On eBay Here. Guild made 4 versions of the Thunderbass; 2-, 3-, and 4-tube preamp models as well as a 4-tube preamp model with a screen voltage regulator; The Quantum. Of these, most were twin 6L6 although some of the 4-tube preamp models and Quantums - like this one- were factory-fitted with 8417s ... a now-extinct tube.
This is what distinguishes the Quantum model; the 6GF7 screen voltage regulator. When the screen voltage gets above 300V, it shunts the excess voltage to ground; drops below and the tube conducts up to 300V stiffening the screen voltage, tightening the sound, and preventing sag. It gets the (edit: 8417s and) 6L6s to act a little like an EL34/6CA7 whose output transformers often have an extra (more costly) set of taps for the screen supply ... this is a value-adding feature at some cost savings:
In the upper left in the magenta circle, that's the 6GF7; it's what makes the amp. All the Thunderbass heads have bias test points; shown on the chassis upper right and on the schematic lower right to allow the bias to be tested without flipping the chassis. Further, there are individual bias pots accessible from the tube deck (lower left) to allow each tube to be independently biased ... allowing the use of mis-matched tubes and facilitating tone-tweaking.
The amp has separate guitar and bass channels each with their own tone controls. The amp has a Master Volume but it pre-dates the popular effects loop feature found on later amps. I bought a 4-tube preamp 8417 Thunderbass several years ago and converted it to 6L6s. This eBay model needs a new power cord and new caps - don't need to have it humming before it can benefit from fresh power. Changing it to 6L6 means changing a couple of power resistors on the DC rail and the bias resistor. Without changing the power transformer, the power supply won't put out the 460V found in the 6L6 versions ... but, besides good tone, that was one of the virtues of the 8417; you only needed 400V to get 50 watts. Of the head series folks, I think this is the one to have.
This is what distinguishes the Quantum model; the 6GF7 screen voltage regulator. When the screen voltage gets above 300V, it shunts the excess voltage to ground; drops below and the tube conducts up to 300V stiffening the screen voltage, tightening the sound, and preventing sag. It gets the (edit: 8417s and) 6L6s to act a little like an EL34/6CA7 whose output transformers often have an extra (more costly) set of taps for the screen supply ... this is a value-adding feature at some cost savings:
In the upper left in the magenta circle, that's the 6GF7; it's what makes the amp. All the Thunderbass heads have bias test points; shown on the chassis upper right and on the schematic lower right to allow the bias to be tested without flipping the chassis. Further, there are individual bias pots accessible from the tube deck (lower left) to allow each tube to be independently biased ... allowing the use of mis-matched tubes and facilitating tone-tweaking.
The amp has separate guitar and bass channels each with their own tone controls. The amp has a Master Volume but it pre-dates the popular effects loop feature found on later amps. I bought a 4-tube preamp 8417 Thunderbass several years ago and converted it to 6L6s. This eBay model needs a new power cord and new caps - don't need to have it humming before it can benefit from fresh power. Changing it to 6L6 means changing a couple of power resistors on the DC rail and the bias resistor. Without changing the power transformer, the power supply won't put out the 460V found in the 6L6 versions ... but, besides good tone, that was one of the virtues of the 8417; you only needed 400V to get 50 watts. Of the head series folks, I think this is the one to have.