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Old 16th November 2006, 04:42 AM   #1
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Default Where is the error with this ohm's law calc?

Using EZ80 rectifier which uses a 6.3V filament transformer. But transformer puts out 7.8 volts. I want to reduce this to 6.3 volts. My understanding is EZ80 is rated at 0.6 amps. So...

7.8-6.3 volts = 1.5 volt drop / 0.6 amps = 2.5 ohm resistor value.

I had a few 1.8 ohm resistors and soldered 1 each to each of two transformer secondary leads....and still measure 7.8 volts. Where is my error?
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Old 16th November 2006, 04:48 AM   #2
hacknet is offline hacknet  Singapore
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did you check that your tube's filament is drawing current?

the way it looks is your tube isnt drawing current.
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Old 16th November 2006, 04:49 AM   #3
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In one acronym: RMS

This site explains it well:

http://www.alpharubicon.com/altenerg...standingAC.htm
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Old 16th November 2006, 08:49 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
you could add two power diodes in series to drop about 1.3 to 1.4Vdc and same in reverse for the other halfwave.
Not quite the same but maybe good enough. Comments to this non-tube guy!

Yes, Hacknet is right, the resistors only drop voltage when they are carrying current. Open circuit voltage does not change (no matter how big the resistors are).
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Old 16th November 2006, 09:34 AM   #5
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Mike,

When you said you measured 7.8v after you added the resistors
was the tube connected? If the tube wasn`t connected there
would be no current draw thus no voltage drop.

If this was the case it brings up another question. When you
measured the transformer voltage was this with the tube connected? Transformers measure a little higher when they
are not connected. With small transformers this open circuite
voltage can easily be 20% or higher than when undrer load.
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Old 16th November 2006, 09:43 AM   #6
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Default Re: Where is the error with this ohm's law calc?

Quote:
Originally posted by riotubes
Using EZ80 rectifier which uses a 6.3V filament transformer. But transformer puts out 7.8 volts. I want to reduce this to 6.3 volts. My understanding is EZ80 is rated at 0.6 amps. So...

7.8-6.3 volts = 1.5 volt drop / 0.6 amps = 2.5 ohm resistor value.

I had a few 1.8 ohm resistors and soldered 1 each to each of two transformer secondary leads....and still measure 7.8 volts. Where is my error?

Did you measure at the transformer? You should indeed have 7.8V there. The 6.3 is at the tube pins....

Jan Didden
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Old 17th November 2006, 04:20 AM   #7
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Ahhh...got it. As many of you suspected, I was testing without the board hooked up to the transformers. I was cautioned to measure the secondaries b4 hooking them up to the circuit to see if voltage was too high. I'm learning. I studied a schematic of the EZ80 and also followed the 6.3v trace and 0v trace to the tube pins. I measured the voltage at the pins: 7.4 without the resistors (down from 7.8v at the secondaries) and 5.4 with the 1.8 ohm resistors in the secondary path prior to the board's wire terminals.

This surprised me as I anticipated needing 2.5 ohm based upon my original calc of 7.8-6.3=1.5 / 0.6 amp = 2.5 ohm requirement. I had only 1.8 ohm resistors on hand. Now I'm down to 5.4 volts. This is more than 10% below the 6.3v requirement. So I conclude that the circuit is drawing a current different from the EZ80s rated current of 0.6 amp. I'm not yet entirely comfortable measuring current. I need to study how to do this.
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Old 17th November 2006, 07:30 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi,
measure the voltage drop across your dropping resistor.
Your measurment accuracy is the worse of the DMM or the resistor accuracy.

I=V/R
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Old 19th November 2006, 01:03 AM   #9
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With the help from y'all, I feel like I've made some progress in learning how to measure voltage in a circuit. I replaced the 1.8 ohm dropper resistors with 1.2 ohm and I've increased voltage from 5.4 to 6.0v. The EZ80 rectifier tubes specs 6.3V for filament...so I think I'm within tolerance and OK.

The one disappointment is that the 6dj8 SRPP circuit of my Dac tube stage requires 200V for B+, and I was measuring 222V after a 6800 ohm resistor on the B+ in the stock circuit (and 283V before this resistor). I want to get closer to 200V. So:

283V-222V / 6800 ohm = 0.00897 A

So 9000 ohm x 0.00897 A = 81V drop or 283-81=202 V

Well, I couldn' find a 9K ohm R, only 7.5K Ohm at the local parts store. I thought this would give me:

7500 x 0.00897 A = 67V drop or 283-67=215V a step in the right direction. I measured 269V !!!! more than the 228V generated with the lower 6800 ohm resistor. Could the resistor's tolerance be that crappy or did I error somewhere?
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Old 19th November 2006, 01:09 AM   #10
Duo is offline Duo  Canada
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I think in your curcuit, connecting the tube filament directly to the transformer winding is the best choice.

That 7.8V will drop a lot when you actually put yourself a load on it. At least try if for long enough to measure what voltage you get at the tube pins. Running a 6.3V tube a little bit hot for a moment won't hurt it.
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