Power tube issues

[sdorer]

I'm having issues with the power amp stage of my new fender build. I have the same voltage on the screen and the plates without the tubes in, but when I put the tubes in the one plate is 340 volts and the other is 360 volts with 360 volts on the center tap of the output transformer. The tube with the lower voltage is running much hotter than the one running 360. I'm a bit stumped. Scrrens are wired parralel and of course are all the same. Also, I'm trying the fixed bias for this amp but I'm getting -150 volts when leo says I should be getting -31. Any thoughts as to troubleshooting this one?

[ampgeek]

Is that -150 VDC on the control grids (g1) of the power tubes? If so, they are way over biased. Assessing different plate voltages on the power tubes wont be productive until they are biased correctly.

Can you point to a schematic that is/or close to your build? Specifically the bias supply and control area.

Dave O.

[sdorer]

It is the fender Vibrolux 5e11. I am using a diode instead of a selenium rectifier. My guess is that is why the voltage is so far off. I was applying no bias to the grids and had the cathodes grounded when making the measurements. I will adjust the resistor voltages to get to -31. Would I increase the resistor before the diode or to ground. I'm not sure how the capacitor and resistor works to ground. I've read a lot but obviously need to keep reading. I appreciate your experience and insight.

[martin manning]

To compensate for the lower voltage drop of a silicon diode you would increase the value of the 6k8 resistor. Be aware that there is no guarantee that -34V is correct. You'll have to adjust the voltage to get the bias point dialed in for your amp based on the power tube current and plate voltage, and finding a setting that works well with the tremolo. Adding a trim pot there would be a good idea. I also see the filter cap there is spec'd at 25V, which is obviously too low. 100V would be better.

Seems like something else is wrong, though, as you shouldn't be able to get 150V from the bias tap. I suggest disconnecting the lead going to the tremolo depth pot and measuring the voltage at the pot (should be zero) and from the bias supply (tubes out).

[JazzGuitarGimp]

I hope you didn't mean to say "selenium rectifier"! Man, I haven't seen one of those in 40 years!

[rp]

I hope you didn't mean to say "selenium rectifier"! Man, I haven't seen one of those in 40 years!

He did but he meant what would have been on the original when the schematic was drawn, me thinks.

Copy the bias circuit from the BF Princeton or BF Deluxe depending where you're getting it from and tweak from there to get the range you want. The farther your tubes are from matched the more difference their plate voltages and current draw will show.