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Old 16th April 2005, 09:23 PM   #1
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Default B+ problem?

I've built an amp but when I fired it up the heater voltages were spot on:5v rectifier(5Z3),2.5v 2A3,6.3v 6sl7.B+ on the other hand is 200v down.I measured the voltage on the tranny 380-0-380 no problem.Same going into rectifier.Measured opt connection to B+.It reads 215v and the HT wiring to rectifier is hot enough to melt solder.Any suggestions as I'm a newbie and lost????????
Its the Morrison Micro circuit I'm building.
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Old 16th April 2005, 09:47 PM   #2
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You're clearly drawing too much current somewhere. Replace the audio circuit with a power resistor dummy load and see if the power supply is operating properly. Do you need help figuring out the size of a dummy load resistor or is Ohm's Law in your toolkit?
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Old 16th April 2005, 09:50 PM   #3
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Originally posted by SY
You're clearly drawing too much current somewhere. Replace the audio circuit with a power resistor

I'm gonna need some help with that please.Every other amp i've built worked ok first go.
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Old 16th April 2005, 10:05 PM   #4
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OK, let's take it step by step. What is the B+ supposed to be and how much current is the circuit supposed to draw?

If you've got a link to the schematic, that will help us walk you through the process.
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Old 16th April 2005, 10:22 PM   #5
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Quote:
Originally posted by SY
OK, let's take it step by step. What is the B+ supposed to be and how much current is the circuit supposed to draw?

If you've got a link to the schematic, that will help us walk you through the process.

Here's the diagram
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Old 16th April 2005, 11:12 PM   #6
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OK, without going through the circuit in detail, one can see that the total draw from B+ will be on the order of 50mA (it might be 40, it might be 80, but we're just approximating). OK, so that means, as far as the power suply is concerened, the load looks like whatever resistance it takes to conduct 50mA (0.05A) at 400V. Pull Ohm's Law out of our mental toolbox: the equivalent resistance is voltage divided by current. So R = V/I = 400/0.050 = 8,000 ohms. So an 8,000 ohm resistor will load down the supply properly (nearest standard values are 8.2K or 7.5K).

Now, we put Ohm's Law back neatly and pull out Ohm's Power Law, power = voltage times current. In this case, P = 400 x 0.05 = 20W. So... disconnect the B+ feed from the circuit, attach an 8.2K (or 7.5K) 25watt power resistor across the power supply output. Check the voltage. If it's 400V like it's supposed to be, the power supply is fine and you need to start troubleshooting the amplifier. Contrariwise, if the voltage is low, then the power supply has a wiring error or a bad part.

Either way, you've cut the problem in half.
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Old 16th April 2005, 11:14 PM   #7
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Default B+ Problem

Thanks a lot.Will give it a try tomorrow.
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Old 16th April 2005, 11:34 PM   #8
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Does this apply for the stereo version or is it 50mA for each channel=100mA? If it's 100mA then dummy load should be 4k @50watts .Is this right?
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Old 17th April 2005, 08:03 PM   #9
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Had another look at the amp today.PSU filter cap WRONG WAY ROUND.DOH!!Should have double-checked it.All ok now.404v B+ and sounding SWEEEEEEEEEET!
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Old 17th April 2005, 08:15 PM   #10
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Originally posted by kianbach
PSU filter cap WRONG WAY ROUND.DOH!!

Ummm.....valves/tubes are so forgiving to the greatest mishaps.....that 5Z had to stomach the lot.....you wouldn't have gotten away as-clean-as that with a SMPS....

richj
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