Over-voltage on heaters... - diyAudio
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Old 29th April 2008, 04:43 PM   #1
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Default Over-voltage on heaters...

Hi,

Transformer was specced for 220V AC, and in Ireland it can vary from 220 to 240 depending on where you are. Hence, at college I measured 227V across my primaries.

That means my heater outs are a little over what tehy should be, 5.6 and 13.4V respectively. I've tried to put a resistor series with one side of the 5V but all it does is do a great deal of heating and not enough dropping of voltage. (.6V/1.95A=~.3Ω)

I could run rectifiers and DC-heat them....but they don't do a xx-12.6 :S

Exactly how bad is it to run them a little over?


L
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Old 29th April 2008, 05:22 PM   #2
Jeb-D. is offline Jeb-D.  United States
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I'm not sure about 5V filament, but 12.6V filaments are usually rated +/- .6V, so you are over the upper limit.

Quote:
I could run rectifiers and DC-heat them....but they don't do a xx-12.6 :S
You could use a xx12 regulator then place a diode between the ground lead and ground. That will raise Vout up a half volt or so.

A regulator is going to dissipate as much power as a resistor will, if placed in series with the filament.

I have the same problem. Most North America transformers are rated for 115V line, but the voltage at my place is 122V. I usually put a power resistor on the primary (ClassA amps only) to drop the 7V. It also reduces mechanical hum, because my mains has some DC voltage. The resistor adds some DCR to the primary, which lowers DC current that flows through the primary. It will bring down the B+ too, if it is a shared transformer.
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Old 29th April 2008, 05:25 PM   #3
cerrem is offline cerrem  United States
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Heaters are designed to see variance in operational voltage and are intended to operated over a typical range of 7% provide no grid current is flowing such as in Class AB2...
Variance of the mains is normal and is part of the power grid operation... In the old days the regulation on US mains was not as good as today...back then it ranged from 100V to 120V and it was refered to as 110V nominal...
Figure 5.86V to 6.75V is normal heater operating range...
You can also look at this from a current point of view.... Two of the same tubes operating in parallel at the same voltage will not always draw the exact same heater current due to process variation....so technically you should look at the overall power of the heater....heaters are not an exact science since there are too many variables...don't get hung up on this...
If you have DC on your mains or you nuetral is more than .3V off of Ground then you need to call the power company or check for a faulty applicance in your home...unless you have the bad luck of living next to an industrial park..

Chris
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Old 29th April 2008, 06:03 PM   #4
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I have lovely mains here (in my house) - 222V (.07v Gnd->N) and bugger all DC so I must run a check here before raising too much fuss .

Worst comes to the worst, I'll just get me a variac, but if college is teaching me one thing - practical things are rarely 100% 'by the book'


L
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Old 29th April 2008, 08:33 PM   #5
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A very simple way to reduce heater voltage is to place 2 diodes in parallel with each other, with each cathode facing the other way (between the PT and all filaments). This will reduce the voltage by 0.7V. Make sure you pick a diode that can handle the current.
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Old 29th April 2008, 09:28 PM   #6
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Will bear that in mind, johnny! I think killing the whole thing at source with a 20Ω resistor on the live input will sort it and help to reduce DC on the line.

L
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