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Old 17th Oct 2016, 9:53 am   #1
zeerust2000
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Default Advice needed on choosing a power transformer

Hi everyone,
I'm presently planning to clone the valve stereo amplifier in a 1968 unit I have, which sounds great to me. It has a 6V4 rectifier, and each channel uses 1/2 of a 12AX7 and a 6M5 power valve. The 6M5 is very close to an EL84. In fact they can be substituted for each other, apparently. My question relates to the power transformer. In the original unit, the secondary HT winding gives 520 VAC unloaded (ie, with rectifier valve removed) and around 495 VAC with the circuit load. So, would it be reasonable to use a Hammond transformer rated at 450 VAC? I'm assuming here that this figure is for a unit under load, as this appears to be how the Hammond specs are given. Any advice will be greatly appreciated. Cheers...
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Old 17th Oct 2016, 1:22 pm   #2
G8HQP Dave
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Default Re: Advice needed on choosing a power transformer

I assume those voltages are across the whole of a centre-tapped secondary?

You need to know what your mains voltage is and how this relates to the expected Hammond primary voltage. Generally, valve circuits can cope with 10% variation.

Just a minor quibble: "450 VAC" is not a "rating" but a parameter or characteristic (what it does - 450V AC rms in this case). A rating is what it can cope with (e.g. for transformers this could be VA 'power' or insulation breakdown).
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Old 17th Oct 2016, 4:53 pm   #3
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Default Re: Advice needed on choosing a power transformer

Hi, Voltage out is important, but also is the current capability of the transformer for both the HT and heaters,
As a very rough guide, if the Hammond transformer is twice the size (volume) of the old unit it will probably do for both channels of the amp.
If it is only the same size as the present one then it will probably do for the new channel only.

Ed
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Old 18th Oct 2016, 2:53 am   #4
zeerust2000
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Default Re: Advice needed on choosing a power transformer

Thanks for the replies. Yes, I measured across the entire. centre tapped, secondary. The mains here in Sydney is supposed to be 240V, although I haven't measured it myself. The Hammond transformers have two primary windings, which are designed to be connected in a variety of ways to accommodate various mains voltages. They do cater for 240V mains. Current capacity is no problem...I've identified a unit which may be suitable, which has 300ma available. More than enough. I'm pretty new to all this, it's a steep learning curve.
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Old 18th Oct 2016, 11:54 am   #5
G8HQP Dave
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Default Re: Advice needed on choosing a power transformer

The mains here in the UK is supposed to be 230V, but in most places it is still 240V and could lawfully be as high as 253V. It could also be below 220V, so if you can do so safely it might be worth measuring your mains voltage. It will vary a bit from day to day and at various times of day, but it probably won't vary very much. It is useful to know whether your mains is typically a bit high or low or about right.

300mA (not 300ma) should be more than enough, but be aware that most transformers are AC rated for a resistive load and you can't draw that same current continuously as DC via a rectifier and capacitor arrangement. There is not much point in using a transformer which is far too large as it will cost more and take up lots of chassis space. Best to choose one which is just a bit too large.
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Old 18th Oct 2016, 3:39 pm   #6
bikerhifinut
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Default Re: Advice needed on choosing a power transformer

Good advice.
I'll second the remark about UK mains, ours frequently hits the legal 253V limit here and it has caused me endless aggravation with mains transformers wound for the nominal 230v. Ed and Mike barker are unfortunately only too well acquainted with my tales of woe. In future if I order a transformer from a manufacturer or supplier I will insist that there's a 250V as well as a 240V tapping on the primary. And if they can't/won't/try to fob me off with BS then it will be no deal.

A.
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Old 19th Oct 2016, 1:36 am   #7
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Default Re: Advice needed on choosing a power transformer

Dont worry about mains voltage in Sydney, in most places its spot on.
300 mA is about 5 times more than you need, even for both channels!!
A transformer giving 100 mA at 500 to 530 volts UNLOADED will be perfect for both channels.
A, 370DAX Hammond is exactly what you need.
http://www.hammondmfg.com/pdf/EDB370DAX.pdf

You might direct a question to, Ed Dinning for furher advice, OR criticism of my choice.

Joe
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Old 19th Oct 2016, 11:12 am   #8
G8HQP Dave
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Default Re: Advice needed on choosing a power transformer

You need about 50mA DC per channel. That 370DAX will be OK if the 100mA rating is for rectified DC, not RMS AC. The mention of 76VA may imply that the current is a DC rating, but it would be helpful if manufacturers were clearer in their datasheets.
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Old 20th Oct 2016, 2:19 am   #9
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Default Re: Advice needed on choosing a power transformer

Dave,
It is a DC rating, it will be perfect for what is required.
In fact I would hazard a guess, that the "1968" design that he has, is derived from a Philips radio amplifier.
The little transformer would even supply a modest pre amp or tone control amp.

Joe
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Old 20th Oct 2016, 6:38 am   #10
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Default Re: Advice needed on choosing a power transformer

50mA is about right. One side of a 12AX7 will draw about 5mA, the 6M5 about 40-50mA on the anode and about 10mA on the screen grid. All the figures are approximate, so with losses etc 50-100mA is about right. See here for more info - http://www.valvewizard.co.uk/psu.html

Hammond are very helpful if you email them asking for any data.

Andy.
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Old 20th Oct 2016, 7:51 am   #11
Craig Sawyers
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Default Re: Advice needed on choosing a power transformer

For capacitor smoothing you need a transformer with a VA rating about 1.5 - 2 times the DC power drawn. This is entirely because of the charging current waveform needed to top the capacitor up. Then you add the heater power and add that to get the overall transformer VA.

Half way decent apps note here with examples http://www.ietlabs.com/pdf/GR_Appnot...r%20Design.pdf

Or look at Schade's classic 1943 paper "analysis of rectifier operation" http://documents.mx/documents/analys...operation.html

Or just analyse it using http://www.duncanamps.com/psud2/
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