The GP12XV is from the Series Six era, before the SMX Series, and before Trace Elliot was sold to Gibson. The GP12XV is the preamp stage of the Hexa, Quatra, and Twin valve amps. It has front and rear panel DI outs, FX loop, rear panel variable crossover, preamp link, and traffic light input gain lamps for a visual representation of your input level. Next to my DIY preamp, it is my favorite preamp for bass.
The “Made in UK” preamps are hard to come by in North America. Excited about finding a pre-Gibson Trace Elliot, I purchased the preamp from a fellow on www.talkbass.com. I later discovered that this particular preamp, serial 71, was being bought and sold on a regular basis. Everyone who had bought it complained about a weak signal. I found that the signal was too weak to drive my QSC 1850HD. I also found that preamp was noisy and it looked like the power supply was a bit fried. I the replaced the faulty power supply components. I found two sources of noise. A constant hum from a bad tube and the other was a wire on the input pot that barely made connection to the PCB. The tube was replaced with a JJ Tubes ECC83S. The loose wire caused intermittent noise and a huge reduction in volume. With the preamp now working as intended, I still found that the the preamp did not supply a hot enough signal to my power amp. After doing some research I found that the preamp was intended to be mated with a power amp that had a much lower input sensitivity. To get a hotter signal to my power amp I needed to modify the preamp circuit for more gain.
I’ve looked everywhere for the GP12XV schematic and the best match that I can find is the GP12 X Series 6 Preamp. All the values appear to be correct except the value of R78, which is 100 Ohm on my board and there is no tube circuit.
Modifying The GP12XV Preamp for a Hotter Output:
To increase the output, I increased the value of R109. This increases the gain of the last opamp stage. Don’t forget to adjust the value of C78 accordingly or you will inadvertently change the cuttoff frequency of the low pass filter.
|Gain increased by 3||R109||C78||High Freq. Limit|
The maximum output should now be around 22dBu before distorting the preamp’s output.