Burning Screen Resistor

[pompeiisneaks]

also mulling that last response, I can think of an OT failure state that could mimic 15 or 100k resistance. what if a winding broke open, but arc'ed across to another winding enough times to create carbon build up that was like a carbon resistor path of 100/200k etc. It still allows conducting, but heavily resistive?

At any rate we're all doing speculation, when in fact we should be helping find the source of the problem. I can't think of other causes. Desoldering the leads and reconnecting causing a 'move' of the problem, to me implies something wrong with the OT wires themselves at a minimum? Maybe a heavily corroded lead on one of the two that isn't providing solid consistent connectivity? The resistance test would be a very good validation of this theory.

~Phil

[pdf64]

also mulling that last response, I can think of an OT failure state that could mimic 15 or 100k resistance. what if a winding broke open, but arc'ed across to another winding enough times to create carbon build up that was like a carbon resistor path of 100/200k etc. It still allows conducting, but heavily resistive?...

Exactly!

I've not ruled the OT out; rather a further test would be beneficial before advising someone to spend $$$ on a replacement.
If it's not the OT, it may be that a test or reporting error has been made, or there's something else going on, eg ultrasonic oscillation.

[bal704]

Resistance between OT Lead 1 and OT CT = 20 ohms. Resistance between OT lead 2 and OT CT = open

[sluckey]

I'm really sorry to hear that. But... if one side of the OT is open circuit there is no way any amount of current could flow through it. So, how are you measuring plate current?

[bal704]

That was a question I had. I have a bias jack hooked to each output socket pin 1/8, and those jacks are connected to a ground jack via a 1R resistor between each bias jack. I've used this bias measurement system before and it seems to work.

I measure 390v on pin 3 of one socket, 0V on pin 3 of the other socket, both wrt ground. However, even on the socket that has 0V on it (and is open), I still measure about 20mA through the bias jack to ground.

Not sure exactly what's happening, but it sounds like my OT is toast in any event.

[pdf64]

... if one side of the OT is open circuit there is no way any amount of current could flow through it. So, how are you measuring plate current?

My guess is that rather than being plate current, the 20mA mentioned is cathode current.
And of that, most will be via g2.
See chart at the bottom of p7 http://www.mif.pg.gda.pl/homepages/fran ... /6L6GC.pdf
At low plate voltage, g2 current will increase.

[sluckey]

And of that, most will be via g2.

Those poor screens are likely bent too. Definitely been stressed.

[bal704]

Does this mean the tubes I had in this are also toast?

[sluckey]

Does this mean the tubes I had in this are also toast?

Maybe. Probably. In your initial post you said you measured 40v across the 470Ω screen resistor. That's 85mA flowing through the screen! Very excessive. Likely to have damaged the screen. With all the back and forth tube swapping they are both likely damaged.

If there's one thing to take away from this expensive experience it should be... Never plug in any power tubes in a fixed bias amp build until you are absolutely sure you have proper negative bias voltage on the grid (pin 5 in this case).

[bal704]

Any way to check the tubes to see if they're good (without a tube tester)? Or is it best to throw them away?

[pdf64]

Unless you've got access to an expensive, high spec tube tester, the only way to check that a tube will work properly at amp like conditions is to check it in an amp.
It may be fine, be prone to shorts, be worn out by the overdissipation etc.
In an amp, check it using a light bulb limiter first.