Thunderbird

Chuck D. Bones

Circuit Wizard
This is a Marshall Superlead emulator designed by the geniuses at RunoffGrove. I bought the board from 1776. Maybe PedalPCB will make one of these too!

I have been battling this one on & off for about a year. Had a noise problem I just couldn't fix. I went so far as to socket the first two stages to see if it was the opamp, it wasn't. Found and replaced a couple of leaky tantalum caps. Turned out I had a bad batch of 10uF 16V. Replaced those, still had the noise. Disabled the charge pump, didn't help. I took another bash at it on Saturday and finally found the problem. One of the 470nF film caps was noisy (the one in the lower left corner, next to the diodes). Replaced that and viola! This pic was shot before I installed the IC socket or replaced the noisy cap. I tried TL072, LM833 & NE5532. They all sounded pretty much the same. I ended up with TL072 because I have the most of those. The rest of the opamps are LM833 in accordance with the BOM.

Instead of the usual FETs, this circuit uses resistors, caps & diodes to model the overdrive characteristics of the various gain stages in a Marshall. At first I was skeptical, but those guys at ROG really know their shit.
The GAIN control sets the front end gain and governs the overall distortion. The TONE control sweeps between the tones of the Marshall's Normal and Bright channels. Basically sounds the same as running thru both amp channels and adjusting the blend with their respective volume controls. The normal Marshall tone stack is there, but it's hardwired. TOP & BOTTOM are treble and bass boost at the end of the chain. Like a real Marshall, the PRESENCE control is fairly subtle and affects the very top end. LEVEL is a master volume at the output. This pedal will play clean, it will roar, and everything in between.

The observant eye will notice that I left out one transistor and resistor. Those are req'd for battery operation only.

Highly recommended.

Thunderbird innards 02.jpg Thunderbird front panel 02.jpg
 
Top notch inside and out and great sleuthing!

The knock on this design was always the noise.
 
I wouldn't looking at it and zooming in as I always do to get a good look, it must be very slight

It looks as straight as you can get it to me

I think a slight imperfection shows the work that went into it and it's kinda like a makers mark

Those guts are pristine as well defo leave it, it's only you that'll notice it

20200908_014604.jpg
Don't even think about it!
 
Last edited:
This was by far the hardest troubleshooting I've had to do on a pedal. I poked around with a scope and headphones, measured DC voltages, removed parts, replaced parts, and swore. Heavy on the swearing. I'd get frustrated and just walk away for months at a time. Every so often, when I was in the right frame of mind, I'd try again. Eventually, persistence paid off. Felt good when it was finally fixed, but it was a hard road getting there.
 
Most excellent and well done on finding that bugger or a cap! I have one of those in the works as well, will see how long it takes me to get it going...
 
Sweet looking build, and congratulations on the epic snipe hunt finally coming to a close.

At first glance, I thought the gain control said "grin." I suppose the two are synonymous in many cases.
 
Back
Top