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Changing Gain to Boost

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  • #1

    Changing Gain to Boost

    I have a Randall RG50TC tube combo. It has two (switchable) gain controls on the overdrive channel. I find this useless - would like one to become a boost (or solo) control. FWIW, a boost pedal does not work - apparently V1 gets slammed as it is, any increase in signal just gets compressed with no noticable change in volume.

    Looking at the attached schematic, you can see both gain circuits are identical. Any ideas on how to modify the one for more boost? Or maybe replacing V1 with a lower gain tube, and in conjunction with modding one of the gain controls, get more headroom. The first half of V1 (not shown) is at the input so it would change the characteristic of the whole amp - normal and overdrive. Not that this isn't a welcomed option. Or I also thought about stealing the boost circuit from the Normal channel, and route it behind V1. Any thoughts?
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  • #2
    It's difficult to say anything wihtout seeing the complete schematic. But generally, if you want to change the volume only, it needs to be done as late in the circuit as possible. I would look at doing something around the master volume control, e.g. having two separate master volumes. Checkout the Soldano SLO100 for how to switch between two master volumes.

    The boost control for the normal channel uses a JFET. JFETs can typically handle a maximum of about 40V before breaking. The normal channel signal level should be designed around this. For the OD channel, it is likely that the signal coming out of V1b is too high and would kill the JFET.

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    • #3
      I don't know if that amp has an effects loop, but if it does it's probably located late in the preamp circuit or just after the preamp. You could probably use a boost pedal or boosted EQ there for a lead/volume boost and avoid reworking the circuit.

      Chuck
      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "If you're not interested in opinions and the experience of others, why even start a thread?
      You can't just expect consent." Helmholtz

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      • #4
        Originally posted by Chuck H View Post
        I don't know if that amp has an effects loop, but if it does it's probably located late in the preamp circuit or just after the preamp. You could probably use a boost pedal or boosted EQ there for a lead/volume boost and avoid reworking the circuit.

        Chuck
        That's what I decided was best for now. I did jumper R25 and got a little more bite (treble boost), but not nearly enough to be useful as a boost.

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