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PostPosted: 01 Feb 2017, 17:08 
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Joined: 10 Sep 2015, 13:36
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The power transformer on my SE amplifier has a 6.3V heater winding rated for 4A current supply.
I want to run some power tubes, two 6CD6GA, which require 2.5A each for the filament heater plus
the 0.3A current for one 12AT7 tube.
I'm thinking of using a separate 6.3V 6A filament transformer to meet the current requirement.
One like this one - http://www.digikey.ca/scripts/DkSearch/ ... 6087361204

My question is - Is it okay to leave the 6.3V winding on the power transformer unloaded ?

OR

Is it feasible to use a filament transformer rated at 3A like this one - http://www.digikey.ca/scripts/DkSearch/ ... 6087361204
to supplement the supply from the power transformer. If yes, how do I wire up this ?

My SE amp is PCB based with screw down terminals for the transformer leads.


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PostPosted: 01 Feb 2017, 21:47 
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Location: Chilliwack, BC
URL's are bad. Please post the digikey part #

Cheers!

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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
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PostPosted: 02 Feb 2017, 07:19 
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Location: Russia Komi republic
No problems, if you leave PT with no load. Transformers calculated with load, so, if you leave 6.3v with no load you can get more voltage b+, but with resistors you can reduce this.

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PostPosted: 02 Feb 2017, 07:47 
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Geek wrote:
URL's are bad. Please post the digikey part #

Cheers!

The URL's work for me. Anyhow here's the DigiKey part numbers -

6A filament transformer - 237-1925-ND
3A filament transformer - 237-1923-ND

zyu wrote:
No problems, if you leave PT with no load. Transformers calculated with load, so, if you leave 6.3v with no load you can get more voltage b+, but with resistors you can reduce this.

Thanks for that response ZYU.
So, apart from an increase in B+ voltage there will be no other downside in leaving the 6.3V tap on the
power transformer permanently unloaded ?


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PostPosted: 02 Feb 2017, 08:20 
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1. It is perfectly fine to leave a winding unloaded. In fact, it is desirable in many cases with older transformers to reduce the heat load. It will have NO downline effect on any of the other windings other than reduced heat.

2. Theoretically, you will be putting 5.3A on a 6A transformer, running it at over 88% of its rating. This is in the David Hafler/Dynaco range of load-to-rating. I get squirrelly when I see a load that is more than about 70% of rating on any transformer - and I am much happier at 60%. Keep in mind that transformers are rated in "Ideal World" conditions - which seldom happen in actual use.

http://www.digikey.ca/product-detail/en ... -ND/455251 Comes immediately to mind if you wish to stick with Digikey. WARNING: This transformer is rated at 115VAC input. Check your line voltage.


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PostPosted: 02 Feb 2017, 09:06 
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If I was asked how to do, most of all, I would use the old power transformer winding to supply 0.3A
For big tubes, I would use a separate transformer.
I do not know if the heater is stabilized? If the heaters is powered by AC, I would choose the transformer on the basis of: a nominal load + 10%. It can be 20%.
then 2.5 + 2.5A + 10% = 5.5A.
the transformer 6A excellent choice.

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PostPosted: 02 Feb 2017, 12:59 
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Love those Triad F-series. The **only** ones I have no problems with at all in my builds that are sensitive to noise.

And as said, no problem.

Cheers!

_________________
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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 02 Feb 2017, 13:21 
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Joined: 10 Sep 2015, 13:36
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Thanks for the helpful responses guys.

Peter W. wrote:
1. It is perfectly fine to leave a winding unloaded. In fact, it is desirable in many cases with older transformers to reduce the heat load. It will have NO downline effect on any of the other windings other than reduced heat.

2. Theoretically, you will be putting 5.3A on a 6A transformer, running it at over 88% of its rating. This is in the David Hafler/Dynaco range of load-to-rating. I get squirrelly when I see a load that is more than about 70% of rating on any transformer - and I am much happier at 60%. Keep in mind that transformers are rated in "Ideal World" conditions - which seldom happen in actual use.

http://www.digikey.ca/product-detail/en ... -ND/455251 Comes immediately to mind if you wish to stick with Digikey. WARNING: This transformer is rated at 115VAC input. Check your line voltage.

I'll think about that transformer you suggest Peter.
The thing is, of the several different output tubes I can use in my SE amp, the 6CD6 is the only one that require
such high heater current, all the others require less that 1.5A each. I will only use the 6CD6 some of the time as
part of the fun of 'tube rolling' :D
So can I regard the 6A transformer as okay under this condition ?
The 115vac input is fine for my mains supply.

zyu wrote:
If I was asked how to do, most of all, I would use the old power transformer winding to supply 0.3A
For big tubes, I would use a separate transformer.
I do not know if the heater is stabilized? If the heaters is powered by AC, I would choose the transformer on the basis of: a nominal load + 10%. It can be 20%.
then 2.5 + 2.5A + 10% = 5.5A.
the transformer 6A excellent choice.

ZYU, as mentioned, my amplifier is PCB based with screw down terminals for the transformer leads.
I am not capable of or want to re-jig the board to separate the heater supply for the preamp and power tubes.


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PostPosted: 02 Feb 2017, 14:45 
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Location: Russia Komi republic
harrydutt wrote:
ZYU, as mentioned, my amplifier is PCB based

I did not notice it. This changes everything.
As I wrote earlier, but with the changes:
2.5+2.5+0.3+10%=5.85А
Still OK. But if you can get something like 6.5-6.6A it will be 80% load.
What I try to tell my calculations?
The larger the stock - the better. But there are a lot of conditions: the transformer size, cost, etc.
Will enough 6A? I think yes. But, as said before, more is better, so it will be calm for you.

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PostPosted: 02 Feb 2017, 15:14 
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Joined: 10 Sep 2015, 13:36
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Okay, I see Mouser has a 6.3V 8A Triad Magnetics transformer so I'll get one from there.
This one - http://ca.mouser.com/ProductDetail/Tria ... 6Th1PZaw== , Mouser part # 553-F69X
This will be a more suitable choice.
Thanks for all the responses.


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