Bogner Shiva Power Tube Pins 1&8 Not grounded?

[chunkitup]

I do not have the schematic for this amp. A friend blew a screen resistor. After replacing both screen resistors with 1k 5w cement resistors (original were 6.5w, I hope 5w will be ok) I was ready to turn on without the tubes in and take some voltages. I was trying to find a place I could place the ground lead from the DMM and knew pins 1&8 should be grounded. I quick tested from pins 1&8 to a ground lug from a tag strip for continuity but got none. I then switched to the ohm setting and it showed an open loop. I tested from the bottom of several cathode resistors to the tag strip and it showed the tag strip was at ground. So why are pins 1&8 not at ground?

[g1]

What kind of power tubes?

It seems maybe the standby switch disconnects the cathodes from ground?

[52 Bill]

Is the switch set to play? In standby the cathodes are un-grounded.

[chunkitup]

So they are ungrounded when off. That would explain it. Why do they do that?

[The Dude]

This might help.

http://spinoo.free.fr/sloclone/Shiva%20power%20amp.png

[Enzo]

They do that as just one of many ways to turn off the power tubes. This silences the amp, which is the purpose of the standby switch.

[Jazz P Bass]

Just a thought.
Screen resistors typically do not 'blow'.
At a 6.5 watt rating, something caused more than that to flow through the resistor.

My guess: ditch the output tubes & install new ones.

[chunkitup]

Yep. The tubes are gone. Going in will be new Rubys which Bogner recommends. But first onto another puzzling thing. The owner had sent this amp to bogner to get fixed for a in and out volume problem. As I go thru the amp almost all the pcb screws were backed out. Several are missing and the owner said there was something rattling around for awhile. Why wouldnt they use lock washers of some sort? On to the pic.
http://i587.photobucket.com/albums/s...ognerShiva.jpg
The bias is sent to pin 5 on both tubes avoiding the grid resistors which are connected to pin 6 where I would expect to see the bias connected then thru the 4.7k to pin 5.

[frus]

well that's just plain wrong

[Jazz P Bass]

The bias is sent to pin 5 on both tubes avoiding the grid resistors which are connected to pin 6 where I would expect to see the bias connected then thru the 4.7k to pin 5.

Good catch.

[chunkitup]

Biasing Question

So I put in a pair of Ruby EL34B STR's. At -59v bias which is the top of the range I get 2.8mA @ 542v which is 15w. 60% of 25 is 15 which is what Ive been reading to bias this amp at. Everything sound about right? The current really jumps up though when I strike a hard chord while the plate voltage doesnt come down very much. I guess that is why you need a robust tube in this amp. Will 5w screen resistors be enough? They did have 6.5w resistors in there which I did not have to replace them.

[The Dude]

Did you mean 28mA or 2.8mA?

[Enzo]

Bias is idle current, the current while playing is not related to bias.

[chunkitup]

Thanks everyone. I need to get out of the sun. Calculating 2.8mA is not .028 but .0028. I cant believe I wasted everyones time. Everything is great with this amp.