Hi, I don’t post much but I get great info from the group. I have a DIY Don Garber/Loftin White type 2A3 amp. My previous homes AC voltage was 123VAC and the PT is 750VAC center tapped. Per Don’s schematic I was within 1% voltage was at all the the critical test points. The PT is setup for 123VAC. My new homes AC is 119VAC. With changing anything or digging into why, my voltages have dropped like 15% and starts with the B+. Where should I start investigating for the reasons. Help is very much appreciated.
The decrease in voltage, in such a case, is the consequence of lower AC plate to plate plus decreased heating of the rectifier.
If I be you, I don't worry about it unless you use it at full power.
If I be you, I don't worry about it unless you use it at full power.
How was the PT setup for an odd voltage? Maybe there's something in the circuit more involved. Actual numbers would be a good start for troubleshooting.
Measurements tubes installed:
120VAC on the PT primary.
741VAC (secondary) cross the plates of the 5V4G rectifier.
2A3 - 2.35VAC across filaments.
6SF5 - 6.59VAC across filaments.
5V4G - 4.86VAC across filaments.
Is it normal to have this 9VAC drop from the spec’d 750VAC PT?
120VAC on the PT primary.
741VAC (secondary) cross the plates of the 5V4G rectifier.
2A3 - 2.35VAC across filaments.
6SF5 - 6.59VAC across filaments.
5V4G - 4.86VAC across filaments.
Is it normal to have this 9VAC drop from the spec’d 750VAC PT?
Did you measure the 750VAC in the old house or are you assuming that? It is important to understand whether the drop of 9V is to be expected.
119VAC / 123VAC = 0.967
750 x 0.967 = 725.6V
But that is just a small part of the story.
The often overlooked factor of Power Mains Voltages:
Crest Factor
Crest factor is according to how much 3rd Harmonic distortion there is of the power mains fundamental frequency.
150Hz third harmonic of 50Hz power mains,
180Hz third harmonic of 60Hz power mains.
A slightly flattened sine wave will have less peak volts, but will still have nearly the same rms volts (flat for lots of time, integrates to more rms power).
Approximation:
With no harmonic distortion, 100VAC has a peak voltage of 141.4 Volts.
With 7% harmonic distortion of 100VAC, the peak voltage is 141.4 x (1 / 1.07) = 132.1 Volts. About 9V difference, a coincidence, or not.
Go back to your old house.
Measure power with a Non-True AC meter (an AC meter than is either average responding, peak responding);
Then measure with a True Rms AC meter.
The Only way the 2 reading will be the same, is if there is No harmonic distortion (pure sine wave power . . . Good Luck).
Then take those 2 meters, and measure your power mains at your new house.
Have Fun!
750 x 0.967 = 725.6V
But that is just a small part of the story.
The often overlooked factor of Power Mains Voltages:
Crest Factor
Crest factor is according to how much 3rd Harmonic distortion there is of the power mains fundamental frequency.
150Hz third harmonic of 50Hz power mains,
180Hz third harmonic of 60Hz power mains.
A slightly flattened sine wave will have less peak volts, but will still have nearly the same rms volts (flat for lots of time, integrates to more rms power).
Approximation:
With no harmonic distortion, 100VAC has a peak voltage of 141.4 Volts.
With 7% harmonic distortion of 100VAC, the peak voltage is 141.4 x (1 / 1.07) = 132.1 Volts. About 9V difference, a coincidence, or not.
Go back to your old house.
Measure power with a Non-True AC meter (an AC meter than is either average responding, peak responding);
Then measure with a True Rms AC meter.
The Only way the 2 reading will be the same, is if there is No harmonic distortion (pure sine wave power . . . Good Luck).
Then take those 2 meters, and measure your power mains at your new house.
Have Fun!
Original measurements 6 years ago:
B+ = 477VDC (today 439VDC)
2A3:
Vp = 469VDV (today 437VDC)
Vk = 195VDC (today 175VDC)
Vp-Vk = 274VDC (today 262VDC)
6SF5:
Vp = 132VDC (today 117VDC)
B+ = 477VDC (today 439VDC)
2A3:
Vp = 469VDV (today 437VDC)
Vk = 195VDC (today 175VDC)
Vp-Vk = 274VDC (today 262VDC)
6SF5:
Vp = 132VDC (today 117VDC)
I never did because the numbers were within 1%. It’s when I went back to finally finish this amp project that I got these new numbers at my new location.
Sorry,
You need to go back to the old house, measure with both a True RMS meter and a Non-True rms reading meter.
Then measure at your new house with the same 2 kinds of meters.
All else is just a guess of the one or more causes of your problem.
Your DC measurements can not prove the one or more reasons that cause the difference.
Some things can not be reverse engineered.
Power Mains are often misunderstood.
Average Responding, Peak Responding, and True RMS, AC meters are often misunderstood.
They all read exactly the same for a pure sine wave; but any distortion makes them all read different voltages.
Power mains: if they are pure sine waves, you get one DC Voltage. If power mains wave is mis-shaped (distorted), you get a different DC Voltage.
Another one . . .
"Grounds are Commonly Misunderstood"
You need to go back to the old house, measure with both a True RMS meter and a Non-True rms reading meter.
Then measure at your new house with the same 2 kinds of meters.
All else is just a guess of the one or more causes of your problem.
Your DC measurements can not prove the one or more reasons that cause the difference.
Some things can not be reverse engineered.
Power Mains are often misunderstood.
Average Responding, Peak Responding, and True RMS, AC meters are often misunderstood.
They all read exactly the same for a pure sine wave; but any distortion makes them all read different voltages.
Power mains: if they are pure sine waves, you get one DC Voltage. If power mains wave is mis-shaped (distorted), you get a different DC Voltage.
Another one . . .
"Grounds are Commonly Misunderstood"
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I never did because the numbers were within 1%. It’s when I went back to finally finish this amp project that I got these new numbers at my new location
Oh lots of fun. Actually with the drops the ratio’s are still getting me close the bias, current and power. Thx!
Nothing to worry about.
Difference between measured and nominal is well within spec and in any case less than 5%
Difference between measured and nominal is well within spec and in any case less than 5%
You could try a boost tranformer to fix the 3% drop and see it that in turn resolves the 15% drop.
https://sound-au.com/articles/buck-xfmr.htm
https://sound-au.com/articles/buck-xfmr.htm
Were these measurements taken with the exact same rectifier tube? If so, is it possible that the rectifier has aged? That could account for some of the difference.
Tom