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Old 31st July 2013, 05:22 AM   #1
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Default Correct measurement of filament voltage.

I've got a tube preamp which uses two 6CG7 tubes.
If I measure the voltage across pins 4,5 of tube A, it reads 6.0vdc, however tube B reads 6.5vdc.

The heater supply for both tubes is from the same source, and if I swap the tubes then the readings are reversed accordingly. If I read the voltage from the heater + and heater - terminals on the PCB, then the voltage is identical.

Does this mean that if we read the voltage across pins 4 & 5 of a (small signal) tube, then we are also seeing a voltage drop due to resistance of the filament?

Last edited by lordearl; 31st July 2013 at 05:25 AM.
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Old 31st July 2013, 05:52 AM   #2
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There is difference between heater voltages of tubes A and B if the voltage is fed first to tube A and then to tube B via thin heater wires.
But with similar tubes there should not be any difference if tubes are swapped.
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Old 31st July 2013, 06:02 AM   #3
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Very strange - it is a Broskie circuit with jumpers between the heaters of the two tubes. The jumpers are quite thick. Although even if this is the case, it doesn't explain why the readings are reversed when I swap the tubes.

Is the 0.5v difference likely to be affecting the performance of the tubes?
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Old 31st July 2013, 06:04 AM   #4
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Quote:
Originally Posted by lordearl View Post
Is the 0.5v difference likely to be affecting the performance of the tubes?
There is maybe something wrong in your test setup, but if the heater voltage of 6CG7 is between 6...6,5 V it works fine.
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Old 31st July 2013, 09:20 AM   #5
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Quote:
Originally Posted by lordearl View Post
[...]If I measure the voltage across pins 4,5 of tube A, it reads 6.0vdc, however tube B reads 6.5vdc.[...]
If I read the voltage from the heater + and heater - terminals on the PCB, then the voltage is identical.
If I understand you correctly, you can only measure a difference if you take the measurement at the tube pins not if you measure at the PCB?!?
This would indicate that one tube has a bad connection to the PCB (oxidized heater pin?) and some voltage is lost at that bad connection. That would also explain why the problem is moving with the tube. In your setup tube A would be the suspect.
Clean the heater pins and see if that changes anything.
Best,
Martin
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Old 31st July 2013, 12:27 PM   #6
TheGimp is offline TheGimp  United States
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Are the heaters in series?
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Old 31st July 2013, 01:08 PM   #7
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Hi mate, yep see pg 16 here. Genius design, I'm using 12.6v from the transformer to get the split 6.3 volts.
http://www.google.com.au/url?sa=t&rc...49967636,d.dGI

Last edited by lordearl; 31st July 2013 at 01:12 PM.
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Old 31st July 2013, 01:29 PM   #8
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One heater has slightly more resistance and is dropping a little more voltage than the other, very typical and normal. Don't sweat it and go listen to some music.

Mike
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Old 31st July 2013, 02:05 PM   #9
kevinkr is offline kevinkr  United States
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Mike nailed it.
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Old 31st July 2013, 03:31 PM   #10
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Excellent! Cheers all.
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