No Voltage drop across plate load resistor

[richierick]

Thanks much for the lessons imparted, and I sometimes just sit and read through the posts as there is so much to learn. I know the basics as I have an AA in Electronic Engineering Technology, but I have mostly done a lot of SS and digital stuff, and now I need held with a Valve/Tube amp problem
I have built a guitar amp using the Deluxe Reverb AB 763 schematic, and I'm having a problem with the driver circuit. There are two resistors on the plate load, one is a 100K and the other is 82K. The 100K works as it should, so there is 178.3 Volts measured there but the 82K still has full incoming voltage (328 Volts). So there's no voltage drop, and I cant figure out why. I would be grateful to hear any thoughts from the Gurus.

 

[JonSnell Electronic]

Are pins 3 + 8 connected together and to the 470R cathode resistor? What is the voltage on the grids? Is the ECC81 faulty?

 

[funk1980]

On the contrary, there's FULL voltage drop. Your triode is not conducting, so the resistor has got the full HT across it. Is the heater working? Defective tube??

 

[Enzo]

I am with Jon on thid one, I wager on the missed connection at the cathode, it is a home build.

 

[Loudthud]

If the grid on the input side (W/82K) is grounded, that would explain the lack of plate current. Measure the grid voltage on both sides relative (connect one lead of the DVM) to the joined cathodes. It should be about -1.5V.

 

[debk]

This may be a dumb question, but at what point did you measure the voltage?
I believe the part of the circuit you are talking about is the 12AT7 tube, and correct me if I am wrong that is the phase splitter

 

[Enzo]

He referred to it as the "driver" , but yes, it is also the phase inverter. One of the two triodes is clearly not conducting.