Help for Main Transformer Spec for CCDA
I am building a CCDA stereo 9-Pin valve pre-amp and looking for a power
transformer to work with.
The following is quoted from the document:
ď The CCDA PCB requires a power transformer(s) to energize its two power supplies.
The heater power supply power transformer must offer at least 1.8 times more current than the heaters will draw. For example, two 12FQ7s will draw 0.6A @12.6v, so the heater power transformer must be able to sustain an AC 1.08A current draw. In addition, with sine waves, the AC voltage equals the peak voltage divided by the square root of 2, i.e. 1.414. Thus, a 10Vac sine wave peaks at 14.14V; a 6.3Vac, 8.9V.
In other words, a sine wave that peaks at 14.14V will produce the same amount of heat in a resistance as a 10Vdc voltage source would produce in the same resistance; thus, we label the 14.14Vpk sine wave as being 10Vac. Thus, in order to get the 16Vdc raw DC voltage that a 12.6V heater voltage regulator requires an input voltage equal to remainder of 16V minus the rectifier loss (about 2V) divided by 1.414, which is roughly 12.6Vac.
The high voltage power transformer must also follow the same rules. Thus, to achieve 300V of raw DC voltage, the transformer secondary must deliver (300V +2V) / 1.414, or about 214Vac. And if 50mA is required, the power transformer must be rated for 50mA x 1.8 (in a full-wave bridge rectifier circuit), or 90mA. Thus, such a transformer VA rating would be rated about 20VA, as 0.9 x 214 =19.71. "
I have very limited knowledge and understanding on the power
transformer and seek your help to work out the spec on the Heater and HV.
The following is my understanding for the secondaries of the main transformer
HV 300 VDC (300v + 2v / 1.414) = 214 VAC, 20VA
Heater 17.8 VDC (16v + 2v / 1.414) = 12.6 VAC
Iím going to use 6DJ8 x 2. The heater voltage is 6.3v and heater current is 365ma.
Correct me if Iím wrong. The heater requires 12.6VAC at 1.3A (365ma x 2 =730ma x 1.8 =1.314A x 12.6VAC) = 16.6 VA.
I have looked at the Sowterís Website Custom Designed Mains Transformers. Its price depends on total power in watts. It says: To obtain the total power in Watts (VA) simply multiply the voltage and current rating of each secondary winding and add them together.
Does it mean: HV = 20(VA) Watts + Heater 16.6(VA) Watts = Total 36.6 Watts? Should I choose the transformer from 35W or 40W?
SOWTER CUSTOM MAINS TRANSFORMERS
I am using solid-state diode for rectification and prefer the centre-tape version for the PT. The transformer main can be used either 120v or 240v. I appreciate your advice on the spec and if you know where to source the power transformer.
I'll look into it.
A couple of questions -
What HT voltage do you want? (I.E., what B+ voltage, 200v? 250v? 300v? something else?)
And do you know of any local (well, Australia, for your purposes) distributor of DIY parts and such? A link would be very helpful.
There has to be an off the shelf transformer that is close enough, or perhaps even perfect. You are not needing anything particularly special or strange. A custom xfmr at that power level will be quite expensive for it's size.
The B+ is 300v.
The CCDA xfmr requires the HV 214V at 90 mA = 19.26 VA or 20 Watts and the heater requires 17.8 VDC (16v + 2v / 1.414) = 12.6 VAC.
Iím not sure the heater VA calculation if I use 6DJ8/6922 x 2. The 6DJ8/6922heater voltage is 6.3v and heater current is 365ma.
Correct me if Iím wrong. The heater requires 12.6VAC at 1.3A (365ma x 2 =730ma x 1.8 =1.314A x 12.6VAC) = 16.6 VA or 17 Watts?
Don't have the link for the supply of a quality xfmr for audio here in Australia.
try evatco: Power Transformer and Choke Prices
You don't need to run a 6DJ8 with a 12v heater, a 6.3v heater winding will be fine.
They draw about .4A each, or .8A in total, and with rectification losses you will find that a 1.5A winding will be fine, and 2A will be better.
You could choose to run them on 12v, but finding a single transformer with B+ and a 12v heater winding will be difficult.
Also, 300v is really quite hot for a 6DJ8, if you look at page 13 of the CCDA manual you will see a suggested range of 100-240v B+, with the given resistor values centered on 200v B+ .
SO, now knowing that you will be needing about 200v B+ and a suitable winding to run the heaters, we can look at this page - Power Transformer and Choke Prices
And we see that the model "368BX" is awfully close to ideal. It's 150-0-150 (giving about 210v B+, perfect!) at 75ma, which will be fine for the current required. The filament winding is 6.3v 2A, which is more than enough.
If you would be interested in overkill, then look to this - EDCOR - XPWR196-240
It's the same voltage range, just much more current. Underrating a transformer has very little downside, and it's chief advantages are that it will run quiet (low mechanical hummm) and cool. Ignore the 5v winding, it being there will cause no harm to anything.
I have no idea what the Import duty and shipping costs would be to OZ, but it's worth a look. Also know that Edcor makes everything to order, and usually take 4-5 weeks before shipping.
Aarrgh... I can't type today. The Hammond I'm talking about is the 369BX. Sorry if there was confusion.
Thanks 6L6 for your very helpful info.
I should use the 150vac to get the 200vdc to run the 6DJ8. The Edcor XPWR196-240 looks good for my application.
From the schematic it has two 150V points and a CT in the middle. The CCDA B+ PCB has two connection points for the AC and one for the CT. Could you please let me know how to wire the XPWR196-240 to get the 150V.
Thanks and regards.
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