BF Champ component function question

[jlatrace]

What is the function of the 220K resistor connecting the PT pin 5 to ground? When I look at other SE amp's they do not hav ethis resistor.

I using the schematic for the power section and have put a "ODS skyliner Amp 124" style tone stack and added the "ODS skyliner AMP 124" style NFB.

When I remove the resistor the amp sounds great. With it it sounds consitpated and weak (very low volume).

I am letting it sit and then I am going to chek voltages and bias.

[Enzo]

It would be helpful to specify a schematic.


Took me a while to find pin 5 on the power transformer. You mean on the 6V6 I assume.

Pin 5 is the control grid of the 6V6 and that resistor is the grid return. Some call it the grid leak resistor, but I prefer to reserve that term for a specific application.

Please show an example of a single ended amp that doesn't have a grid resistor. The grid either must be connected to ground or to a bias supply through a resistor.

[jlatrace]

Well you are right, the resistor is necessary to properly cathode bias the amp. Trial & error and monitoring the bias, with and without, proved me wrong. Thanks for the straight forward reply.

[Enzo]

The bias of a power tube refers to the relationship between the cathode and grid, but not necessarily the voltage on either. In other words ground a grid and put +30v on the cathode is exactly the same thing as grounding the cathode and putting -30v on the grid. Either way the grid is 30v more negative than the cathode.

That resistor references the grid to either ground or bias voltage. In the case of cathode bias, tube current causes the positive voltage at the cathode, but unless the grid is referenced to ground, there is no way for that bias relationship to form.

[jlatrace]

Thanks, your help in increasing my understanding is very much appreciated.