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In Reply to: RE: Excess Line Voltage vs Amplifier Sound posted by sayntjack on January 08, 2017 at 17:57:25
Bucking transformer.
I made one out of a TRIAD USA 6.3v/4A filament transformer, a Leviton hospital-grade outlet, small metal outlet box, and fused it too boot.
Steve
Follow Ups:
Hi Krankkall. Regarding your 6.3V/4A buck transformer approach, I assume that the transformer is a 120V pri, 6.3V sec and that you have connected the secondary in series with the primary in subtractive polarity to get ~114V. Given the secondary rating of 6.3V/4A indicates that the transformer is 25VA capacity (6.3V x 4A). If this is so, then the current rating of the 120V winding is only 0.21A (I=25VA/120V) which is inadequate for the typical 1.5 - 3 amps a typical 20-50W vintage amp is likely to draw.
It's the secondary rating that counts in a bucking arrangement since its that winding that is in series with the mains. In the subject case, the rating would be 4A Max although I'd be inclined to derate that to ~ 3A to stay on the conservative side of things.
Hi Steve O, I disagree, the windings must be rated for the current that is passing thru them. In the case where someone is taking the 5V secondary winding of say a Dynaco ST70 amp and put it in series with the primary there would be no problem as the transformer primary winding current is already rated for the amp's load and the current rating of the 5V secondary is more than adequate. In the case of what I think Steve was doing using a stand alone transformer, the VA of the transformer must be high enough that the current rating of the 120V primary winding can carry the current being drawn by the load. Jack
...using a simple single primary/single secondary transformer in a buck (or boost) config, it is the secondary current rating that counts because the secondary must pass full current drawn by the device for which it's bucking voltage. Since we know the secondary volts and amps we also know its VA rating. Neglecting losses, in this case the VA rating of the primary = VA rating of the secondary. If the the VA rating of the secondary is not exceeded it will not be for the primary either. So effectively, secondary current rating becomes the limiting factor: the primary goes along for the ride with its current reduced by the factor Vsec/Vpri , again neglecting losses.
If I get time I'll wire up something, make some measurements and report back!
This is pretty basic stuff. To establish the rated full load current of the windings of a two winding single phase transformer, take the VA rating of the transformer and divide it by the rated winding voltage.
For example, for a 50VA transformer with a 120V primary and a 6.3 volt secondary, the rated full load currents are: Primary, 50/120 = 0.42A, Secondary, 50/6.3 = 7.9A.
So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary in subtractive polarity (120-6.3 vs 120+6.3) and you needed it to be capable of supplying 3 amps to your load then the transformer would need to be rated 360VA minimum (120V x 3A) to have a primary winding capable of 3 amps.
To verify, I took a 135VA transformer, 120V pri, 6.3V sec. With my line voltage I got 128V additive polarity & 114V subtractive. Wired in subtractive polarity I connected a 60W light bulb and measured 474mA in the wire to the bulb. In the incoming wire to the transformer primary from the wall outlet I measured 480mA - a little higher as expected because of the transformer magnetizing current.
... "So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary"This isn't a conventional circuit for a stand alone bucking transformer. Conventional arrangement places primary directly across the mains and the secondary in series with the load with the combo across the mains.
I too set up a circuit using a tiny 115V - 6.3V/1.5A filament transformer in buck mode supplying 100V/1.5A to a resistive load (150VA or W). Measurements were just as I described previously and the 10VA transformer handled the 150VA load for 1hr w/o issue. i.e. no transformer buzzing and normal operating temp.
Will edit in circuit diagram and meas later.
Test circuit:
Edits: 01/10/17
Yes, that would be great if you could post the circuit diagram as our discussion is a little in the dark without it.
Hi Steve. I'm not seeing your posted diagram but no matter. I have set up my buck circuit again & retaken measurements. Results - I'm more than a little embarrassed. My statement regarding calculation of rated winding currents of a transformer is correct - the rest, regarding current thru the primary 120 Volt winding in the buck configuration is not correct. I apologize to all. With the 60W 120 volt bulb bucked with the 120 / 6.3 V transformer to operate at 114V from the 122V incoming, the lamp current thru the 6.3V winding was 474mA and the current going into the 120V winding was 58mA. I'm scratching my head a little on this but again I apologize to all. Jack
I see your diagram - same circuit as mine. Thanks Steve.
Largest tube amp I have is 8 wpc, works for me.
Steve
Hi Steve. I assume that the 8W that you refer to is the power output of the amp, which does not directly relate to the input power / current. With the output tube plate dissipation, filament power and power factor of the transformer are taken into consideration, it is likely that your amp is drawing at least 0.5 amps at 120V - twice the current rating of the buck transformer 120V winding if it is a 25VA transformer.
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