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Old 6th Aug 2013, 8:44 pm   #1
crackleypot
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Question Power HT transformer suggestions comments

My valve amp blew up, rather the mains power transformer went into melt down (big hum and blew fuse) so I'm in need of a replacement. I've spotted some other threads (messages) that recommend transformer winding companies JML/VVT for example.
But my question to you people is, this transformer is going to be a lot of money and I'm not 100% sure of the HT secondary voltage needed. The DC HT on the circuit diagram is 420V (with semiconductor full wave rectifier) so I figure x0.7 = 300Vac. At present there is no low R series HT resistor (to limit inrush current) the HT cap is 32uF and the amp is about 40 W.
I'm sort of guessing what the current will be, current thinking (get the pun? ) is 190mA.
Do you think 300Vac is right?

Also just to confuse things, the amp also drives a 115Vac shaded pole motor from the primary which is used as an auto transformer. That is the usual 2 x 110V series windings, and the tap taken for the motor which I guess is about 14W. Question, how will the transformer need to be designed in order to accommodate this loading?
My guess is just add the load to the overall rating and use wire gauge for the primary for that VA rating, for example, if the tx is 100VA for the secondary load, add say 20VA making a transformer of 120VA (theory values).

What do you think?
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Old 6th Aug 2013, 9:08 pm   #2
Alan Stepney
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Default Re: Power HT transformer suggestions comments

I would contact Mike Barker (Murphymad, on here) who has rewound several transformers for me.
With one of them, there was uncertainty what the HT winding should be (it had several variants), but he worked it out from the remains of the existing winding and the end result was excellent.

Ed Dinning also rewinds transformers, and I have heard good reports on his work (but havent seen it personally).
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Old 6th Aug 2013, 9:26 pm   #3
mole42uk
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Default Re: Power HT transformer suggestions comments

Quote:
Originally Posted by Alan Stepney View Post

Ed Dinning also rewinds transformers, and I have heard good reports on his work (but havent seen it personally).
Ed's work is excellent - I have one of his transformers in my Sussex valve tester.

Richard
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Old 6th Aug 2013, 10:03 pm   #4
'LIVEWIRE?'
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Default Re: Power HT transformer suggestions comments

I always understood (or have I mis-remembered?) that the formula for a PSU using a full wave bridge rectifier was AC Voltage x 1.414 = DC Voltage, but, on the circuit diagram for a vintage valve radio, the Transformer secondary voltage is given as 236v, with the HT at the reservoir being 275v. This is approximately 1.165 x the AC. The supply filter is the commonly used type with a series resistor between the reservoir & smoothing capacitors, and the voltages quoted are 'on -load', with the current drain being 110mA. Off-load, the HT would presumably be higher (haven't checked it!), but would it really be 330volts?
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Old 6th Aug 2013, 11:15 pm   #5
TrevorG3VLF
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Default Re: Power HT transformer suggestions comments

What output valves does it use?
Supply with a normal voltage for these.
Rewind with the same gauge and number of turns.
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Old 6th Aug 2013, 11:28 pm   #6
Herald1360
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Default Re: Power HT transformer suggestions comments

Off load, the dc voltage on the reservoir capacitor of a capacitor input filter PSU will be 1.414 (sqrt 2) times the rms ac input to the rectifier, less a bit for rectifier volt drop.

On load, it all changes. The voltage will drop as load increases because of rectifier losses, transformer losses and simply the droop in voltage on the capacitor as it waits for the next charging pulse from the rectifier.

Various tables and curves such as those by Schade helped ordinary mortals to design things in the past, but nowadays it's much easier simply to use one of the many Spice based simulation packages- a little trickier if you must use a valve rectifier, though some models exist, but a reasonable approximation with a suitable silicon diode and series resistor should be close enough.

The 236V ac - 275V dc on load sounds perfectly plausible in practice, given likely values for the various loss generating factors in a typical valve rectifier circuit. What the highest "surge" voltage seen on switch on would be will depend on the type of rectifier and whether it heats up faster or slower than the rest of the circuit. Yes it could be around 330V worst case or indeed off load.
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Old 6th Aug 2013, 11:46 pm   #7
Richard
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Default Re: Power HT transformer suggestions comments

What amp is it?
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