Expanding the Gallien-Krueger 800RB

this bute came in with “scratchy” volume, MV and tone controls … measurements revealed output DC offsets between 30mV and 137mV on most of the LF353 op-amps … replacing these by 8-pin sockets and inserting an OPA2134 in the front and TI-NE5532’s all the way to the back brought the preamp’s noise floor down quite dramatically; and offsets to negligible levels …

the next round of mods involved cutting the lines leading to each input of the 300w and 100w power amps … a 1/4″ shorting jack is used to open the 300w line as it is perfect for slaving other preamps, like my 360+ preamp (Lite version shown) // when there is nothing plugged into the jack the switching function ties the input of the 300w amplifier to the 300w MV pot (stock mode) … the switch next to the slave input jack either ties the 100w amplifier input to what the 300w input sees, or to the 100w MV pot (stock mode) …

when an external preamp is plugged into the slave input the 300w MV control has no longer any effect, nor does the jFET boost stage (hence the reason for doing all this) as it is avoiding all that … the 100w amplifier can now be used to feed another speaker // in one case feeding the same signal that is sent to the input of the 300w amp, and in the other from the GK preamp … so, with this head alone, you could conceivably have one player pumping bass through a preamp into the 300w side and another instrument plugged into the front of the GK and driving its own speaker with the 100w amplifier … independently …

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