will a PTX without center tap work?

djn · 19th October 2010, 05:01 PM

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.

Doz · 19th October 2010, 05:13 PM

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)

djn · 19th October 2010, 05:55 PM

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.

DF96 · 19th October 2010, 06:10 PM

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?

djn · 19th October 2010, 06:26 PM

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?

Lingwendil · 19th October 2010, 06:39 PM

Do a Choke-input hybrid bridge to drop a bit of voltage and you should be good.

djn · 19th October 2010, 06:49 PM

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?

ChrisA · 19th October 2010, 06:53 PM

Quote:

Originally Posted by djn

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.

jrenkin · 19th October 2010, 06:54 PM

Quote:

Originally Posted by djn

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?

djn · 19th October 2010, 07:28 PM

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.

19th October 2010, 05:01 PM	#1
djn diyAudio Member Join Date: Oct 2004	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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19th October 2010, 05:13 PM	#2
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	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)

19th October 2010, 05:55 PM	#3
djn diyAudio Member Join Date: Oct 2004	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.

19th October 2010, 06:10 PM	#4
DF96 diyAudio Member Join Date: May 2007	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?

19th October 2010, 06:26 PM	#5
djn diyAudio Member Join Date: Oct 2004	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?

19th October 2010, 06:39 PM	#6
Lingwendil diyAudio Member Join Date: Apr 2008	Do a Choke-input hybrid bridge to drop a bit of voltage and you should be good.

19th October 2010, 07:28 PM	#10
djn diyAudio Member Join Date: Oct 2004	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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