Fetzer 6418 booster

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Chrisfromthepast

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Mar 5, 2018
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Hi, long time lurker here, stuck on a project that should be simple.
I’ve got a 9v guitar booster that I thought would be simple enough to make in an afternoon, but the sound is not great.
I’ve got a fet input stage whose purpose is to provide a clean boost to the subminiature pentode stage.
I selected the source and drain resistors based on this article:

http://www.runoffgroove.com/fetzervalve.html

I like the sound of the fet stage, though I liked it a little better with the source resistor bypassed.
Maybe 3 of those simple stages in a row would give me the overdrive Im looking for.

Currently though, the pentode stage is farty.
Program material sounds very bad on the low end, but is ok about 200hz.
Guitar pickups have similar issues, with neck position humbuckers sounding like trashy farts, but twangier pickups or positions sound pretty good.

Can I stick a capacitor somewhere and get a better low end response?
I tried the tube with 20v supply and it wasn’t different. I tried the tube in triode mode and it wasn’t much different.
I have 9 more tubes, should I swap and see if one of the other ones sounds better?
Should I be testing this type of tube before I used it.

Thanks for the help!
I’ve never worked with these valves before

I set the heater to 1.25, the fet is 2n5457
The tube is 6418 is made for low power consumption. It has a 30v max b+, but in running it on a 9v battery.

I have a schematic,  but the upload folder is full. I’ll figure that out shortly
 

rackmonkey

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I’ve built several of those Fetzer valve type pedals. You’ve obviously found that it’s more of a clean boost with some hair than a crunch machine. As I started adding stages to play with the possibilities, I came across this:

https://guitarpcb.com/wp-content/uploads/2018/05/BD_3-Time-Champ-v1.pdf

Basically, 3 stages of Fetzer valves (early version) on a cheap PCB. The schematic above will certainly give you the crunch you’re looking for. Maybe too much. But you can play with FET choices and pot placement/gain staging to get whatever you want though.

As far as the pentode stage goes, those low voltage tubes are different enough animals from common high voltage tubes that things can get a little tricky. From your description of conditions it sounds like the signal going to the 6418 is too hot, but I’d have to see what you’ve got going to really help. Did you stick with a circuit from the datasheet or RDH? Once you get a schematic posted we can help you out with that one.
 

Chrisfromthepast

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Mar 5, 2018
Messages
32
Thanks!

The circuit you linked to seems like it will be exactly the sound Im seeking without all the fuss and microphonics of these little tubes.

Here’s a link to the circuit that didn’t sound great when overdriving the pentode:

https://imgur.com/gallery/25jQZcy
Fet is 2n5457, heater is 1.25v, valve is 6418, these details are missing from my scribble.

I can see from the tube’s datasheet that even with 15v rails, the maximum audio input is 0.8v. So, it looks like I can’t even use it as the first gain stage without humbucker farts.

Since clean glassy pentode tone is going to be impossible with this tube, Im going to try some clipping to stay under +/- .7v signal levels.
I am going to try a couple of parallel/ opposite polarity silicone diodes to ground just before the pentode stage.

Like this:
https://i.imgur.com/g1ZzvNb.jpg

Do you think that would work?

Sorry for the chicken scratch, shoulda been a doctor...

Thanks for the help!
 

john12ax7

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What you describe is often when a tube grid starts conducting.  Look up "blocking distortion". You're proposed diodes could be an effective solution to limit the signal,  would also give you some clipping you might like.

Your can also experiment with the coupling cap feeding the tube.  You often need to strip out a lot of low end from the guitar signal to avoid unpleasant distortion.
 

Chrisfromthepast

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Mar 5, 2018
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Thanks for hooking me up with the right search term!
Blocking distortion is definitely the sound Im hearing.
Based on my limited reading, 68n is a really large coupling capacitor and Im going to swap that for 47n.
A corner freq of 34hz is fine for guitar, I imagine.

In the current build, I’ve omitted a grid stopper resistor because I was ignorantly treating it like a glass jfet. Could that make a difference? 6.8k?

As it happens, the 2 diodes to ground between gain stages keep the tube in a safe input range and deliver a nice crunch.
Its good enough sounding that Im going to stick this one in an enclosure and build up a second one with the capacitor change.

Thanks for the support!
Im glad to get a little bit of experience with these tubes that I originally bought to play around with the p48 frankentube project. Its nice to be able to use a stable power supply on my first build with these quirky lil things.
 

john12ax7

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A grid stopper can be useful,  as it will limit the current if the tube grid goes into conduction.  But also it will create high frequency rolloff in combination with the tube input capacitance, this can help distortion to not be too harsh and fizzy sounding.

For the cap feel free to experiment and push the rolloff  frequency much higher,  it is often necessary for a distortion  stage.  You might see something like. 002u into 500k for 159Hz in a high gain amp.

The way a designer chooses to rolloff the low end and high end creating the distortion is part of what makes different amps sound different.
 

Chrisfromthepast

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Mar 5, 2018
Messages
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I’ve had rev 2 of this little circuit at a friend of mine’s home studio.
The goal was simply to ween him off of the virtual guitar amps he was using in Logic.
We’re building more involved guitar preamp as time allows, but this little circuit really opened his mind about what is possible.
Drew the layout with sharpie, etched with ferric chloride.
its hideous, but its just a proof of concept.5E98A4F7-E77F-4858-8BF8-2D6AD6D89F08.jpeg
 
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Chrisfromthepast

Active member
Joined
Mar 5, 2018
Messages
32

All the guitars on this EP were tracked through the guitar pedal discussed in this thread, going into a clean solid state amp, sm57, nothing fancy.
I didn’t track or mix it, just brought by some mics and toys.
 

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