With a low-power tube combo amp, usually I am only interested in the power-amp section, which is bounded by two, usually hypothetical, jacks: the input of this black box is the Effects Return jack, and the output of this black box is the Speaker Out jack. This turns the combo amp into a power-tube saturation processor module, as follows:
Bypassing the combo's inadequate preamp - Even with a clean-channel-only preamp with few gain stages, like the Pro Jr., the presence of preamp tubes presents the risk of unwanted preamp clipping, self-noise, and EQ coloration. You want to inject the signal from a separate, good preamp chain directly into the combo's tube power-amp section. Some small combo amps have an effects loop - such as the VC508. This is easiest - simply ignore the preamp and "guitar input" jack, and connect your preamp's output to the amp's Effects Return jack; in the VC508 you can simply plug a 1/4" plug halfway into the jack. If there is no effects loop, the universal standard way to do this is to patch in through the center pin of the Volume pot -- this pin goes directly to the beginning of the actual power amp. You might need to use a coupling capacitor or transformer, but I have managed to simply hardwire a connection successfully. Torres has some kits for installing an effects loop.
Bypassing the combo's inadequate speaker and cabinet - Ideally, the combo has an External Speaker jack and a way of disconnecting the internal speaker, so you can drive a serious cabinet instead of the combo's speaker. Usually, though, the combo's speaker is hardwired, and there is no Ext Spk jack, so you have to install some sort of jack, or plug-and-jack, connector. This way, the combo's speaker is disconnected, and you can send the power amp's output into a serious cabinet or a load and cabinet simulation filter.
These two techniques isolate the tube power amp; combined with a miked cabinet or a load-and-filter, this turns your combo into a power-tube saturation processor, an amp in a box. I like having a Send cable and a Return cable heading off into the other room, turning the low-wattage tube power-amp and guitar speaker into a silent, abstract "black box" processor, isolated from any preamp concerns, and ready for post-amp processing.
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