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Old 1st July 2012, 10:35 PM   #11
cbdb is offline cbdb  Canada
diyAudio Member
Join Date: Oct 2008
Location: Vancouver
The light bulb in series with the power transformer (between the power plug and the wall outlet) is a great way to limit the current without limiting the voltage (until you draw too much current). Bigger bulbs pass more current.
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Old 2nd July 2012, 08:21 AM   #12
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Join Date: Feb 2002
Location: Lindau

Here is my advice how to test:

First check with the power turned off:
Measure DC resistance from each tube pin to ground and from each tube pin to B+ and see if the DCR reading corresponds with the expected values from the schematic.

If all reads ok, unplug all tubes and turn the power on. Measure heater voltages on the respective pins and the voltage on the tube rectifier plates.
readings will be higher than expected since the transformer is unloaded.

Then turn off and pug in the signal tubes. Turn on and check if the heater voltage is within limits ( I shoot for a range between nominal value and -5%)

You can do the same test for the rectifier tube and first unsolder the HV leads from the power transformer.

If all tests ok, power up with just the recitifer tube plugged in. Do this test only if your capacitors can handle the increased voltage! As has been mentioned the voltage will be substantially higher when the PSU is unloaded.
Alternatively, solder in a bleeder resistor which will draw enough current.

Measure the B+ voltage and also check if it arrives at the anode pins of signal tubes.

If that is also ok, test with all tubes installed. Measure cathode voltages and check if bias currents are within limits. Also measure all plate voltages.

If all static readings are ok, next test would be to run a frequency sweep to see if frequency response is ok.

Best regards

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