Peavey Rage 158 modification (with Boost!)

The Peavey rage 158 "transtube"-series amp is a nice little guitar combo with an 8" blue marvel speaker and 15Watts of output power. It's based on the TDA2040 power amp chip and it sounds really good. There's also a "modern/vintage" mid-shift EQ button as well. This little amp utilizes no opamps for the preamp section which is a rare thing. The preamp section is designed exactly like a tube preamp. Each "gain stage" of its preamp is an equivalent of one half or one triode section of a 12AX7 tube. Two transistors are essentially replacing one triode of a preamp tube. Therefore, the laws of tube amp modification apply here as well!
Here's a link for the rage/blazer 158 schematic (the Blazer model has a built in reverb effect):
https://peavey.com/forum/viewtopic.php?t=35705

Boost Mod:
I wanted to add a bit of boost to this little amp as Peavey is well known for its high gain and crunch tones.The boost has to be switchable as the amp produces a nice variety of clean tones as well. The boost works for the clean and the OD channel. Beware that this can add many deciBels to the clean signal so make sure to turn down the volume of your amp before engaging the "boost" button. All you have to do is locate the R5 resistor (maybe it's marked R18 on the Blazer model or older rage models) on the PCB of the amp. The "boost" control bypasses this resistor with a 2.2uF/50V small electrolytic capacitor to ground. It's like bypassing the cathode of a 12ax7 tube which adds gain to this stage. By placing a switch between the positive pin of the 2.2uF capacitor and the R5 you can turn the boost on/off. You could even connect a 25kOhms trimmer/pot in series with the boost switch to control how much boost you want to apply to the signal of your guitar. The amp sounds like a big tube amp that's overdriven hard when the boost switch is On. The valve-like tones are really good and there's plenty of gain for soloing and riffing with this little amp. You can even connect it to a bigger speaker cab (4 to 16 Ohms) and rock out (see the following Cab-out mod)!

Here's the original schematic and the boost circuit:

Pics:

Cabinet-output mod:

That's an easy one. You can add an external speaker/cab output to your amp simply by connecting the speaker wires of your power amp to a Plastic (NOT all speaker outputs are referenced to ground)  female Jack and mount it on the chassis. You could then (Option A) wire a male TRS jack to the internal speaker terminals if you like to use this small 8" speaker as a test cab for other small amps. There's option B where you could wire the internal speaker to the female jack if you don't plan on using it as a small cab. The amp sounds huge through a 2X12" Cab with Celestion V30s! I highly recommend using this small amp with a external cab!
Here are the schematics for these applications:

 Option A:

Option B:


Cheers,
Thanos