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DX-100 Slow Loss Of Grid Current




 
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Author Topic: DX-100 Slow Loss Of Grid Current  (Read 178 times)
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W3MMR
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« on: April 04, 2021, 05:54:24 AM »

  On the DX-100 this morning, all was well... I was seeing an oscillation on my carrier envelope and im running 6146b's, so I wanted to swap in a pair of 6146a's. I saw no difference. So, I put the 6146b's back in, fired it up, keyed the rig, tuned it up, plate current shot thru the roof and popped the inline fuse on the power cord I installed.... Sh*t...

  So, I go to pull the 6146b's back out, and noticed that I messed up the anode cap, busted the glass, and the tube lost vacuum.  I put the 6146A's BACK in the rig...

  Now when I key the rig, after about 10 seconds, the grid current will start to fall. Not real fast, but after 1 minute, it will fall down to almost nothing and obviously the carrier falls along with it. Plate current does NOT fall. And when this happens, grid current is pulled all the way past zero to the edge of the meter.
Seems to me something is heating up... So, Only thing found so far is?? Plate choke seems to be charred. Spec'd at 1uh, tested at 1.2uh. That bottom ring Pi ring? You can slide up and down and the inner part is all black and charred. I wonder if this Is heating up and causing grid current to drop. I don't think it would, but I dont know much. Again, plate current remains the same throughout all of this but I can barely get 250ma of current no matter how much I turn the loading up.. also put in known good 6146s, 5763, 12by7a, and 6au6a. No effect. Everything else in the 5763 and 12by7a circuits test good. Screen dropping resistors in the final section tested good.

I have a spare Ranger plate choke, but I cant find the value for it anywhere before I rip it out of my parts rig. Not sure if it would be sufficient beings it only has 1 6146 final.

Perry
W3MMR
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** Studio "A" - Anan 200D, Shure SM7B, AL80BX, Internal & External Processing <--> Studio "B" - Heath-Kit DX100, D-10/4, National NC-303 **
W2PFY
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« Reply #1 on: April 04, 2021, 11:29:19 AM »

I think you have a grid to cathode short in one or both of your 6146 tubes. That is what happens with the 4-250 etc family as well. Not fun but 99 percent of the time, that is what it is. The big indicator is the meter going negative past the zero or low mark on the dial face. 

73

Terry
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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #2 on: April 04, 2021, 12:12:04 PM »

Tooobes. Like Erry-Tay said!  Wink
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W3MMR
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« Reply #3 on: April 04, 2021, 07:44:46 PM »

I need to buy a tube tester.. I tried a bunch of different tubes, but not the right combination! A bunch went in the trash today!

Tubes it was...

A new pair is being bought...

73
Perry
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** Studio "A" - Anan 200D, Shure SM7B, AL80BX, Internal & External Processing <--> Studio "B" - Heath-Kit DX100, D-10/4, National NC-303 **
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