Is there an easy way to test Screen Grid resistors?

[pine]

I have an LSC 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.

 

[ja22y]

that would be the last place I'd check, those resistors don't really drift that much. Can you define 'underpowered'? I assume you have checked fx send pot in the back, speaker impedance plugs, etc ... Even in 100w with the master/solo pots max it's still 'underpowered'? How old are the pwr tubes? For my uses those Mesa (Russian) tubes lasted only 2 years and started to degrade after a year or so. I'd try new pwr tubes first if they are close to 1 1/2 years.

 

[pine]

that would be the last place I'd check, those resistors don't really drift that much. Can you define 'underpowered'? I assume you have checked fx send pot in the back, speaker impedance plugs, etc ... Even in 100w with the master/solo pots max it's still 'underpowered'? How old are the pwr tubes? For my uses those Mesa (Russian) tubes lasted only 2 years and started to degrade after a year or so. I'd try new pwr tubes first if they are close to 1 1/2 years.

I have it at 100 watts, 6L6s, output at 3:00 and Master (clean) at noon, effects send at 2:00, and it's not really that loud, at least, not what I'd expect from a Mesa. The gain channel is considerably louder; with the output at 3:00 as before, I am at 50 watts and the master at 9:00 and it's as loud as the clean side.

 

[ja22y]

couple things to consider:
1. issue in the channel: the gain and master pots are reversed (stock) for ch 1 and 2 so for the same gain setting, ch1 will produce the same volume as ch 2 but ch 1 master needs to at about 2 to 3 notch higher. You can try to max the gain and master for both ch and see if the levels are the same.
2. If 50w sounds louder than 100w then may be the 2 outer pwr tubes are not up to spec. Try swapping the outer/inner pair. The 100w now should be louder.
Those are the easy things to try. BTW for 6l6 pin 4 is the grid. I don't have the schematic so I don't know what value the grid resistor should be and if anything else is connected to the grid. But whatever the value is it should be consistent for all 4 tubes give or take 10% for tolerance.

 

[nemesys]

This sounds normal for a couple of reasons.

50 watt mode can easily sound louder than 100 watts, depending on other settings. First, 100 watts of output power is only 3db louder than 50 watts. I'm assuming here that your second channel is set with some dirt rather than clean, and that your first channel is clean. An overdriven preamp with two extra gain stages can easily generate more than 3db of gain. A good test would be whether the volume increases in the same channel when you go from 100 to 50 watts. The relative position of the channel masters to eachother isn't necessarily relevant -- the pots have different tapers, as ja22y said.

The LSC is a very warm-sounding amp, so the apparent volume can be tricky. I have an F-50 as well, which is much brighter, and as such sounds quite loud with the master at 8:00. I thought my LSC was pretty quiet when I first got it, because it sounded about as loud with the master closer to 10:00. Then, I ran them together, and discovered that I needed to turn the F-50's master up to 11:00 to make it as loud as the LSC. It was a bit of a revelation -- the F-50 with the master at 9:00 drowns out my whole band; another guitarist, bass, drums, 2400-watt PA, everything. I thought the amps weren't in phase at first, and I switched the LSC's loop on and off to make sure.