Blackface Bassman Fetzer Valve (schem)

[PenPen]

In my search for a dark booster for another project, I built a lot of different booster designs. I had looked at the Fetzer Valve, but I didn't want something Fender sparkley, so I kind of ignored it. I finally decided to take the schem for the Blackface Bassman and try replicating it the way the ROG guys do with other amps. Once I was done, I realized it actually was just a tweaked Fetzer Valve. It sounds good on its own, so I decided to draw it up and post it in case anyone else finds it interesting.



After hearing it I wanted to tack this on to a Ruby to see how it sounds, but I'm encountering some problems taming the headphone output. I made it with a standard stompbox output and ran it into my amp, and I tamed that enough so it isn't clipping the LM386, but converting it to standard Ruby output to speaker or headphone burps, fizzles, and fuzzes badly. I would post a schem of how I did it, but its late and I'm too tired to draw up another schem and discuss this problem right now, so I'll post that at some other time after I experiment a bit more with it.

Enjoy the schem, I'd be interested in anyone who actually OWNS a Bassman to say how close this sounds to one. I don't have one to compare to, but the sound is nice and deep, still retaining some of the sparkle. I like it a lot.

[Dragonfly]

i hope you don't mind...i whipped up a vero layout for it...untested, but should be just fine

AC

[wampcat1]

i hope you don't mind...i whipped up a vero layout for it...untested, but should be just fine

AC

Has anyone told you that you're the man? Cause YOU ARE THE MAN!! 
Thanks guys, great stuff!

Brian

[PenPen]

Indeed, Dragonfly is the man. Yet again, thank you for putting out a layout for my schem. It appears that once a layout is posted, the chances of someone actually building one goes up quite a bit. And getting people to build our designs is one of the reasons we post our schems here, right?

[Arn C.]

Thanks PenPen!!!!

Peace!
Arn C.

[Dragonfly]

It appears that once a layout is posted, the chances of someone actually building one goes up quite a bit. And getting people to build our designs is one of the reasons we post our schems here, right?

...exactly...

[spinoza]

Bumpy:

I'm building one this week!
To be continued...

[PenPen]

Cool. Some extra tips on this, since I haven't updated the schem in a while, try tacking on an output buffer, or even a boost stage at the end. There is some volume loss from the tone stack, but it will still be more than unity. Lastly, replacing R6 with a 10k pot wired as a variable resistor should give you some control over the bass level.

[AL]

Hmmm? Interesting. I hope I can find time to build it. Right now I'm working on a Bassman. Just curious - why the 10uf for the value of C1? The standard Fender value is 25uf.

Thanks

AL

[PenPen]

Good point on that cap. I don't remember the reason, though a quick look through my schems for the various Bassman amps showed values of 25uF for guitar and 5uF for bass, and perhaps I compromised with 10uF. I'm not sure. 25uF should sound fine in there, perhaps even better.

[AL]

Thanks for the response. I did a quick check on the Aiken Amps site and here's what he had to say about the cap - for amps. Using Ck & Rk (C1 & R3 on your schem).

http://www.aikenamps.com/

Capacitor Ck is used to bypass the cathode resistance to ground for AC signals, which results in a higher gain.  Without Ck, there is negative feedback, or degeneration, which reduces the gain of the stage and increases the output impedance.  If Ck is not large in comparison to Rk, it will affect the frequency response of the stage, by introducing a "shelving" response, where the stage gain is boosted at higher frequencies compared to lower frequencies.  The "breakpoint" of the frequency response is controlled by the value of Ck, in conjunction with the cathode impedance.

So, apparently adjusting this affects the frequency which would be why the bass amp was 5uf and guitar 25uf... or maybe I'm stupid and reading that wrong. 


AL

[PenPen]

Nope, you are exactly right. It affects the gain at certain frequencies, thus changing the perceived tone of the stage.

As an aside, I plan to do some experimenting with this circuit soon, I intend to try tacking on Jack's Mosfet boost to the end of it to see how that sounds. If its good I'll be posting an updated schem.

[B Tremblay]

Check out the revised and expanded Fetzer Valve article for more information on how that bypass cap can affect the sound:
http://runoffgroove.com/fetzervalve.html

[spinoza]

Oh noese! I just came back from addison (that's where I buy my stuff in Montreal, don't know if there's a better place) and read about the 5uF cap... the thing is I'm gonna use this effect for my girlfriend who plays bass in a hip-hop band, so she needs it very very bass-y.

Anyway, I'll try it with the 10uF, if she turns the tone all the way down I'm pretty sure she'll get that "reggae-dub" sound.

@Penpen: good thing to know but it doesn't really matter for that particular projet, it will be used more as a pre-amp. I might even want to stick it in an old bass I got and make it an active pickup. The fetzer tone would use the bass' tone pot (which is not working anyway) and the volume would be, well, the volume

[spinoza]

Oh and yeah, I bought a couple LM386-N1... I thought it was a triode, not a chip! I assume that it's the same triode, but in a chip format? So my question is: how do you wire that to the circuit?

[AL]

An interesting side note (to me) about the bypass cap (C1). The Flipster, which is a sim of an Ampeg SB-12 does not have a bypass cap for C1. I have an SB-12 and a couple of Bassmans and the Ampeg is by far my favorite guitar amp. The SB-12 is also supposed to be used as a bass amp. But, the Ampeg Gemini 1 is also missing this cap. Hmmmm? Anyone care to hazard a guess as to what is happening here?

AL

[petemoore]

And getting people to build our designs is one of the reasons we post our schems here, right?
  This is what enables the common experience which we all can truly enjoy and appreciate.
  Thank you for the excellent contributions Al, Dragonfly...!!
  A little board with this on it could go...anywhere !!
 
     
 
 
 

[AL]

B Tremblay - Thanks !! That's a lot of very good info on the Fetzer Valve. I just printed it out and hope I get some time to read it this weekend. Good stuff.

I think I answered (or rather Aiken answered) my question concerning the missing bypass cap (C1 - CK on Aikens pages) on the cathode in some earlier Ampegs. Here's what Aiken had to say:

This circuit is similar to the bypassed cathode configuration with the exception of the removal of the cathode bypass capacitor, Ck.  Removing this capacitor adds degenerative negative feedback at the cathode, which lowers the gain of the stage and increases the output impedance at the plate.  The frequency response due to the input and output coupling capacitors remains the same as for the bypassed configuration.

That might explain a few things too. It has been my understanding that Ampegs were designed from a jazz standpoint and were (as a lot of manufacturers were at the time) trying to reduce distortion. This would be one way of doing it.

I am in the process right now of rewiring a silverface bassman that was given to me. The Normal channel is going to the older AA864 circuit. But, I am wanting to rewire a bass channel to an SB-12. There are two version of the SB-12 (7868 and 6L6). I'm wondering how this will sound and which version will sound better? The preamp stages are a little different in the two and there is an extra gain stage in the Fender. Plus the 7868 is a 19watt tube and the 6L6 is 30watts.

Thanks PenPen - you've got me thinking...

AL