Is there 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.

[steve25]

You have two ways to check these resistors, a safe way and a amp powered up way. For the safe way first unplug the amp and turn the stanby switch on for 3 minutes to drain out the power supply, than take your meter set for ohms and jump it across pins number 4 and 6 on the socket. when you are lokking down on the socket pin 4 will be counter clockwise fron the keyway.
Unless you had a output tube short chances are that these 470 ohmers are good.
If the amp plays clean but lacks power pull the driver( phase inverter tube) tube next to the output tubes and swap it with a preamp tube, and check for a difference.
To test these resistors live set you meter to read at least 500 VDC if its not a autoranging type and place it between pin 4 and ground to check for volatge.
PS in thinking about this, I do not know if mesa uses pin 6 as a tie point for this resistor, so it may be best to just be carefull and check pin 4 for voltage.

[yankeerob]

Hey Pine - welcome to TFF - if the Lone Star is anything like the DC-3 we've had in re construction and design (v. likely) then you've got one hell of a lot of switching going on - most probs of this nature are down to the radial electrolytics in the power supply for the switching matrix - they're only rated at 85° and with 10 bottles glowing just underneath 'em it ain't the smartest design work I've ever seen - I hate these amps for that very reason - you have to take the whole damned thing apart to get at the solder side of the boards so good luck - I do - however - doubt it's anything to do with grid stoppers as it's usually something to do with all that switching...

[jerryjg]

Hey Pine - welcome to TFF - if the Lone Star is anything like the DC-3 we've had in re construction and design (v. likely) then you've got one hell of a lot of switching going on - most probs of this nature are down to the radial electrolytics in the power supply for the switching matrix - they're only rated at 85° and with 10 bottles glowing just underneath 'em it ain't the smartest design work I've ever seen - I hate these amps for that very reason - you have to take the whole damned thing apart to get at the solder side of the boards so good luck - I do - however - doubt it's anything to do with grid stoppers as it's usually something to do with all that switching...

Hi Yankee. Does that mean what i think it means? Meaning to get at the solder side of the board ,you must desolder all of the main connections to the baord, fix the problem, and then resolder them all again?
Cause thats the way it is on my old Acoustic Control 165 amp Boogie clone , and I would never consider keeping it- too hard to work on!