#1
So, I've been hired to make an overdrive pedal for a friend. He gave me one requirement. It has to sound like absolute shit, and should have ridiculous amounts of saturation and grit.

I repeat:

*THIS PEDAL IS NOT SUPPOSED TO SOUND PRETTY*

So, I'm personally a complete tube ***** and I wanted to try something unorthodox with the design. I began searching for tubes that were inherently unstable in critical gain situations. My search led me to the --BN6-type tubes. Specifically the one with the 4.3V heater. My original idea was to create a simple class-A preamp, but instead of using 12AX7s/ECC83's or 12AT7's or whatever, I'd use a tube that most would think I was batshit crazy to use. Seeing as 4BN6s were typically used in old television sets and FM receivers, I thought, "Oh, the hell with it." and decided they'd be a prime candidate. All was going well until I suddenly clued into the fact that BN6 tubes aren't exactly similar in function to 12AX7s and other 9-pin dual-triodes.

So, my question... Can anybody either direct me to a 9-pin dual-triode that would give gut-wrenching sonic characteristics to this device, or could somebody give me advice on how to design a preamp around the 7-pin construction of the BN6-type tubes?

Any help would be greatly appreciated.
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#2
This, is interesting, i will be watching this thread.

Can I just ask, why did you chose to use tubes for this?
#3
Quote by kacper_j
This, is interesting, i will be watching this thread.

Can I just ask, why did you chose to use tubes for this?


Well, I personally just like the way that tubes clip as opposed to transistors. I was looking around at tubes, and these old FM-receiver tubes can be as cheap as 4 bucks a pop, so I didn't see any reason why not too. I can open a tube amp and actually relate to some of the contents, but I look at an SS circuit (and don't ask me why) and my mind goes blank. It's really just more of a personal preference thing than anything. And I'll be sure to post updates as I progress with the design and build.
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#4
^well, he did say he was a tube whore and wanted to try something unorthodox. this sounds pretty unorthodox.

I don't know much about tubes, but I recommend doing the exact opposite of what all the mojo sucking tropical fish hunting tone freaks that think yellow LEDs sound the best for clipping say - use cheap ceramic caps on everything you can.

I'm thinking you could probably use a regular tube and just give it a LOT of distortion, and maybe throw a cap in there to send a lot of the highs to ground to make it really muddy. or you could go the other way and filter out a lot of bass and mids so it just sounds like an icepick in your earhole. maybe throw some diode clipping in on top of the tube distortion?
#5
I wanted to avoid having to use diodes for clipping. I know they're used in the majority of boost pedals, but I want to make this sound more organic, if you will. Also, good point on the ceramic caps. I'm going to use them wherever I can, but unfortunately they don't always have very high values. I'm avoiding using electrolytic types, but from my experience, they're the ones that typically have higher capacitance that ceramics may not be able to handle. I'll keep it in mind though.
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#6
Why not use two or three JFET gain stages to boost the signal, then use a couple tube stages to get the clipping you want?

To use the 4BN6: http://tubedata.tigahost.com/tubedata/sheets/127/4/4BN6.pdf
Look up designing with small signal pentodes.

Use poly film caps, ceramics have their uses, but generally film is good. Electrolytics can be useful as well, PS filtering caps and cathode resistor bypass caps are two prime examples.
Last edited by blandguitar at Aug 16, 2010,
#7
Quote by blandguitar
Why not use two or three JFET gain stages to boost the signal, then use a couple tube stages to get the clipping you want?

To use the 4BN6: http://tubedata.tigahost.com/tubedata/sheets/127/4/4BN6.pdf
Look up designing with small signal pentodes.

Use poly film caps, ceramics have their uses, but generally film is good. Electrolytics can be useful as well, PS filtering caps and cathode resistor bypass caps are two prime examples.

Thanks very much for the info.

But do you think using more than one transistor stage would really be necessary? I know that JFETs are generally lower noise and output than MOSFETs. However this is going to be put in front of an amplifier rather than a line level mixer. I don't want the pedal's output to be >10-15dB than the input. I was thinking of including a pad at the end of the circuit, but would any of this be necessary? I also found those old data sheets before, they're very useful though. Thanks.
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#8
To design a gain stage, they're pretty necessary, and simplify the process. Two JFET stages allows you to shoot up gain out the wahzoo, this means distorting the tubes will be easier. you can always add a volume pot at the end of the preamp to control the output level.
#9
Quote by blandguitar
To design a gain stage, they're pretty necessary, and simplify the process. Two JFET stages allows you to shoot up gain out the wahzoo, this means distorting the tubes will be easier. you can always add a volume pot at the end of the preamp to control the output level.


So, say I was using two SS stages before the tubes. After this I use a pentode and a twin triode. Would this be possible? And if so, what effects would using either the pentode or the triode before the other have? I know pentodes are typically used for power rather than preamp gain, and that their amplification factors can be over 1000-fold (I assure you that I won't come anywhere close to this, however...). So would I benefit from placing a triode before/after the pentode, or would I benefit more from my original idea of using only pentodes. I'll look into using JFETs for voltage boosting before the tubes, however.
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#10
It's defintely possible, and the pentode will most certain push the triode(s) into distortion after two gain stages before it. There are two different types of pentodes, power and small signal, power pentodes are used in power amps, whereas small signal are just there to provide a shi*load of gain. With an EF86, gains of 100-200 can be obtained pretty easily.

If you want distortion, use the pentode before the triodes. You'll get distortion. A guitar can put out up to a 4v pk-pk signal, enough to drive the pentode into overdrive, this isn't extremely typical, but can be achieved. With two JFET stages, you can not only save space and power, but give the signal a hearty boost, then pentode to supply gain, and tubey distortion. The last two triode stages will distort pretty heavily. You said you didn't want it to be pretty. You could always cut it back to 1 JFET stage, either option should suffice.
#11
So say I wanted to still give a large boost, but nothing horribly insane. If I had two latch switches, one for bypass/on, another to swap in/out one gain stage. Would it be more wise to add this function to one of the JFET stages or the final tube?
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