Checking Bias

[zozoe]

OK now, After draing all the filter caps, with my handy DVM, which pins on the power tube do I check the voltage over? I was told to check the MV across the 1 ohm resistors & on each power tube cathode.... (the sizable green ones)...
Kindly highlight the pins for me, or am I to put the leads across that resistor, then power up?
Thnx ॐ

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[LOUDthud]

You need to observe where the "key" points on the center pin of the octal socket. Looking at the bottom of the socket inside the chassis, the "key" points between pins 1 and 8. Pin 1 is on the left and pin 8 is on the right when the key is pointing toward you. On a 6L6GC or 6V6GT there is no connection to pin 1 but some amps use the terminal on the socket as a tie point. Note: On old versions of the 6L6 {nothing} or 6V6 {nothing} where there was a metal can over the tube instead of glass, the metal tube was connected to pin 1. This is why you see pin 1 grounded on some really old Fender Tweed amps and layouts.

The 1 Ohm resistor typically goes from pin 8 to ground. You measure cathode current by measuring the Voltage across the 1 Ohm resistor and use Ohm's law to determine the current. I=E/R. A measurement of 20mV equates to 20mA if you are using a 1 Ohm resistor.

[Stevem]

The reason for using a precision 1% tolerance 1 ohm resistor is that it does the ohms law calculation for you.
The MV you read on your meter when it’s set for DC volts is the exact amount of MA ( current ) the tube is idling at.

If resistance wise your 1 ohm resistors check in at less then one ohm then you should dive into ohms law to see more closely what your outputs are idling at.

Note also that if your amp has a standby by switch you can use that to drain the power supply off.
Just turn the amp off by the main power switch and leave the standby switch in the play / on position.

[zozoe]

Well understood SteveM.... I'm getting what seems to be conflicting info, or just my misunderstanding co.ing into light,,, BUT, isn't 'running them hot' what's implied when one slightly increases the plate voltage in MV, across pin 8 (1ohm res.) & ground? thnx

[Stevem]

Pin 8 is the cathode of the output tube, not the plate.
The plate voltage as seen on pin 3 will change as the tube is adjusted by its bias to idle hotter or colder.

A higher reading across the 1 ohm resistor will make the tube idle hotter and in turn you will see the plate voltage drop down because your loading the power supply heavier and yanking more current out of it.

Here are some idle current recommendations in reference to plate voltage for the 6L6GC tubes you have.

Note how the higher the plate voltage used the less your idle current can be.

I would shoot for the 60% number at most .

[Phil_S]

If the sizable green resistors are running between pin 8 (cathode) and ground, then clip one meter leads onto each side of the 1Ω resistor and read mV. That is the same as your bias current, as explained above. Draining filter caps is not relevant to checking bias. You need to power up the amp to do the check. Be careful not to electrocute your self. That's why I suggest clips instead of naked probes.

[zozoe]

Here are some idle current recommendations in reference to plate voltage for the 6L6GC tubes you have.

fwiw, they're 6V6's & I'm just want to "bias a little hotter for a touch more crunch"...? I'm so lame with the terminology but my ears could explain it better...
thnx

[Stevem]

I would not idle a 6V6 any hotter then .020 A ( 20 ma) especially if the amp has a power that voltage wise was made for 6L6 outputs.