DeVille bias pot problem

[wcrankshaw]

A friend of mine had me take a look at his Hot Rod DeVille because he changed the tubes, and thought maybe the bias was messed up. Just didn't sound as good, kind of buzzy, etc. So I watched the tubedepot video on biasing those amps, and checked it out. The bias pot is reading no more than 10-12 mV on standby, and when turned on it isn't any different. Turning the pot to the extremes makes no difference either. Do you think this is a bad bias trimmer, or is there a bigger issue somewhere else? Any help would be great. Thanks!

[Jazz P Bass]

Fender HR Deville

You need to check Pin #5 of the output tubes.
It should be -54 Vdc.

[wcrankshaw]

Thanks for the reply. Pin 5 is measuring 0-VDC with the bias pot hard right, and -1.1 VDC with it hard left. Sounds like a problem.

[52 Bill]

Double check your readings, if the bias supply was that far off, the output tubes would be red plating like crazy.

[acorkos]

resolder all the pins on both power tubes....failing solder joints is a common problem on fender hot rod amps

[Prairie Dawg]

A friend of mine had me take a look at his Hot Rod DeVille because he changed the tubes, and thought maybe the bias was messed up. Just didn't sound as good, kind of buzzy, etc. So I watched the tubedepot video on biasing those amps, and checked it out. The bias pot is reading no more than 10-12 mV on standby, and when turned on it isn't any different. Turning the pot to the extremes makes no difference either. Do you think this is a bad bias trimmer, or is there a bigger issue somewhere else? Any help would be great. Thanks!

The bias supply is one of the simplest things there is in a guitar amp so it ought to be pretty simple to trouble shoot it. You've got a supply, either from a discrete winding on the power transformer or from one of the high voltage leads, a dropping resistor, a diode, a capacitor to smooth out the wrinkles, a couple of other odd resistors and then delivery to the grids thru some 220k resistors.

Get your schematic out. Identify the components on the board, and work your way from the AC pickoff through the diode and then the rest of the way to the grids, always making sure that you're measuring AC ahead of the diode and DC behind it. If your readings are correct it will be simple enough to locate, and you might do well to check that diode for proper function and the resistors for value. Remember to reset your multimeter for AC or DC as the case may be. It helps avoid WTF moments.

Remember that you have to have the tubes installed to accurately make your measurements, which means you've got to have a load like a speaker connected to the amp. Also there are significant voltages in there that'll ring your bell if you're not careful.

[booj]

Hot Rod DeVille.pdf

[wcrankshaw]

So it seems there may be a bigger issue. I checked voltages comparing to the schematic at a lot of places, and they all seemed extremely low. There is however sound coming out with a guitar plugged. It just sounds cold, and not quite right, and the drive channel is pretty weird. There is a buzz coming from the chassis (potentially transformer) when the amp is switched to on. Switched to standby, buzz goes away. Buzz is definitely not a rattle, loose screw, etc. Any ideas?

[wcrankshaw]

Green, Filament is only reading .189VAC on each tube. Red, HV is only reading 6.3VAC.

This sounds like sort of a power transformer problem doesn't it.