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Old 26th March 2005, 01:11 PM   #1
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Default Mysterious runaway output tube

I just changed all the driver and splitter tube sockets in my power amp. Every voltage I know of checks out, yet suddenly, one of the driver tubes starts to run away as I bring up the VAC with a Variac. In checking, I've done the following.

1) Switched tubes.
2) Check for bias voltage on the grid.
3) Tightened the grid socket on the driver tube and checked the pin for continuity.
4) Checked the voltages on the spitter tube.
5) Checked for shorts on splitter and output tube.
6) Checked other tubes, which operate fine. (I have a single bias supply).

As yet I have not

1) put a meter on the grid tube pin itself when the amp is running to check for voltage
2) changed the coupling cap, which appears to be operating normally within the circuit.

Obviously, I've missed something. But is there something else that can cause a tube to runaway that I'm missing? Is there a place to look that I should focus on?
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Old 26th March 2005, 01:29 PM   #2
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What happened when you switched tubes? Did the problem remain at the same socket or did it follow the tube?
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Old 26th March 2005, 02:00 PM   #3
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Sorry. Should have made that clear. The problem remained at the socket.
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Old 26th March 2005, 05:06 PM   #4
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New information. I thought it was just one tube because I was so quick to pull back, but it's both on one side. Again, all checks reveal nothing out of the ordinary. As I ran it up again, counting on the sound of the transformer to tell me the tubes were running away, I found ample bias on the grid.

It seems like it almost has to be a socket (I'm ordering new ones). Can one bad bias pin affect both tubes? Maybe so.

I have no other ideas unless I'm just missing something.
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Old 26th March 2005, 06:27 PM   #5
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Try the coupling caps, leaky maybe?

I doubt a loose socket would cause trouble; it would have to disconnect the grid, which would kill audio (if you've been testing with signal). It would also go suddenly, since the contact resistance is either very high (open) or very low (if even in the kohms range)

Tim
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Old 26th March 2005, 06:56 PM   #6
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Is there a common cathode resistor/cap?
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Old 26th March 2005, 07:46 PM   #7
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No common cathode resistor or cap.

Meanwhile, I've since switched the tubes--the two from the left to the right. The problem remained on the original side.

I also changed one of the coupling caps. (Will try the other). Nope. But I don't think this is it.

I've also taken readings from the pin side of the socket. All are normal.

I guess my question is, what other than going off bias, can cause tubes to run away?

It's something freaky, something I did when changing tube sockets. There has to be a short somewhere, but I have no idea where.

Thanks so far for all the replies.
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Old 26th March 2005, 07:54 PM   #8
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Pull the tubes, let everything discharge, then start to check continuity from the top side of the socket to the rest of the circuitry. Check coupling caps for leakage (especially the one you didn't replace!).
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Old 26th March 2005, 08:03 PM   #9
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Why not? I can't think of anything else to do. Thanks.
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