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On the Breadboard - 1st Impressions

12/22/2000 2:47 PM
Doug H
On the Breadboard - 1st Impressions
I got this thing breadboarded last night, this is the "BK Butler" version with the 12V transformer:  
Overall I like it a lot! This is definitely -not- a high gain circuit. It sounds a lot like ZZ Top's first album - tight and warm. It's not buzzy at all, maybe Aron's pedal is the newer version. The cool part is it has a -lot- of harmonics and sustain with minimal distortion. It has a very "live" feel to it, every time I hold a note it just wants to keep going and decay into octaves, very harmonically rich. I love the asymmetry of it too, if you short the 1K resistor going from +supply to the opamp it just gets too sterile. It has a lot of headroom too, it cleans up real nice with your guitar volume knob. I measured + and - 15V on the rails going to the tube. Speaking of the tube, it works well with a 12AX7 or 12AU7. The 12AU7 is actually a little louder and has better low-freq response, but not as much highs. I ended up going with one of my trusty Telefunken 12AX7's and it's very sweet.  
On the down side, when the gain is maxed you get some trashy harmonics when you try to play complex chords, some sort of intermod problem there. I am working on that, I'm going to see if I can fix that.  
I reduced the .033u input cap to .01u and that helped. Perhaps reducing the .047u coupling cap to .022u would help as well. I will be experimenting with that kind of stuff.  
I used a TL072 because that's all I have right now. I set it up like the orig circuit with the buffer and inverting gain stage. The diodes going from the supply rails to input do, as Frank mentioned, some sort of limiting. It sounds like they keep it from hitting the rails with the opamp. I'm going to sim that again and see if I can figure out exactly what's going on. Strangely enough, that's adding to the intermod distortion somewhat too. With this TL072, I found pulling the diode going to the -15V rail (pin 4) really added a lot of harmonics and smoothed it out. It got into some high gain territory there and really sounded cool! It was hard to get out of that tone when I backed the gain down though, but it might be cool to stick a switch on there or something for that. Anyway, lots more playing and experimenting to do.  
Overall, a very nice little circuit!  
12/22/2000 3:02 PM
Eric H

Great post, Doug, thanks for the details --I'm working on a hi-voltage tube pre right now and am waiting for some parts, and time. Should have something to report a couple of days after Xmas.  
BTW I had the same idea (the connected junction boxes) only mine would have three, using the hollow threaded connectors to connect them --like a mini moon-base :)
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12/23/2000 11:58 PM
Doug H

"BTW I had the same idea (the connected junction boxes) only mine would have three, using the hollow threaded connectors to connect them --like a mini moon-base :)"
Great idea, Eric! I will try that instead. A lot easier than drilling more holes in this stuff.  
Let us know how your hi-voltage box works out. I'm curious about it.;)  
12/22/2000 9:41 PM
Frank Clarke

Hi Doug,  
In Circuitmaker, the diodes don't do anything. Since Since they do something in real life, I'm still intrigued.  
The 1k resistor to the + power supply would shift the bias on the signal going into the grid, maybe this is the "Bias" control on the rackmount version? Sounds like a 1k pot might be interesting.  
I should build this, maybe a 24VAC wall wart with zeners with a B+ doubling switch. Or not. I'll need to design the box first, "stomp" and "well ventilated tube" being almost contradictions.  
In the time I spent simulating, I could have built the thing. I'm trying to avoid shopping.
12/22/2000 10:39 PM
Doug H

"In Circuitmaker, the diodes don't do anything. Since Since they do something in real life, I'm still intrigued."
Yeah, they don't do anything in Spice either. But I played with it some more this afternoon and they definitely allow more highs through. For that matter, the 1K resistor does the same sort of thing. In Spice, the TL072 behaved very differently than the RC4558 wrt the 1K resistor. Don't know if I can trust that or not (I would love to have a scope...).  
Yes the 1K affects the "asymmetry" of the clipping. May have to try a pot there. What's funny is the B+ rail is already cut down a little (~12V) compared to the -15V rail. I think that's due to the 470 vs 1K series resistor in the rectifier?? Don't know why he used the 1K too but it does affect the sound in a good way, IMO.  
Some of the complaints I've heard about the "high gain" version (presumably the 9v version?) make me think the reason this sounds good is because of the increased headroom. That and it doesn't try to be anything it's not. It's a decent low-medium gain overdrive which is real responsive and "tubey". Cutting B+ down to 9V would definitely add distortion, but I suspect it adds the buzz I've been hearing about too.  
I'm getting a little mushiness from the power supply right now but I suspect I've got a bad connection on the breadboard. Next I want to try ditching the buffer and using a non-inverting stage, and adding an extra triode stage just to see what happens.  
12/23/2000 6:10 AM
phil ruetz
Re: rackmount bias control
I have never actually tried to verify this inside my own Chandler rackmount, but I strongly suspect that the "Bias" control is a 5k or 10k pot used as a cathode resistor on one half of the 12AX7. This would account for the "sweet spot" and for the relative uselessness of most of the pot's range.  
Anybody else have any hunches?  
-- phil
12/23/2000 7:28 AM
Frank Clarke

The 10k/10k at the could shift the input signal up and down a bit, but a cathode resistor would change the bias of the tube. I wonder if the useless setting is 10k or 0k :). Contour could be anything, I suppose. The circuit is simple enough to add presets for the main resistor values.  
With a wall wart, it could fit in a pretty small box, I'd like to try the "Real tube sound in a stomp box" without having to use a hammer. Luckily, overheating here in Canada can be a real bonus this time of year.
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