Is ther an easy way to diagnose screen grid resistors?

[pine]

I have a Mesa Lone Star Classic and it seems to be underpowered. Is there a quick way I can test the screen grid resistors without pulling the chassis out? I read somewhere about measuring impedance on one of the pins of the power tube sockets to see if there is resistance. I believe the resistor is supposed to be 470 ohm, I just don't know which pins to measure to diagnose this.

[52 Bill]

I read somewhere about measuring impedance on one of the pins of the power tube sockets to see if there is resistance. I believe the resistor is supposed to be 470 ohm, I just don't know which pins to measure to diagnose this.

I'm not familiar with that amp, what output tubes does it use?

If it uses 6L6's or EL34's, you could try reading the resistance between pins 3 and 4 of the power tube sockets. This reading would include the screen grid resistance plus the resistance of one half of the output transformer primary winding. This would at least let you know an approximate value for the resistor.

Be sure that the amp is off and the caps discharged before doing this.

[bnwitt]

Pull the chassis. It's not that hard and you should take a look around.

[Enzo]

Or similar to Bill's idea, measure from pin 4 to pin 4 of any two sockets. If there are no screen resistors you get zero ohms. if there are screen resistors, the readings between any two sockets ought to be twice what one screen resistor is. By doing several combinations of sockets you can see if any one socket is out of whack from the others.

Without pulling the chassis, but pulling the power tubes, I routinely stick a probe in pin 4 of the end socket on some Marshall and go down th row expecting to see approximately 2k each time. 1k screen resistors.

[52 Bill]

Or similar to Bill's idea, measure from pin 4 to pin 4 of any two sockets. If there are no screen resistors you get zero ohms. if there are screen resistors, the readings between any two sockets ought to be twice what one screen resistor is.

That's a great idea! Enzo, you've done it again.

[pine]

Or similar to Bill's idea, measure from pin 4 to pin 4 of any two sockets. If there are no screen resistors you get zero ohms. if there are screen resistors, the readings between any two sockets ought to be twice what one screen resistor is. By doing several combinations of sockets you can see if any one socket is out of whack from the others.

Without pulling the chassis, but pulling the power tubes, I routinely stick a probe in pin 4 of the end socket on some Marshall and go down th row expecting to see approximately 2k each time. 1k screen resistors.

I'm putting a probe in pin 4 (this amp will take either 6L6s or EL34s) and the resistance between the two outer or the two inner tubes is roughly 940 ohms, and between any inner and any outer tube is way higher, around 1.6 K.

[52 Bill]

I'm putting a probe in pin 4 (this amp will take either 6L6s or EL34s) and the resistance between the two outer or the two inner tubes is roughly 940 ohms, and between any inner and any outer tube is way higher, around 1.6 K.

The 470 ohm value makes sense here: 470 + 470 = 940.
Unless there is something different about the way that the screen grids are powered in this amp (and it could be in a Mesa) there should still only be 940 ohms from inner to outer sockets.

Someone else with more specific info might know the answer to this one.