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Old 30th September 2007, 10:30 PM   #1
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Default Measuring Cathode Current

Hi,

I am embarking on my Dynaco Mk3 build. I am going to put a cathode current meter in the circuit. I have attached a schematic with proposed positions for the meter labeled M1, M2 and M3.

I presume that it does not matter where the meter is placed but would someone be kind enough to check my schematic and advise as to whether any position is preferable.

Thanks,

Rob
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Old 30th September 2007, 11:16 PM   #2
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Since there is no provision for balancing currents from each KT88, why bother?

Secondly, your M1 and M3 positions won't indicate cathode current anyway, will have a potential for exposure to high voltages, and I wouldn't be so happy to be seeing wiring that carries a high level signal going every which way.

You'd be better off adding a small sense resistor to each cathode (10R is good) and measuring the voltage drop across that resistor to get the cathode current. No dangerous voltages there, and not much signal either, even if you panel mount a meter for that purpose.
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Old 1st October 2007, 02:42 AM   #3
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Hi Miles,

You are correct about biasing individual tubes but the resting cathode current is biased with a 10K variable resistor. I want to be able to bias the cathode current without opening the amplifier up and I figured that a panel meter with externally accessible 10K pots would be the best option. I can see your point about the high voltage signal wire but I do have 200mA high voltage panel meters (the cathode current at idle is about 140mA) which would work but if there is a better way to proceed then I am all ears.

As an aside, I don't see why the current through points M1 and M2 is not the same as M3?

Rob
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Old 1st October 2007, 06:46 AM   #4
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I would advise not putting any sort of meter in the plate circuit of the finals. It could cause instability. To monitor cathode currents, just add a 10R sense resistor to each cathode between the cathode pin and the shared cathode resistor. You could then connect a meter across the 10R sense resistors with a SPDT switch (or a SP3T with a neutral position that disconnected the meter for normal operation.

If you want to just measure the total cathode current, then add the 10R sense resistor in series with the shared cathode resistor. Either way will avoid dangerous voltages, and the possibility of instability.
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Old 1st October 2007, 09:55 AM   #5
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OK Miles,

I am coming around to your way of thinking. Why can't I just replace the shared 11.2R resister with a 10R resistor and then aim for a voltage drop of 1.4v (for an idle current of 140mA) across the resistor? In other words, why add the 10R resistor in series at all, why not just replace the 11.2R?

Thanks for your patience with this. It takes me a while to abandon a plan but I can see that your suggestion has more merit and is certainly safer than my original idea!

Cheers,

Rob
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Old 1st October 2007, 10:55 AM   #6
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Quote:
Originally posted by Rob11966
OK Miles,

I am coming around to your way of thinking. Why can't I just replace the shared 11.2R resister with a 10R resistor and then aim for a voltage drop of 1.4v (for an idle current of 140mA) across the resistor? In other words, why add the 10R resistor in series at all, why not just replace the 11.2R?

Thanks for your patience with this. It takes me a while to abandon a plan but I can see that your suggestion has more merit and is certainly safer than my original idea!

Cheers,

Rob

Err,-

Is that 11.2 Ohm, or is it 112 ohm in the schematic....?

Looks like 112 ohm.....Maybe Im wrong though....

Personally I would add the 10R at the 'earthy' end of the cathode res. as Miles suggests.
To monitor the current at the anode circuits will give you an incorrect reading as it does not include the screen current, even though the screen-cuurent isnt Huge.
Its also much less 'safe' and with high-level signal swings on the wiring to and from the meter, more prone to pick-up in preceeding stages, possibly could cause instability/oscillations--Not good....
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Old 1st October 2007, 11:23 AM   #7
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Default Now I need a new Panel Meter

Hi Alastair,

Yes, definitely 11.2 Ohm. Decimal point is clear in the high definition original. I accept the points that you make - now I have to find a 0-2v panel meter. I was going to go with the retro look but may have to settle for digital.

Any suggestions for retro analogue panel meter suppliers will be gratefully accepted!!

Cheers,

Rob
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Old 1st October 2007, 11:44 AM   #8
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G'Day Rob,

Do you need a meter at all? Could you put a test point and just attach your multimeter when you need to check or adjust the bias?

Anyway, good luck with the build. Hope it looks and sounds as great as your last project!

Chris
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