Why 2.2k screen grid resistors?

[bluesguitar]

I'm checking out the chassis of a '66 twin reverb that looks basically untouched. However on the power tube sockets there are newer 2.2k screen grid resistors, and some 10 ohm resistors on pin 8 to ground. Anyone know why they didn't stay with the usual 470 ohm screen grid resistors? Also, I've heard of 1 ohm resistors to ground on pin 8, but why 10 ohm?

[Reeltarded]

The 2k2 goes easy on the tubes and the 10R is one order of magnitude away from 1R but just as useful. Move the decimal.

The moral of the story is that people use what they have on hand. Those 2k2s might not sound best. Where is that thread about the screens going slightly positive? lol

[bluesguitar]

The 2k2 goes easy on the tubes and the 10R is one order of magnitude away from 1R but just as useful. Move the decimal.

The moral of the story is that people use what they have on hand. Those 2k2s might not sound best. Where is that thread about the screens going slightly positive? lol

Thanks for the response. I understand that these resistors off the cathode pin allow you to measure the bias. This would be very handy especially in a twin with four 6L6's. I usually use a bias tool that goes between the tube and socket, but it only measures two tubes at a time. How exactly do you measure "across the resistor"? I know how to measure plate voltage with the negative lead clamped to the chassis and the positive on pin 3. Do I do the same thing on pin 8 to get a bias reading?

[tubeswell]

How exactly do you measure "across the resistor"?... I know how to measure plate voltage with the negative lead clamped to the chassis and the positive on pin 3. Do I do the same thing on pin 8 to get a bias reading?

Yes. One meter lead goes to one end of the resistor, the other lead to the other end. For 1R or 10R bias resistors, set the R-meter to the 'mV' setting to get a reading.

[Leo_Gnardo]

I usually use a bias tool that goes between the tube and socket, but it only measures two tubes at a time.

And what's inside that "bias tool?" Ta-daaaah! A resistor from pin 8 of the plug end to pin 8 of the socket end. Depending on who made it, either 1 ohm or 10. If it's 1 ohm then 1 mV on your meter means 1 mA of bias current. If it's the 10 ohm version, then read 10 mV for each mA of bias current. Don't know which? Measure it with your ohm meter.

The only reason I can think of to choose 10 ohm, is you can get an extra digit of accuracy from your digital voltmeter. Other than that, it's the same thing. Of course it doesn't hurt to have 1% precision measuring resistors whether it's 1 or 10 ohms. In fact any value say 1 to 10 ohms would do - 1 or 10 just makes the math easy to do in your head.

As for 2K2 screen grid resistors, these will limit the amount of power your Twin can produce to maybe 50 - 60W. On the other hand output tubes will probably last near forever. So if somebody wanted their '66 Twin to be ultra reliable, they might have done that. By coincidence I'm about to get to work on a 66 Twin right now. Looks like all original.

[Emetal]

And what's inside that "bias tool?" Ta-daaaah! A resistor from pin 8 of the plug end to pin 8 of the socket end.

Well not necessarily, some tools have simple wire going to your amp meter (set to mA) and back. If you measure in mA no need for a resistor. I do not even use a bias tool, I simply un-solder ground from pin 8 and (temporarily) solder my amp meter in series, between pin 8 and the ground... But yes the more expensive bias adapters have several resistors in them for taking voltage and amp measurements with the same tool...