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Old 1st November 2006, 07:48 PM   #21
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Hi Ed,

If this were my amplifier and I wanted to measure output tube current, here's what I would do.

I'd install a low value carbon comp resistor in each tube's cathode to ground. By low value I mean around 4.7 ohm ( watt) or something close to that. I could then measure the small voltage drop across that resistor and use simple ohms law to calculate the current. (I=E/R or voltage divided by resistance)

For a current of 50mA, you would measure 0.235 volts across 4.7 ohms. And you could even make that cathode point available with a feed-through standoff on the chassis near the tube for future measurments. And do not be concerned about adding a tiny resistance in the cathode cirucit. Pragmatically it will have no effect on the sound or circuit performance.

If you'd prefer not to alter the amp in any way, then you could make an adaptor socket to place under the tube that would make the cathode and resistor available for your test probe. It they're octal base tubes it's easy. Just use a defective tube's base as your plug with a socket mounted on top.

victor
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Old 1st November 2006, 08:31 PM   #22
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Thanks - Got it!
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Old 1st November 2006, 10:53 PM   #23
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Default Proximity?

The way this thing is laid out, the output tubes are right next to the transformers. On some other amps I see from the same period, they are not nearly as close, like Fisher x100's or even Scott's 299d. One opt has got the rectifier next to it also. The problem is compounded a bit also because the replacement JJ 7591's are larger than original stock (jj's on left) and can actually TOUCH the laminations on the transformers if tilted. Perhaps Tubelab is correct and this is simply a radiant heat thing. After 20 or so minutes the far sides of the tranformers are much cooler than those facing the tubes.
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Old 2nd November 2006, 12:36 AM   #24
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The tube current can be measured without adding any resistors.

With the amp off, and everything discharged (I would measure it after it has been off overnight) measure the DC resistance of each half of the OPT primary. Write down the numbers. Turn the amp on, with no signal applied, measure the voltage across the same primary halves. Take the measurements after the readings have stabilized, but before the transformers heat up, the resistance will change slightly. You can use ohms law to compute the plate current. The screen current can not be easily measured, but can be assumed to be 5 to 7 mA per tube.

It is normal for the dc resistances of each transformer half to be slightly different. It is also normal for the currents to be slightly different. Adjust the balance pot for minimum hum.

I set a black RF attenuator on top of the electrolytic cap on one of my SimpleSE boards, This puts it about 1/4 inch from ONE 6L6 tube. It was too hot to touch after about an hour.
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