Festival Series Roadmaster - Distortion mod.

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digi2t
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Joined: Tue Sep 07, 2021 2:03 pm

Festival Series Roadmaster - Distortion mod.

Post by digi2t » Tue Sep 07, 2021 3:12 pm

It's been one of my bucket list amps for a long time. Finally managed to get my hands on one, complete with original 4x12 cab a few months ago. It was everything that I expected... except for the distortion mode. It had this gating decay, where the distortion would just cut out, leaving you with just clean. The gate was early enough in the decay that even at very low volume levels, you could easily pick it up. Also, the distortion itself was nothing to write home about.

So, staring at the schematic for a few days, it finally dawned on me that there is a Bazz Fuss circuit sitting smack dab between the Bright and Normal channels. Audio probing backwards from the Normal channel validated that this section was the cause of the distortion, as well as the decay gating. If you look at the preamp schematic, it's a lone transistor, with a diode between the B and C legs, 100K collector resistor, and two 0.1uF caps. What's NOT shown on the schemo is a 470pF cap parallel to the diode (I've drawn it in below). The cap is there from the factory on my unit. An initial test to see if maybe a high value resistor (1M) across the diode would help, did improve the decay. The gate was still there, but my ears could still pick it up near the very end.

After doing some research on the Bazz Fuss, and it's many incarnations, I dove in and decided to try some mods on this section, in an effort to at least improve the decay. First, I decided to ditch the BJT and try a Darlington version of the circuit. I started with an MPSA13, 10K collector resistor, FD100 diode, replaced the 470pF ceramic with a silver mica, and the two 0.1uF's with metalized polys.

Huge difference.

Decay was much improved, but so was the distortion. Lots more bark and bite, but... I was now overloading the Normal channel when using the Bright in front. The worst spot was with the Normal channel at around 7, as soon as the Bright went past 4, HUMMMMMM. I decided to tweak the collector resistor. With a 10K pot on the collector, bringing the resistance down to 1K did the trick. I still had much better distortion, with very good decay, and I could go anywhere I wanted with the Bright and Normal channels.

Finally, I decided to try another combo; 2N5306 (gain a little over 40000 on my Peak Atlas), and a 1N5061 diode (0.650 vF). I stuck with the 1K resistor on the collector. The distortion jumped up another notch quality-wise. Remembering the 1M resistor test from my first test, I then wired in a 2M pot parallel to the diode. Dialing down slowly, around 1.5M resistance, the decay now started to bloom into beautiful harmonic overtones. I soldered in a 1.5M resistor to the bottom side of the board.

The distortion, now, is not to be trifled with. It puts a big smile on my face. Just guitar, cable, amp. Happiness.

Here is the section of the schematic showing the distortion section, board placement, original schematic, and the mods;

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