Smoking 7.5k grid resistors on AC30/4 build - Oh, the Stink.

[chopstuck]

I tied the 7.5K grid resistor from pin 2 to 8. I tied the 110 ohm screen resistor from pin 6 to 9. Where may the current be flowing ? I have no voltage leaking across my driver circuit .01uf caps.
Thanks for the input so far. I have no preamp tubes in.

[Structo]

According to my data, pins 1, 6 & 8 are internally connected.

[chopstuck]

Thanks Structo, I will check these old tubes with my meter and see. It would be great if that were true. I've been pulling my hair out on this problem.... just the grey ones...

[tonestack]

I tied the 7.5K grid resistor from pin 2 to 8. I tied the 110 ohm screen resistor from pin 6 to 9. Where may the current be flowing ? I have no voltage leaking across my driver circuit .01uf caps.
Thanks for the input so far. I have no preamp tubes in.

Using Pin #6 as a tie point is also a not a good practice. Pin #6 is internally tied to Pin #9 on the 7189A. Basically, it is not a good practice to use any of the "unused" pins on an EL84 socket as tie points, as pins 1 and 6 are connected to other pins on the 7189A and the data sheets for the 7189A specifically warn against using pin #8. I use noval sockets that have a tie-point in the middle of the socket for the screen grid resistor and solder and shrink wrap the grid stopper resistor to my screen grid wire.

[chopstuck]

Turns out checking all my power tube sockets for continuity helped me find my problem. One of them was shorted pin 8 to 7. I cleaned that socket, (I know I would have to replace it soon), and re-tested it with the tube in. Hmm..... short again. I tested the newest of my old USA tubes and it was shorted or tied 8 to 7. I replaced it with a known good tube (Tested for shorts first) and no squeal. I guess that tube worked in the other amp. This ac30/4 amp sound glorious with the junk 6BQ5's an old RCA 12ax7 and a newish JJ EF806. Well thanks again for all the suggestions. It really helped me on where to look. I need to have a little more faith in my work. I always figure I screwed something up. - I'm usually right about that. Next time I will take your advice and not tie anything to unused pins. I like my grid stoppers on the board anyway. Looks neater.

[tonestack]

Turns out checking all my power tube sockets for continuity helped me find my problem. One of them was shorted pin 8 to 7. I cleaned that socket, (I know I would have to replace it soon), and re-tested it with the tube in. Hmm..... short again. I tested the newest of my old USA tubes and it was shorted or tied 8 to 7.

I may have a defective tube. As I mentioned earlier, the data sheets for the 7189A specifically state that pin 8 should not be used as a tie point.

http://tdsl.duncanamps.com/link.php?target=0825A880


Additionally, if you read the data sheets for the 6BQ5, you will find that pins 1, 6, and 8 are internally connected. The only time it is safe to use an unused pin as a tie point is when the data sheets define the pin as "not connected."

[img:600:785]http://tdsl.duncanamps.com/dcigna/tubes ... bq5-3g.gif[/img]
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