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Peter Maziar (mosfet)
Username: mosfet

Registered: 11-2006
Posted on Saturday, January 26, 2008 - 07:51 pm:   

Posted on Saturday, January 26, 2008 - 07:40 pm:

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Hey all! I have come up with a great mod for the HD 150 solid state distortion stage. I call it 'THE LED DRIVE'.I did it to channel 2. I used two different LEDs instead of the 1N914 diodes in the clipping stage. By hooking up a breadboard, I tested all kinds of LEDs. Small ones sounded harsh, small green ones being the harshest. The best sounding pair were both 5mm, one was red with a higher breakdown voltage, one was an old infra red one (from an old scrap remote control) at arond.95 Vbd. Due to the different breakdown voltages, clipping is assymetrical (cool). I replaced the 22K resistors R28 and R30 with a dual-ganged 80K pot to control the gain of both ZTK109 transistors driving the LEDs, thus the term 'LED DRIVE'. I mounted the LED DRIVE pot in Channel 1 input 2 - unscrewed the nut to the jack, pulled it through into the amp chassis, replace the nut and washer, and wrapped it in a small plastic baggie and an elastic band. Didn't even need to desolder it. Mounted the pot in the hole. Scoped results, 1KHz sine wave input: using the LED DRIVE, with the LED DRIVE at 0, the result is a pure square wave (with pre volume at max), just like stock. Turn the dial to the right, and distorted signal becomes progressively rounded, introducing even harmonics from the LEDs, and notching the signal (as observed on my scope). On ten, harmonics are at the maximum. Works great, with a lot of character. Everything from pure clean, to brown, to maximum harmonic gain available now. notes: Best used with tone controls on my guitars rolled down about 75% to filter out the solid state edge. I also like having the phaser set to manual (speed knob pulled out), depth at 2, and speed at 4. THE LED DRIVE ROCKS! AN EASY MUST-DO MOD! PLEASE TRY IT AND LET ME KNOW HOW YOU LIKE IT! A DATS ALL FOLKS!
Ed Goforth (ed_goforth)
Username: ed_goforth

Registered: 06-2006
Posted on Tuesday, January 29, 2008 - 07:10 pm:   

Hi Peter, Do you have a sound clip of the mod? I thought about trying something like what you came up with, sounds great! A dual pot, one for each LED in place of the 22k? Thanks for sharing.
Peter Maziar (mosfet)
Username: mosfet

Registered: 11-2006
Posted on Wednesday, January 30, 2008 - 01:06 pm:   

No sound clips, sorry. It does sound way better though. Hopefully the transistors will be able to handle the extra gain. Wish me luck, Ed.
Peter Maziar (mosfet)
Username: mosfet

Registered: 11-2006
Posted on Monday, March 01, 2010 - 03:44 pm:   

Update: 4 years later, and the LED drive is still working. It goes from stock sounding to nicely browned. IMHO, a very good mod.
Rick Viola (goodrat)
Username: goodrat

Registered: 05-2011
Posted on Tuesday, May 03, 2011 - 08:06 pm:   

In my projects, I use 3mm red. Large LEDs have more capacitance and will cut highs too much.