Strange biasing troubles

[shzmm]

Hello,
I have a problem with a new build-- I've included a schematic but it's basically an AC4 with a DC cathode follower and tone stack. Anyhow, the trouble I'm having is biasing the el84. When following the schematic, I get around 300V B+, 240V on the plate, 17V on the cathode, and 270V on the screen.
I've tried bringing the screen down with more and more resistance until the screen and plate meet at around 220V. This pushes the cathode up to 22V.
I've tried replacing the tube but get roughly the same voltages. I've also tried to change values in the cathode but as all three are interdependent nothing is really solved.
Out of desperation, I've tried to replace the output transformer as well but that didn't work either. It somehow just doesn't seem normal to drop so much voltage across the OT, does it?
About the only thing I can think of is that I may either have a bad power transformer or the tube socket needs to be replaced.
I should say that the transformer is salvaged from a reel to reel deck.
Any suggestions would be most welcome

[txstrat]

I've learned from this forum that it's not that a big deal if the screen is higher than the plate. But 30v seems too much anyhow. Where have you raised the resistance to drop the voltages?
I'd raise the resitor after first filter cap in the B+ line of your schematic to 5k or even 10k and see what happens.

[tubeswell]

What's the DC resistance of your OT primary?

[shzmm]

Hello and thanks for the replies

The DC resistance of the output transformer is 750 Ohms. This seems to be about the same as other SE el84 OT that I have.
As for the screen being higher, I don't know. I assumed the opposite that it could kill the tube. Maybe a few volts wouldn't be such a big deal but 30 Volts seems like alot. It would be great to get confirmation on that though.
I have increased the dropping resistance like you said but this threw the cathode much much higher.
Hope I can figure this out, it's quite frustrating.

[txstrat]

Are you trying to get your voltages exactly (or very close) to the vox schematic?
I wouldn't bother with the voltages as long as they are within 10% up or down. I would decrease the cathode resistor to 180 ohms for a start, to see if it changes the voltages on the plate and screen enough.

[shzmm]

Hi again,
No I'm not really trying to match the values on the schematic per se, but I would like to fall into safer territory. I have tried adjusting the cathode resistor down to 180 150 even 100. Strangely, the 180 brings the voltage up on the cathode.
I'm having a difficult time getting my head around this one. I have the feeling something must be defective but I'm not sure where to look. Any more feedback is more than welcome.

[txstrat]

Don't get stuck on the cathode voltage.
What are the voltages on the plate and screen with a 180 ohms cathode resistor?

[shzmm]

Thanks for sticking with me on this one--

OK so I decided to take a deep breath and start over again. This time I followed your suggestion of dropping the B+ before the screen. I first tried 2k then 2.7k which pretty much got the screen and plate in the same ballpark.
Then with a 180 ohm on the cathode, everything dropped a bit. Now I have around 305 as B+, 270V on the screen and plate and around 8.5V on the cathode. I suppose I can live with that.
Sometimes I guess I just a get a little stuck in the tweaking without following a regular pattern. I should probably start taking notes as I go along.
I'm also going to have to back to the textbooks and study how dropping 30V on the output transformer works.

Thanks again for your help!

[txstrat]

Glad you sorted it out.