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Old 19th October 2010, 05:01 PM   #1
djn is offline djn  United States
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Default will a PTX without center tap work?

I have been collecting parts for a spud amp and have a PTX with 6.3v and 5v but the two 380v legs are not center taped. So, can this be used in an amp? I've not seen a scheem without a center tape on the main primaries.
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Old 19th October 2010, 05:13 PM   #2
Doz is offline Doz  United Kingdom
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Yes, use a bridge rectifier. Your off load voltage will be 1.414 times the primary ie. 537.32 volts, so you need to ensure your filter capacitor can cope with this (and your tubes for that matter)
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Old 19th October 2010, 05:55 PM   #3
djn is offline djn  United States
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Thanks Doz. This amp chassis is a work of art (I am a metal sculptor) so it is pretty much set up for a tube rec. I think I should just wait for a CT PTX of the approp. voltage. The tubes really can't go that high anyway so I would have to knock it down....to much trouble.
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Old 19th October 2010, 06:10 PM   #4
DF96 is offline DF96  England
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A choke input PSU would give you 90% of the transformer voltage, instead of 140% with capacitor input. Do you have room for a choke?
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Old 19th October 2010, 06:26 PM   #5
djn is offline djn  United States
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Right now I have two 8mh chokes in a CLCLC configuration. So I might be able to switch that around???

Stupid question, but does a bridge rectifer HAVE to be SS or can it be tube as well?
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Old 19th October 2010, 06:39 PM   #6
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Do a Choke-input hybrid bridge to drop a bit of voltage and you should be good.
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Old 19th October 2010, 06:49 PM   #7
djn is offline djn  United States
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OK, I just whipped this up in PSU II with the two chokes I have and it looks a little low for the tubes I am using. They are the 6LR8. I need to think about 1. if this is enough voltage and 2. do I have room for 2 rec tubes.

Are there any rectifier tubes that have 4 elements in them?
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Old 19th October 2010, 06:53 PM   #8
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by djn View Post
I have been collecting parts for a spud amp and have a PTX with 6.3v and 5v but the two 380v legs are not center taped. So, can this be used in an amp? I've not seen a scheem without a center tape on the main primaries.
A bridge recifier requires four diodes. I would work fine. But with a center tap you only need two diodes. Back in the old days of tube rectifiers no one wanted to build a full wave bridge because it doubled the number of rectifier tubes. But now that solid state diodes cost just 10 cents each it is not such a big deal.

So I'd say use the non-tapped transformer if you are going with SS diodes. If using rectifier tubes, get a CT transformer.
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Old 19th October 2010, 06:54 PM   #9
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by djn View Post
OK, I just whipped this up in PSU II with the two chokes I have and it looks a little low for the tubes I am using. They are the 6LR8. I need to think about 1. if this is enough voltage and 2. do I have room for 2 rec tubes.

Are there any rectifier tubes that have 4 elements in them?
You could try a hybrid bridge rectifier.

The Valve Wizard

Maybe?
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Old 19th October 2010, 07:28 PM   #10
djn is offline djn  United States
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That is an interesting idea J. I don't know why it wouldn't work. I don't think I can modle it in PSU but maybe a little bench testing would be in order.
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