Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on July 17, 2012, 01:55:42 pm

Title: Peavey 6505+ - Question about heaters and V3b cathode (pin8)
Post by: six el six on July 17, 2012, 01:55:42 pm

I have both ac and dc voltages on my heaters all over this amp.

I am trying to understand how this heater supply circuit works.

1. Could someone explain to me what's going on here or point me to the part of Merlin's power supply book that would cover this type of heater circuit?

2. Also, what's up with the cathode on v3b?

I've got 147vdc there. Why?

Voltages and schematic attached.

Thanks,
six

 

Title: Re: Peavey 6505+ - Question about heaters and V3b cathode (pin8)
Post by: sluckey on July 17, 2012, 02:55:08 pm

Quote

I have both ac and dc voltages on my heaters all over this amp.

It is a crazy looking filament scheme. Some tubes are heated with DC and others are heated with AC. Even the AC is referenced to +24vdc. Peavey probably used dc filaments in sensitive or high gain circuits to reduce noise.

V3B is a cathode follower. That high voltage on the cathode is typical. There's also a high voltage on the grid.

Title: Re: Peavey 6505+ - Question about heaters and V3b cathode (pin8)
Post by: six el six on July 17, 2012, 03:10:52 pm

Quote from: sluckey on July 17, 2012, 02:55:08 pm

Quote

V3B is a cathode follower. That high voltage on the cathode is typical. There's also a high voltage on the grid.

I understand that V3b is a cathode follower.

What I don't understand is where the 147vdc is coming from??  :w2:

Is the schematic incomplete?

Title: Re: Peavey 6505+ - Question about heaters and V3b cathode (pin8)
Post by: sluckey on July 17, 2012, 03:42:00 pm

Quote

What I don't understand is where the 147vdc is coming from?? 

The tube plate is connected directly to a B+ source. The tube can be thought of as a variable resistor between plate and cathode. The grid controls the amount of resistance (or conductance) and allows more or less current to pass thru the tube. That current passes thru the cathode resistor then from cathode to plate inside the tube. The current causes a voltage drop across the cathode resistor. For example, 1ma of current thru a 100K resistor will cause a voltage drop of 100V across that resistor.