More about Rivera amps: the ‘Ninja Boost’ MOD

Those of you who are regular readers of our blogs will know that we have visited the subject of providing quality service to Rivera amplifiers (USA) in a couple of previous blogs. This time, we are not discussing a fault as such, but addressing a design limitation with a (relatively) simple MOD.

As we have discussed previously, the classic Rivera amp models have two discrete preamp channels, one with ‘British’ voicing, and one with ‘California’ voicing, plus master reverb, FX loop and prescence controls. Both channels have gain boosts, activated from ‘pull-on’ functions on the front panel controls. On the USA (green LED) channel this is known as ‘Ninja Boost’.

The Ninja Boost feature when activated provides a substantial gain boost. In a live performance situation, activated from the footswitch unit, this gain boost is actually unmanageable, which is the whole point of this blog. We were recently approached by a local Central Coast (NSW) rock guitarist, Mr Greg Taylor, to provide a MOD so that the level of boost can be preset to a more appropriate level from a suitably located additional control.

The gain boost in the USA channel takes place between the 2nd & 3rd gain stages, where an interstage attenuator is partially bypassed by a fixed resistor when the boost is activated. Each gain stage uses one half of a 12AX7 twin-triode valve (tube). This attenuator and associated switching circuitry is located on the main board.

Inspection of the chassis and board revealed that we could snip one end of the abovementioned fixed resistor, close to the input grid of the 12AX7 gain stage. Having done this, we could run a pair of shielded cables to a pot located in a spare position on the rear panel. The pot is wired up as a variable resistance in series with the existing fixed resistor, meaning that only 2 of the 3 lugs are connected. Thus we control the amount of boost by varying the insertion loss of the attenuator circuit. Because this is a series attenautor, as opposed to a volume control, a pot with a linear or reverse-logarithmic taper is more suitable than a logarithmic taper, giving a smoother sweep.

We use a high quality shielded cable (eg, Canare), rather than ordinary hook-up wire, so as not to introduce any additional hum & noise when the boost is activated. The shields of both cables are earthed (grounded) at one end only, in this case to the body of the pot, to avoid creating an earth-loop (ground-loop). Heatshrink is used at both ends to prevent accidental short circuits, and also to create a stable connection to the resistor that we snipped at one end – the other end of this resistor is still connected to the board. The whole job is neatly routed into the existing wiring and held in place with nylon cable ties. Although we have performed an intrusive modification, the amp has not been degraded in any way, from the perspective of reliability or additional noise.

So how did the MOD perform in the final play test ?? The amount of boost is now variable from a just perceptable boost at minimum (anti-clockwise) setting, to the original level of Ninja Boost at the maximum (fully clockwise) setting, all in a smooth, linear sweep preset from the rear panel. Thanks to Greg Taylor for his continued custom and for the many challenges he has provided us over the years ! Coming soon we have a blog about the legendary Wasp amplifiers, modding current Fender amps for blues-harp players, and mods to the Fender Blues Junior amps.

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9 Responses to “More about Rivera amps: the ‘Ninja Boost’ MOD”

  1. Marco Sabattini Says:

    Quite interesting, I’ve studied my M60 schematic and I’ve found that 220k bypass resistor you’re talking about. If not a top secret matter, is it a 1M pot that you’ve used for this mod, right? Thanks in advance 😉

    • ivanrichards Says:

      Yes, a 1 meg pot, linear or reverse log taper works best. Please refer all hands-on servicing of high-voltage electronic equipment to suitably qualified personnel. regards, Ivan R.

  2. Marco Sabattini Says:

    Big thanks Ivan, it works for me like a charm! I’ve followed your indications by letter and now I can play on CH2 with master at 10 engaging the ninja boost at a decent volume when required. By the way, I am an amp tech since 1992, but I hadn’t thought to modify my little personal beast until now. I’ve also found out an interesting mod for brighten up the slavemaster channel on this amp class ( too muddy for me ), consisting of an IC and a cap replacements,
    you can mail if you like. Cheers, Marco.

    • samsavvas Says:

      Hi Ivan and Marco,
      Ivan – thanks for outlining the boost volume mod. Will have my techo install it for me.

      Marco – I’d very much like to see your ‘slavemaster’ channel mod and read a bit more about it. Have you documented it somewhere or can you provide more details? I have an R-55.

      thanks,
      Savvas

      • Marco Sabattini Says:

        Hi guys, maybe it’s not a big mod for slavemaster “channel”, however it just open things when you change c403 from 120pf to 47p (see Rivera M Series schematic here http://www.prowessamplifiers.com/schematics/misc/Rivera_Knucklehead.html ), it’s the cap between negative input and output of the first opamp in this circuit. Also replacing TL072 chip with an RC4559 may help to get some brighteness from this “muddy” (at least for me) onboard overdrive. I’ve noticed that my M60 had 120pF, maybe some later revisions have a 47pF already. The only defect I’ve noticed it’s just a little noise from the new opamp..at loud volumes.

        Boost on the gain channel it’s difficult to achieve (it’s a double face pcb circuit, damn..) however it can be pleasant to add a bright switch on CH1 Volume. Just sold a wire from the lug of C104 connected to the 1M volume pot (take care of shielding) and install a switch on the back to open/close a contact for a silver mica 500pF cap soldered to the center tap of the volume pot (I’ve done it soldering it to a lug of PH101, follow the schematic above). Essentially you get the same function you have on CH2 when you pull out the treble pot. Have fun and take care of the same recommendations on voltages, shielding, etc..or just refer to an expert tech ;D

  3. Gwen Says:

    Hi ! Thank’s Ivan , for this mod ! I would be interested by the “slavemaster” mod , Marco . 😉

    Regards .
    Gwen

  4. rockon1@optonline.net Says:

    How about a boost mod for the gain channel? Most of my playing is done there and it would be extremely helpful. TIA Bob

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