Just wanted to close this thread. Thank you Jay, John and Tony for helping me through this issue with your suggestions. Before going back and trying all testing again I decided to replace the output transformer as if it was shorting (and it was) I couldn’t really test it properly. I proceeded to put a new transformer in and that fixed the excessive current draw and the amplifier is playing beautifully. Thanks again! Mauro
Sent from my iPhone
On Mar 7, 2024, at 5:13 PM, Jay Hennigan via Jukebox-list jukebox-list@lists.netlojix.com wrote:
On 3/7/24 06:02, M De Simone via Jukebox-list wrote:
I’ll explain how I came to suspect the heater circuit. After completely rebuilding the amplifier and testing all tubes, I plugged the amplifier in on my test setup which runs though an ammeter and during initial test the reading climbed much faster and higher than normal, so I quickly shut it down and started troubleshooting. First I injected 115 v to the power transformer through a variac (slowly) and tested 5v heater of 5U4 and high voltage was all good with hardly any current being drawn. Then I isolated the 6.3v by injecting that voltage to the tube heater circuit and this caused the ammeter needle movement to read close to 3 amps. I did disconnect the output transformer centre tap but the result was the same. Later tonight I’m going to repeat this test just to confirm the outcome as I tested many different scenarios and forget exactly the outcomes. Tested with and without tubes isolating the heater line only.
Are you saying that the current rises to 3 amps drawn from the AC line when the tube heater pins are connected via the Jones plug, even with all of the tubes removed, or are you measuring the current with an ammeter on the 6.3 volt heater circuit itself? The heater current of all of the tubes added up is a little over 3 amps at 6.3 volts, but this will only add about 0.16 amps to the AC line current as it's stepped down by the transformer in the WSR.
I suspect that the rise in current isn't due to the heater circuit, but instead from the high voltage pulling excessive current when the tubes are hot and conducting.
With all of the tubes inserted except for the 5U4, is the current drawn from the AC line excessive? In this scenario can you see the filaments glowing in the tubes or are they dark?
It indeed sounds like the output transformer is defective. With power off and the center tap disconnected, what is the resistance from the center tap of the transformer to chassis? It should be near infinite. I suspect that the output transformer has shorted turns and possibly a short to ground from the primary winding.
-- Jay Hennigan - jay@west.net Network Engineering - CCIE #7880 503 897-8550 - WB6RDV
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