Not getting a mA reading in one of the pwr tube sockets

BrentSP

Silver Supporting Member
Messages
2,040
3 tubes show a mA reading but one of the sockets just shows 0.00...........I tried swapping out the tubes using the same socket and still 0.00. What does this mean? Am I running on 3 tubes?
Also I get a plate voltage between 550 - 560 in all four of the sockets just not mA reading in one. Help!

BTW: this is in a Bogner XTC
 

John Phillips

Member
Messages
13,038
If you are gettting a plate voltage reading but no current (despite trying different tubes), then you almost certainly have a screen-grid resistor failure on that socket.

Yes, you will be running on three tubes.
 

BrentSP

Silver Supporting Member
Messages
2,040
Originally posted by John Phillips
If you are gettting a plate voltage reading but no current (despite trying different tubes), then you almost certainly have a screen-grid resistor failure on that socket.

Yes, you will be running on three tubes.
Now I'm not reading the pins as I dont' know how to do that. My mA and PV readings are coming from a Weber Bias Rite. I don't know if this has anything to do with it. By the way all four of the tube lights up and get just as hot as the other tubes.
 

John Phillips

Member
Messages
13,038
The tube will heat up as normal if the filament is working - which is completely unrelated to the high-voltage stuff.

If you're getting correct readings on the other three tubes, the Bias Rite is working OK.

If you're getting a plate voltage reading on the suspect socket, both the plate connection and the cathode (ground) connection must be OK. If the tube isn't running away into overload (you'd see the plate glowing red, and measure a very large current), the grid connection - which supplies the bias - must be OK. That just leaves the screen-grid. Without this supplied with the right voltage, the tube will pass no current.

Chances are that the resistor is cracked or burned out - sometimes they just fail internally for no very obvious reason too.

The only other possibility is a bad connection between the tube socket and the tube pin, but actually a failed resistor is more likely.

Check to see what happens when you flip from standby to on - if the tube is passing current at all, you'll see the glow inside the plate change, usually slightly brighter and a small amount of blue.
 

Lonely Raven

Member
Messages
1,003
John, I hope you get thanked regularly for your technical
help!

You help bring things down to a level us beginners can
understand rather then browbeating the reader like some
seem to enjoy doing!

Thanks!!
 

BrentSP

Silver Supporting Member
Messages
2,040
Thanks John. I talked to Mark Cameron and I may just send it to Bogner and let them tune it up for me.
 

Greggy

Senior Member
Messages
13,439
Originally posted by Lonely Raven
John, I hope you get thanked regularly for your technical
help!

You help bring things down to a level us beginners can
understand rather then browbeating the reader like some
seem to enjoy doing!

Thanks!!
Amen to that.:)
 




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