Blues Deville low distorted sound

[g1]

Indeed I have seen it a few times, prevents the power tubes from drawing current, thus low, distorted output when one is open.

Ok, now I'm really confused.
If the 1K5 control grid resistor opens, you will lose the negative bias voltage and the tube will red plate. So rather than not drawing current, it will draw excessive current.
The 470R screen grid resistor (R57 & R58) opening will result in 'prevents the power tubes from drawing current'.

[Tone Meister]

Ok, now I'm really confused.
If the 1K5 control grid resistor opens, you will lose the negative bias voltage and the tube will red plate. So rather than not drawing current, it will draw excessive current.
The 470R screen grid resistor (R57 & R58) opening will result in 'prevents the power tubes from drawing current'.

Yeah, you're right - I do have their functions backwards. But I am indeed speaking of failed open 1.5k grid resistors in many of the Fender Blues and Hot Rod series amps. I have long ago gotten to the point that I change both PI plate resitors, the 1.5K grids, and the 470R screens any time I pull back the tube board on one of them. And of course, touch up the solder on the pins for all the tube sockets. Quick and easy insurance against future failures.

[J M Fahey]

What´s wrong with actually SCOPING the thing?

[alexradium]

What´s wrong with actually SCOPING the thing?

don't tell me,when i see that moron youtuber Guitologist that loses time chasing ghosts in Mesa Boogie amps and starts subbing parts randomly....he goes for the "feeling",like a blind man in an orgy (Frank Drebin).

[nosaj]

What´s wrong with actually SCOPING the thing?

Why bust out the big guns when a DVM will tell you if the resistor is open?

nosaj

[SoulFetish]

What´s wrong with actually SCOPING the thing?

Right! Even though my first assumption would be that the output stage was conducting only half the phase after hearing the complaint (much like the others). I could immediately confirm this by scoping the output. Then quickly determine if it was one of the tubes before I even took the back off the amp.
The two tools which I've come to trust and rely when troubleshooting an amplifier is my analog panel ammeter which monitors current drawing from the mains supply and being able to observe and analyze waveforms on my scope. Even at my home shop where I haven't installed a panel meter yet, I feel uncomfortable not having it.

[Jazz P Bass]

If the tube pins are readily accessible, I have gotten into the habit of measuring pin 1 /3 & pin 6 /8 of the preamp tubes. If nothing else, just to see how well they are conducting (if at all).

A simple check of the PI circuit would have revealed the open resistor.

FYI: Fender PI DC voltages (As an average/ dependent on B+ Vdc)
Pin 1: 264
Pin 2: 23
Pin 3: 36

Pin 6: 241
Pin 7: 23
Pin 8: 36