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|21st November 2003, 09:54 PM||#2|
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Okay good to see you having a crack at this one. I mention mains transformer first. The lower impedance offered is a real advantage in large current operated amps like my fixed bias 100W+100W KT88 amp. I used a 550VA toroid mains transformer with good success and very good regulation around 3% @ 560V DC. However with high powers such as this design it is imperative to use a soft start 230V AC inrush control (as you will blow fuses) and use silicon recifiers on the toroid output. You can use high value electroytics. If your power is lower then soft start may be not necessary and anti surge fuses may suffice.
There are some important points to bear in mind using toroids..
1. The toroid will probably be a custom made item and be more expensive than E/I types.
2. If you know anything about magnetics then get the manufacturer to reduce B (energising magnetic flux) to about 1.2T. The reason I mention this is because nearly all are wound for the max 1.6T ( near saturation to save copper so turns) and this will reduce the stray magnetic field even further. The integer in any toroid design is the 6.3V heater winding whcih has to be the nearest whole number.
Contrary to belief, toroids do produce a stray hum field, mainly with poorly wound coils and if you are using a steel chassis then the flux travels easily in it. That's why insist on lower core magnetising level. Remember if you are making a amp, then it's always worth getting the signal to noise ratio as low as poss. hence my remark in point 4.
Insist all windings are evenly wound round the core.
In your application the increased number of turns will have nearly no effect on regulation performance.
3. This is your time to draw up a specification.turns /current etc.
The 6.4V winding is the most critical. Generally this winding with very few turns cannot be spaced ideally round the toroid with the leakage flux spewing out the side. This
4. Most important: Always design amp stages with input tubes furthest away from the transformer. In the case of a toroid leave the wires long enough and with enough room so you can rotate the transformer in the chassis when you finished the construction.
You will then discover the optimum place for min hum.
Otherwise you should have little or no problems.
Toroid for output transformers: I believe Plitron do some designs but with exotic materials. May be possible to use toroids with automatic bias but not fixed bias. It pays to look at their website.
Since I design with High power, I avoid toroid output transformers despite the specifications looking better on paper. Equalising the currents in each push pull half for high powers does pose problems with complex solutions.
Ps I get -70dB/SN (A weighted) ratio from that 100W+100W amp.
and excellent LF note beat performance as the HT stays up.
Hope this helps.
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