tempted to increase the filter cap value at the first node. Most of the EL84 designs you see have rectifier tubes. My little 6BQ5 amp has switchable tube/ss rectification and the mod calls for a 100uf cap in the diode version. YMMV, Skip
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