6L6 wiring question

[Tdale]

Probably a simple and stupid question..but.

I see on many layouts that the grid resistors that go to lug 5 and 4, also connect to lug 6 and 1, which are unconnected lugs as far as I understand. Are lug 6 and 1 just used as a solder lug to connect the resistors and wires, in stead of using a terminal strip?

Also, lug 8, which are the cathodes, should be connected to ground, but I see in some schems that there is a 1 ohm resistor between them and ground. Any reason for this? How do you wire the cathodes?

Tommy

[Funkalicousgroove]

Pin 1 is a Supressor grid on a Pentode like an EL34, but on a Beam Tetrode like a 6L6/KT66/6550 it is just an un-used pin, so some people like to use this pin to as a terminal to connect the grid stopper resistor to pin 5. On a Pentode like an EL34 it should be grounded. I wire all my amps this way so that I can swap tubes with a simple bias adjustment.

Pin 6 is also used as a Terminal to connect the Screen Resistor to pin 4(I always use 1K 5W so that I can swap tubes freely).

the 1ohm resistor is there to take bias measurements; Per Ohm's Law it takes 1V to push 1A across 1ohm, so by having this resistor there you can set your DMM to mVDC and read in millivolts how much current the tube is drawing.

Before the question comes about:
A 6L6 is a 30W tube, so to bias it using the 1 ohm resistor you need that reading from the cathode(pin 8 ) to find out how many watts the tube is dissipating at idle(no signal). the formula is: 30/Plate voltage=Max current draw, for class AB tubes should be biased between 60% and 70% of max static dissipation (at idle), SO, if you're using 6L6 and you have 475VDC on the plates (Pin3) then the math would go like this: 30/475= .063. .063x .7= .044, .063x .6= .0378. So on pin 8 with a 1ohm resistor to ground your DMM (set to the DCmV setting) should read between .0378 and .044 to be properly biased.

[Tdale]

Just to make sure.. Am I supposed to measure Volts or Ampere across the 1 Ohm resistor? I seem to remember that P=U*I (effect=voltage*current)... and If we turn that around, it says Current = Effect/Voltage..

I might be wrong..

Tommy

[Tdale]

Hmm, since the resistor is 1 Ohm, I guess volts and ampere will read the same anyway...

Tommy

[Funkalicousgroove]

Exactly!! You will be reading DC Millivots, but it wil correspond directly to milliamps because it is going through a 1 ohm resistor and there is a 1/1/1 ratio.

[jazzyjoepass]

And of course you'll need a minimum 1% tolerance band 1 OHM resistor for accurate readings !

And I thought 0.03A is already high but at least now I know that that's just OK bias level.

But also that depends what type of 6L6 power tube ... but that should be for another topic discussion .... cheers ....

[Funkalicousgroove]

.03 is High if you have over 500V on the plates!! it's all relative to your plate voltage, if you only have 375 volts on the plates you're looking for .048ma up to .056ma.

[jazzyjoepass]

Right. I forgot about that - it is relative to the plate voltage.

You just reminded me to check my plate volts and do a rebias!