Blues Junior Rebuild -- PT question

[olddawg]

The led pilot lamp will draw negligible current. It's not anywhere near a filament lamp draw. If it bothers you, you can alway take the lamp off of the mains after the switch.

[rdh006]

I am taking this slowly, reading all I can find, and trying to learn as I go. I am starting by trying to recreate the power supply without the PCB, using the Blues Jr PT and components. I want to be sure that I understand this correctly, and also make sure that no mistakes are made with the high voltage. In the ideal world, I would find somebody locally to check this over for me, but there is nobody with the required knowledge. I am hoping therefore to get some clarification from the experts on this forum.

I have constructed a bridge rectifier as shown in the drawing in post number 13 above, re-labelled here -- thanks for the help above.



I have the PT secondaries connected to the points marked PT. Point 4, marked with a ground symbol, is connected to chassis ground. There is no connection yet to filter capacitors. At this early stage, I would like to understand what measurements can be taken, if any, to ensure that the circuit is correct.

I am seeing the following:

Point 1 to ground = 310 VAC
Point 2 to ground = 347 VAC
Point 3 to ground = 269 VDC

But my DMM also shows 560 VAC across Point 3 to ground.

I have 117 VAC on the house mains.

With no load present, and the PT specs unknown, can any inferences be drawn?

For example, can I figure out anything about the PT spec from a measurement across 1 and 2?

And I expected there would be no AC voltage across the rectifier output, points 3 and 4. Is this a problem?

Much appreciate the help provided here.

[g1]

Measure AC between 1 and 2. This will tell you your unloaded AC output of the PT.
You are measuring AC at point 3 because you do not have a filter cap connected yet, the rectified DC still has a lot of AC present without the cap to smooth it.
Find your unloaded AC between 1 and 2, then you can figure the peak voltage and figure out what kind of voltage rating you need for your filter cap.

[rdh006]

Thanks g-one. I have avoided measuring between 1 and 3 because my meter has a maximum of 500 VAC, and I worry about exceeding that, given that Point 1 to ground = 310 VAC and Point 2 to ground = 347 VAC. But your post makes me think -- according to the schematic, the first filter cap in the Blues Jr is rated for 450 V.

Then I see that there is a .1 cap ahead of the rectifier which is rated for 630 V, and I'm not sure how that plays into this. Does any of this tell you anything about whether the measurement you suggest is likely to be within the threshold of my meter?

[Enzo]

First do this test: Measure the voltage across a 9v battery with your meter set for AC volts. it might read something at first, but we want to see it settle down to zero in a second or so. If instead you get some steady voltage reading, then your meter is not able to read AC when DC is present.


The Blues Junior drawing says the high voltage supply is 329vDC, and that means the AC producing it will be about 235vAC. That is what you should see between 1 and 2. If you are using the Fender transformer, I see no reason to think the voltages will be any different. Oh maybe a small load related variation, but from the stock 329vDC does it matter really if it winds up at 339vDC or 319vDC?

[rdh006]

Thanks, Enzo.

I connected my meter across a 9v battery. It reads 8.84 when set for DC. Setting it for AC, it doesn't go to zero, it reads 18.4 steady. Would you be kind enough to expand on what this means in this context?

I see 262vAC between 1 and 2.

You arrived at 235vAC by dividing the 329vDC from the Blues Jr drawing by 1.4? If that is the case, multiplying this reading of 262vAC by 1.4 would result in a high voltage of 362vDC. The post above from Justin indicates that there is some leeway and that this might be OK -- I don't yet understand well enough to know.

[Enzo]

The fact that your meter measures 18vAC on the 9v battery means your meter cannot read AC when DC is present.

1 and 2 are the secondary wires. Did you measure AC with them not connected to any circuit? That is the only way you will get close to an accurate reading. Also be aware that voltage will fall once it has a circuit loading it.

If you are making a circuit from scratch, what would be wrong with 362vDC? Nothing I can think of. I bet it would be lower in practice.