Hello all,
I have a Digitech gsp 2101 tube pre amp that runs into a Marshall 9100 dual monoblock tube amp. My issue is that channel A does not have as much output as channel B.
I have owned both of these since new in the early 90's and love them together. I assumed that because of their age the issue was probably the tubes, but I did a swap to test first.
I swapped the output tubes (5881's) between the 2 monoblocks and channel A still had lower output, then I swapped the 81's, same result, then the other 2 tubes, same result.
I turned my attention to the pre amp and swapped the 12ax7's, same result. So now I am befuddled.
Does anyone have any ideas? Thanks in advance for any contributions!
I have a Digitech gsp 2101 tube pre amp that runs into a Marshall 9100 dual monoblock tube amp. My issue is that channel A does not have as much output as channel B.
I have owned both of these since new in the early 90's and love them together. I assumed that because of their age the issue was probably the tubes, but I did a swap to test first.
I swapped the output tubes (5881's) between the 2 monoblocks and channel A still had lower output, then I swapped the 81's, same result, then the other 2 tubes, same result.
I turned my attention to the pre amp and swapped the 12ax7's, same result. So now I am befuddled.
Does anyone have any ideas? Thanks in advance for any contributions!
Nigel
Thanks for the reply. Im afraid, however you just spoke a little greek to me, but I will do a little googling and see if I can figure that out. I say this because I am not a tech, just your average hobby player. I am pretty mechanically inclined though so with a little research and guidance I can probably get the troubleshooting part down. Unless I need something other than a standard volt/ohm meter to measure those items you mentioned.
Thanks for the reply. Im afraid, however you just spoke a little greek to me, but I will do a little googling and see if I can figure that out. I say this because I am not a tech, just your average hobby player. I am pretty mechanically inclined though so with a little research and guidance I can probably get the troubleshooting part down. Unless I need something other than a standard volt/ohm meter to measure those items you mentioned.
Most tubes have an Anode and a Cathode ...... The Anode is where the Positive voltage is applied and the Cathode is where the Negitive voltage is applied ......
The anode and cathode have resistors that are used to set the Bias of the Tube , the Cathode resistor isn"t usually a problem in Preamp tubes as there is generally only a few volts across this resistor but the Anode resistor (usually called the Plate resistor) generally has a couple hundreds of volts across it so it can get quite hot and over time the high temp causes the resistors value change which causes the bias to change and a misbiased tube will have lower signal output .......
I"m sure my explanation isn"t 100% correct but it is how I understand it .....
Cheers
The anode and cathode have resistors that are used to set the Bias of the Tube , the Cathode resistor isn"t usually a problem in Preamp tubes as there is generally only a few volts across this resistor but the Anode resistor (usually called the Plate resistor) generally has a couple hundreds of volts across it so it can get quite hot and over time the high temp causes the resistors value change which causes the bias to change and a misbiased tube will have lower signal output .......
I"m sure my explanation isn"t 100% correct but it is how I understand it .....
Cheers
Ok...that sounds logical. I assume my next step is to find out where in my amp that resistor is located. Then test it with a meter. If anyone happens to know where it is on this amp that would be a great help, but I will start searching for youtube vids, or schematics or something in the meantime.
They connect to pins 1 and 6 of the normal ECCxx type valves used, it's best to measure the voltage on those pins when it's running, rather than try and measure the resistances.
It only takes a few seconds to check them with a volt-meter - but you need to be aware of the high voltages in the unit.
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