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Old 27th June 2011, 01:14 PM   #1
bst is offline bst  United States
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Default Best method of reducing line voltage into vintage transformers?

I have a number of vintage tube power transformers which specify a 115vac primary input voltage. However, the voltage at my wall outlets measures an average of 125v, with occasional drifts as high as 127v.

The transformers I'd like to use are decent quality -- Stancor, Hammond, Triad, and the like -- and some are exceptionally well-made, like the potted UTC mil-spec pulls from vintage power supplies.

I'd like to drop the secondary output voltages back to where they belong, especially the filament windings. I've read of three methods to do this, and would like to know which is the best.

The first is to insert two 12-volt zeners (of adequate current rating) in one primary leg of the transformer. This brings the input voltage back down to a reasonable value, and the secondaries back into spec.

The second method is to add a bucking transformer to the primary, and thus reduce the input voltage.

The third method is to ignore the overvoltage on the primary, and reduce the plate secondary by using a lower value first capacitor right after the rectifier, or going to a straight choke-input filter. This method also requires putting zeners on the filament windings to drop them to the proper levels.

Are vintage transformers typically safe to operate at a continuous 110% rated voltage on the primaries, or is this an unwise practice? Thanks in advance for your observations.
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Old 27th June 2011, 01:44 PM   #2
AndrewT is offline AndrewT  Scotland
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an auto transformer or isolating transformer with multi tapped primary could be used to correct the voltage fed to your system.
It could even be made to automatically change the tappings as the voltage varies throughout the day.

Whether any of your transformers would be overloaded by high supply voltage would come down to you testing each transformer for primary current on zero load to see how it varies with supply voltage.
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Old 27th June 2011, 01:50 PM   #3
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Use a 120 to 12 volt transformer. Place the secondary in series with the primary. One way you get 110 or so the other way 130. As you speculated the bucking transformer is the way to go. You probably only need a 3 amp 12 volt transformer.
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Old 27th June 2011, 02:52 PM   #4
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If you had an unused rectifier winding, you can use it in series with the primary to reduce the voltage. I did this with my ST70 - converted over to SS rectification, so the 5V winding was no longer needed. Inserted in series with the primary winding, viola- reduced excitation with no additional parts required.

I would say 110% is too high on a continuous basis. You draw additional excitation current and risk saturation of the core. This will tend to flat top your secondary voltages and makes the core run excessively warm.
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Old 27th June 2011, 05:21 PM   #5
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Originally Posted by zigzagflux View Post
If you had an unused rectifier winding, you can use it in series with the primary to reduce the voltage. I did this with my ST70 - converted over to SS rectification, so the 5V winding was no longer needed. Inserted in series with the primary winding, viola- reduced excitation with no additional parts required.

I would say 110% is too high on a continuous basis. You draw additional excitation current and risk saturation of the core. This will tend to flat top your secondary voltages and makes the core run excessively warm.
Bad news is you loose safety isolation! There should be an extra layer of insulation between the primary and secondary layers.
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Old 27th June 2011, 06:00 PM   #6
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Doesn't bother me in the least- been running for years. If you want to get creative you can place the rectifier winding in the neutral end, so it essentially sits at ground potential. I would only be concerned if the transformer was shielded, since I eliminate the benefits of the shield.
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Old 27th June 2011, 09:35 PM   #7
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I'd also suggest the 12V bucking transformer as emminently simple, and safe, and achieving exactly what you want (re. primary voltage). It's worth having some margin in VA rating, but a 12V 3A rating may be able to come down if you don't have a large amplifier transformer, which may make it easier to fit a new transformer.

Best to get a new modern transformer, where the secondary has the same insulation/former mechanism as the primary.

Ciao, Tim
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Old 27th June 2011, 11:12 PM   #8
ChrisA is offline ChrisA  United States
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Quote:
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Doesn't bother me in the least- been running for years.
This is the same logic as in the following

Quote:
I've been driving with out using my seat belt for years and I'm not dead.
While this might be true, it does not prove that non-use of seat belts is safe, it only shows that you are lucky.

Connecting the primary to the secondary will work fine, until a certain kind of failure happens, perhaps after years of use vibration causes insulation to fail or maybe a strong power surge breaks something. A well designed amp should remain safe even after a component failure.


One common trick for dropping voltage is a zener diode in series with B+. But I've put a resistor in the heater circuit and also raising the value of a resistor in the B+ supply CRC filter not only drops voltage but improves filtering. This puts the extra volts to good use rather than only wasting power
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Old 28th June 2011, 07:14 AM   #9
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I would do none of that. What's the use of having yet another (auto)transformer, leaking hum around your nice circuit? I would jump at the chance to place an appropriate NTC thermistor in series with the filaments, and have a nice start-up delay for the heaters.
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Old 28th June 2011, 07:22 AM   #10
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Quote:
Originally Posted by bst View Post

Are vintage transformers typically safe to operate at a continuous 110% rated voltage on the primaries, or is this an unwise practice? Thanks in advance for your observations.
any transformer should have insulation to at least 2x and preferably 3x nominal voltage. 10% is insignificant.

And I'm with costis vis a vis solution
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