High/Low Power Switch Problem JCM900 MKIII

[SixStringSting628]

Hello everyone. I have a problem with my High/Low power switch on my 1990 JCM900 MKIII 2100. I decided to try out the Low power (50w) mode the other day and noticed that it seemed louder than the 100watt mode. I decided to measure the volume with a decibel meter. The meter I used was able to record the average and peak dB over any period of time, so on both power switch settings and same amp settings, I played the same thing and recorded the data. With the current volume setting I had, this is what I measured: the Low power mode (50w) had an average of 87.1 dB and a max of 91.3 dB. The High power mode (100w) had an average of 84.6 dB and a max of 88.5 dB. I’m aware that half power should be around 3 dB quieter due to triode operation. I am getting the opposite volumes here. The power switch is installed onto the circuit board so I don’t think that it could be installed upside down or wired wrong. The amp is stock, besides replacing the original filter caps 3 months ago. Power tubes are EL34s and aren’t very old (to my knowledge). Anyone have any ideas as to why this is?

UPDATE: Pulled the chassis out and found that the circuit board on the High/Low power switch had been drilled out and kanoodled with, so perhaps a mod was done to make the 50w mode better? Can’t figure out how to upload pics right now, keeps saying file too large

 

[South Park]

If the half power is on the preamp Chanel you could be getting some bleed over on the circuit board . One of the preamp tubes might be putting out to much power try moving them around . I don’t think you can do much

 

[Old Punker]

3 dB is a pretty narrow band for evaluation. I am no expert but I believe you would need to have the Exact same input on both power modes to do a valid comparison, so you can achieve this by using a looper for example. If you don't use the exact same (pre-recorded in some way) input then you are relying on yourself to produce the exact same output from your guitar for both tests, which is very difficult to achieve. Also, I think you may notice more of a difference between the two power modes at higher MV settings.

 

[mickeydg5]

The High/Low toggle switches the screens from pentode to triode connections.
It is possible that the solder connections or the internal contacts are faulty.

 

[Chris-in-LA]

Hello everyone. I have a problem with my High/Low power switch on my 1990 JCM900 MKIII 2100. I decided to try out the Low power (50w) mode the other day and noticed that it seemed louder than the 100watt mode. I decided to measure the volume with a decibel meter. The meter I used was able to record the average and peak dB over any period of time, so on both power switch settings and same amp settings, I played the same thing and recorded the data. With the current volume setting I had, this is what I measured: the Low power mode (50w) had an average of 87.1 dB and a max of 91.3 dB. The High power mode (100w) had an average of 84.6 dB and a max of 88.5 dB. I’m aware that half power should be around 3 dB quieter due to triode operation. I am getting the opposite volumes here. The power switch is installed onto the circuit board so I don’t think that it could be installed upside down or wired wrong. The amp is stock, besides replacing the original filter caps 3 months ago. Power tubes are EL34s and aren’t very old (to my knowledge). Anyone have any ideas as to why this is?

UPDATE: Pulled the chassis out and found that the circuit board on the High/Low power switch had been drilled out and kanoodled with, so perhaps a mod was done to make the 50w mode better? Can’t figure out how to upload pics right now, keeps saying file too large

One way to make a file smaller with iOS is to email the picture to yourself. That usually cuts their size dramatically. I usually copy the pic from the email and paste it into thread, that avoids using the Upload a File button.

 

[SixStringSting628]

One way to make a file smaller with iOS is to email the picture to yourself. That usually cuts their size dramatically. I usually copy the pic from the email and paste it into thread, that avoids using the Upload a File button.

Oh okay thanks! I’ll try that in a bit!

 

[Gunner64]

Hello everyone. I have a problem with my High/Low power switch on my 1990 JCM900 MKIII 2100. I decided to try out the Low power (50w) mode the other day and noticed that it seemed louder than the 100watt mode. I decided to measure the volume with a decibel meter. The meter I used was able to record the average and peak dB over any period of time, so on both power switch settings and same amp settings, I played the same thing and recorded the data. With the current volume setting I had, this is what I measured: the Low power mode (50w) had an average of 87.1 dB and a max of 91.3 dB. The High power mode (100w) had an average of 84.6 dB and a max of 88.5 dB. I’m aware that half power should be around 3 dB quieter due to triode operation. I am getting the opposite volumes here. The power switch is installed onto the circuit board so I don’t think that it could be installed upside down or wired wrong. The amp is stock, besides replacing the original filter caps 3 months ago. Power tubes are EL34s and aren’t very old (to my knowledge). Anyone have any ideas as to why this is?

UPDATE: Pulled the chassis out and found that the circuit board on the High/Low power switch had been drilled out and kanoodled with, so perhaps a mod was done to make the 50w mode better? Can’t figure out how to upload pics right now, keeps saying file too large

The switch probably was replaced at one time. It probably shorted , overheated and damaged the pads or traces on the pcb. Or someone replaced the switch and damaged them when soldering/desoldering. That's probably the reason the pcb is drilled out.

Maybe the switch is hardwired in pentode mode only. And perhaps when they hardwired the switch because of the pad damage they wired it backwards.

I had a 2100 SL-X that the switch went bad on and the amp started to hum in triode mode, pentode mode it was fine but as soon as I switched to low power it hummed, rather loudly at that.

I replaced the switch. I had no pad damage but I can tell you the switch has rather large lugs, and takes some heat to desolder. If ones not careful getting the old switch out the pads could lift pretty easy.

 

[SixStringSting628]

The switch probably was replaced at one time. It probably shorted , overheated and damaged the pads or traces on the pcb. Or someone replaced the switch and damaged them when soldering/desoldering. That's probably the reason the pcb is drilled out.

Maybe the switch is hardwired in pentode mode only. And perhaps when they hardwired the switch because of the pad damage they wired it backwards.

I had a 2100 SL-X that the switch went bad on and the amp started to hum in triode mode, pentode mode it was fine but as soon as I switched to low power it hummed, rather loudly at that.

I replaced the switch. I had no pad damage but I can tell you the switch has rather large lugs, and takes some heat to desolder. If ones not careful getting the old switch out the pads could lift pretty easy.

I pulled the PCB yesterday and found out more. The chassis is charred below the board. Also, the white wire connected to the long prong on the switch is soldered to pin 3 on power tube number 3 (when looking at the back of the amp). And the red wire connected to the short prong on the switch is connected to pin 3 on power tube number 2. Also, the track (trace?) on the board is drilled out where the 2 middle prongs on the switch are supposed to connect to.

 

[SixStringSting628]

The switch probably was replaced at one time. It probably shorted , overheated and damaged the pads or traces on the pcb. Or someone replaced the switch and damaged them when soldering/desoldering. That's probably the reason the pcb is drilled out.

Maybe the switch is hardwired in pentode mode only. And perhaps when they hardwired the switch because of the pad damage they wired it backwards.

I had a 2100 SL-X that the switch went bad on and the amp started to hum in triode mode, pentode mode it was fine but as soon as I switched to low power it hummed, rather loudly at that.

I replaced the switch. I had no pad damage but I can tell you the switch has rather large lugs, and takes some heat to desolder. If ones not careful getting the old switch out the pads could lift pretty easy.

 

[SixStringSting628]

The switch probably was replaced at one time. It probably shorted , overheated and damaged the pads or traces on the pcb. Or someone replaced the switch and damaged them when soldering/desoldering. That's probably the reason the pcb is drilled out.

Maybe the switch is hardwired in pentode mode only. And perhaps when they hardwired the switch because of the pad damage they wired it backwards.

I had a 2100 SL-X that the switch went bad on and the amp started to hum in triode mode, pentode mode it was fine but as soon as I switched to low power it hummed, rather loudly at that.

I replaced the switch. I had no pad damage but I can tell you the switch has rather large lugs, and takes some heat to desolder. If ones not careful getting the old switch out the pads could lift pretty easy.

 

[SixStringSting628]

Here are some pics

 

[Gutch220]

wow. that's terrible