On 2024/03/07 6:02 a.m., 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. I’d like to blame the output transformer but I don’t think that would affect the test I did. Probably a simple solution but I’m just not seeing it. Thanks for the suggestions so far!
Make some notes on the tests performed as you do them...
Disconnecting the center tap of the output transformer does not stop the current rise? Leave it disconnected and pull the 5U4, leaving the other tubes all plugged in.
Does the current rise as the filaments warm up with the B+ off?
If yes, then something on the 6.3V circuit, probably a tube with a heater to cathode short. Perhaps the cathode of one tube is accidentally shorted to ground?
If not, then troubleshoot the B+ starting with the choke - it may have an internal short to ground. You did replace the electrolytics C7, C13, C6-x, and C43 in your amp rebuild?
John :-#)#
Sent from my iPhone
On Mar 7, 2024, at 8:25 AM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
This just came to me. If your fuse in the amp is blowing, it 's not the heaters. They get their 6.3 volt power from the selection receiver. Unless the fuse that blows is in the selection receiver?
On 3/6/2024 7:57 PM, M De Simone via Jukebox-list wrote: I’m currently working on a V200 amplifier and can’t figure out where the current draw is coming from. I’ve isolated it to the 6.3 heater circuit but can’t find any issue with it. The only problem is the humidity adjustment is missing the wire wound section so it’s basically not doing anything. I put 2 resistors in place as a replacement but when I fire up the amp my amp meter starts rising and I shut it down between 2 and 3 amps. Usually when texting one of these amplifiers it slowly rises to about 1.5 amps and holds steady there. What am I missing here?
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