Rectifier tube and too much current in 6L6's

[dscottguitars]

Hi,  I switched on my rectifier tube 5U4GB, in this new amp and the plate voltage drops to about 310v.  The current in the tubes goes over 100mA and the plates get red.  I have used this same setup before 5 times without any problems so I don't know what could be wrong. I tried a different tube and same results.  Any suggestions??


Thanks,


Daniel

[eleventeen]

Gotta be something wrong with your bias on the output tubes. Whatever level of bias your circuit sets up should be present (per your schemo) even with the amp in standby; eg, no B+ applied to the 6L6 tubes. Ergo, clamp your voltmeter probe onto one or the other pin 5 on those 6L6 outputs and power up the amp (but leave it in stby) and see what those bias volts are. Should be maybe -45 to -50 volts.


Perhaps you've mis-wired your bias selection arrangement. I suppose it's also possible that you have a leaky .1 uf cap between one of the plates for V4 and the bias deal you have....and the leaking plate voltage is swamping the negative volts associated with output-tube-bias. Unlikely but not impossible. You are, I take it, using at least 400 volt caps for those coupling caps? 200 volt caps won't cut it there.


But you should be able to check bias volts in standby, so no red 6L6 plates. Make sure your adjustment range on your bias pots is like -30 to -60 volts.

[sluckey]

You have 110µF input cap. Drop that down to 40 or 50 and see it that helps.

[dscottguitars]

I can try that, but I've used the same power supply before without such a drop in voltage...

[dscottguitars]

I found the problem!!   Thanks eleventeen.  I checked the bias voltage w/o the standby switched in and there's no voltage.  That's because I have the tap coming from the diodes that are fed by the switch so when it is switched into tube rectifier mode, there's no power going to the diodes.  I thought this would be neater that my previous builds.  I had the board made and didn't realize that change...


Thanks!!!

[dscottguitars]

you can see the one diode going to the bias circuit from the red wires on the board...  Thanks again!!

[eleventeen]

Good going. Technically, Sluckey is right (though it shouldn't IMO be the cause of your issue) wrt the input filter cap being too large. Probably a hefty 5U4 or 5R4 can take it and probably a 5Y3...where the filaments are widely separated from the plates and there is no indirect cathode. But such a big cap can blow (short out) an (expensive) GZ34 or 5V4 and short them out. You'd see sparks inside the rectifier tube.