Vulcan Build report - Thanks Joe for a great circuit.

[will]

Hi,

I built Joe’s Vulcan over the weekend. I it sounds awesome!   The Vulcan is very tube like, more so than any other non-tube circuit I have built.  It has good sustain, a Marshall like crunch on chords, great sound with my strat single coils. The circuit has very low noise at reasonable settings. I must say I love the sound of this circuit. Cranked it’s just like an extremely overdriven amp; you hear extreme gain pickup noise along with the pulsating sound folding over itself.

It’s interesting, each time I build an effect I start messing around with the guitar trying all sorts of songs riffs to see what type of song seems to fit. I just started to play the Beatles “I want you so bad it’s driving me mad” rift, which I had never played before. This sounds great on a wide variety of songs. I think this will be my main distortion - overdrive circuit once I tinker with it a bit more and box it.

I do think the circuit has too much gain; to me the best sound is when the gain control is quite low. The volume control needs to be set quite low as well. I put a 200K resistor in series with the output volume control to allow me to be able to set the volume on low. It was either off or too loud on.  I am currently just using linear trim pots for the volume and gain controls. I do pickup a radio station when I am adjusting the trim pots, but none during use.  I guess the screwdriver acts like an antenna.

I disconnected the 22uf emitter cap on Q2 (middle transistor) to reduce the gain. It helped but, it would be nice to have the circuit set so I can use my volume control to get from clean to overdriven. I can’t get totally clean with this circuit. It seems to jump into high gain.  :?

I’m wondering of reducing the size of the 3, 22uf emitter caps or hooking them through 5k pots to ground. So I should be able to adjust the gain of each gain stage from clean to the present level.

Thanks Joe for a great circuit.

Regards,
Will

[RDV]

I think if you use linear instead of audio for the 1Meg drive pot that it will be easier to use the lower end of the gain. An audio pot will bring the gain up very fast at first, then gradually. A linear would bring the gain up more evenly, I would think.

Regards

RDV

[Gary]

Try disconnecting the third stage emiiter bypass cap.

[Ed Rembold]

not wanting to be fussy, but....

Audio pots will bring the gain/volume up more
gradually than Linear.

Ed R.

[Arn C.]

I am in the process of building Joes Vulcan, hopefully I will get it done today and get a chance to try it out tonight, I will let you know how mine works.   Thanks for the schematic Joe!!!  

Arn C.

[Arn C.]

Is there a "rule of thumb" when it comes to transistor placement according to Hfe(gain) of the transistor?

The three 2n5089's that I have are roughly  748, 764, 785
Is there a specific way to install these into this Vulcan circuit ? Highest gain to lowest or lowest gain to highest?

Thanks!
Arn C.

[WGTP]

If there is too much gain (I guess that can happen) maybe 2 stages are all that is needed.  Also, on some of Joe's stuff he has added a .001uf cap in parallel with the 10K resistor to V+ for high end roll off and noise reduction.  Just thinking before I build.  

[Arn C.]

I guess I can always put in some 25K trimpots in place of the 10K resistors to the collectors and adjust from there.

Peace!
Arn C.

[smoguzbenjamin]

On intuition, try doing them from low to high gain, Arn You could also attenuate the gain of each stage, sounds like a pretty broad spectrum of sounds you could get like that.

[Arn C.]

Smoguzbenjamin,
   In fact that is the way I put them,  from high gain to low gain and I did not put in the trimpots.  I am going to try it this way first.   I used all metal film caps and resistors except for the 22uf caps which I used tantalum caps...

Peace!
Arn C.

[Arn C.]

Will,
  How about trying a lower value pot for the volume pot.  Try 50K,  25K, 10K .   I would try 50K first.   I can't test mine here at work.  Maybe you can throw one in and let us know if it makes a difference.  

Arn C.

ps  Mine is all built and ready to try when I get home!

[smoguzbenjamin]

Arn, you've got nothing to do at work I get the impression

[bwanasonic]

I'm pretty sure when cascading gain stages you want to go from lowest (input) to highest (output).  

Kerry M

[will]

Hi,

Many thanks for your responses; I will put in audio tapers into my final version (when I box the circuit). I almost always use trim pots while I am building and tweaking the circuits as they are cheap and give a good idea what would be a reasonable end value. Also most circuits I build remain unboxed until proven to work great in jamming with others.  

I tried lowering the gain by putting a 500 ohm resistor in series with the 22uf emitter caps for the 1st stage and the 3rd gain stage. I can now get a clean signal with my volume control set at about 2. At volume setting of 10 I get a great sounding distortion. In between it’s quite good but at times I notice a subtle distortion anomaly.

It is kind of hard to explain. It seems the circuit is doing some good smooth creamy Santana type distortion, which is great. It seems blended on top a harsher low gain type of clipping that seems to hang on as an after effect. Currently in my circuit I have reduced the gain in the 1st and 3rd stages and the 2nd (middle) stage is maxed out. The hfe of the 2nd stage 2N5089 transistor is a little over 600.

I’m thinking that maybe I should reduce the gain of the 2nd stage then max out the gain in the last stage so the end result is creamier. (I wrote this before I saw your responses) Or I wonder if I need to taper the frequency response before the massive clipping, perhaps there is too much bass clipping. However I do like the overall full sound balance. This circuit has great potential.

I’m also wondering if maybe I should put in a FET for the last gain stage.

Regards,
Will

[Arn C.]

Maybe Joe Davisson can pop in and give us a hand on this......

"Hey Joe, Where ya goin' with that gun in your hand?".........

Arn C.

I was going through some transistors and I found some 2n5225's
with hfe of   344, 346, 349.   If mine is way too high a gain, I will try these next...

Arn, you've got nothing to do at work I get the impression

Quiet day at the job today!!!!    I usually just leave my computer logged on to the forum all day, then when I pop in and out, I can just refresh and jump right in!!!    And what about you Smoguzbenjamin?  Aren't you supposed to be in school?  Oh yeah, I forgot, you are on the other side of the world!!!!!!

[Transmogrifox]

definitely try a FET in the final stage.  It never hurts, you may like it.

About the AM radio problem:  Before you box it and finalize the project, use a well-filtered power supply.  Sometimes those larger value (470 uF) supply caps have a low enough self resonant frequency that theydon't actually filter the power supply at radio frequencies.  The common-emitter type transistor amplifier (like in the vulcan, fuzz face, etc.) IS a square-law multiplier, so you can inadvertently heterodyne radio frequencies to the audio range, so you hear AM radio (because this heterodyning demodulates it) and also static.

A simple way to get RF out of the power supply is to put a small resistor (47 ohm or so) in series with the battery, and also put a .1uF tantalum capacitor in parallel with a large value cap (470 uF).  This will also make the distortion sound better because you won't be modulating your signal with the audible (60-120-180 Hz) power supply noise.

[will]

Hi,

Also, on some of Joe's stuff he has added a .001uf cap in parallel with the 10K resistor to V+ for high end roll off and noise reduction.  Just thinking before I build.  

Actually with this circuit personally I’m not sure you need much if any HF roll off. I tried cutting some treble in my power amp and it made the sound a bit muddy. It seems nicely balanced as is.

I guess I can always put in some 25K trimpots in place of the 10K resistors to the collectors and adjust from there.
.

I’m not sure how much that is going to buy you. You will be able to bias the circuit a bit better using trim pots on the collectors. On mine the bias is a bit low but it seems to work quite well. You will not change the gain of the stage as the 22uf emitter cap is essentially grounding the transistor’s emitter for audio frequencies. Thus the limiting factor is the transistors (hfe) max gain. If you put a trim pot in series with the emitter caps, OR replace the 1K emitter resistor with a 1K emitter trim pot connected to the 22uf cap similar to the fuzz face type circuits, you can limit and control the maximum gain for the stage.

About the AM radio problem:  Before you box it and finalize the project, use a well-filtered power supply.  Sometimes those larger value (470 uF) supply caps have a low enough self resonant frequency that theydon't actually filter the power supply at radio frequencies.  The common-emitter type transistor amplifier (like in the vulcan, fuzz face, etc.) IS a square-law multiplier, so you can inadvertently heterodyne radio frequencies to the audio range, so you hear AM radio (because this heterodyning demodulates it) and also static.

A simple way to get RF out of the power supply is to put a small resistor (47 ohm or so) in series with the battery, and also put a .1uF tantalum capacitor in parallel with a large value cap (470 uF).  This will also make the distortion sound better because you won't be modulating your signal with the audible (60-120-180 Hz) power supply noise.

I haven’t yet put any filtering on the power supply. The radio is not a problem as it only occurs when I adjust the trim pots with a jeweler style metal screwdriver. When I put into a box with regular pots I’m sure it will work great. You do offer great advice on power supply design.  I’m not familiar with heterodyning.

I do like the sound of FET’s clipping so I may give it a try. I’m going to work on this a bit further with it’s existing configuration. I’m also out of JFET’s right now. I cleared out all the local electronic retailers.

Who know maybe a Vulcanized FET or Mosfet achieves stompbox nirvana?  :wink:

Regards,
Will

[Transmogrifox]

For this type of circuit using transistor amplifiers with the resistors tied to the power supply, I highly recommend power supply filtering.  What I mentioned above is simple, but likely adequate...or just add a .1 uF in parallel with the supply for good measure, and here's why...

An easy lesson in heterodyning is this:

Suppose you have a cosine wave at one frequency, f1:

A*cos(2*pi*f1*t)

Multiply it by a cosine of another frequency:

B*cos(2*pi*f2*t)

If you multiply the two together, you get:

A*B*[ cos(2*pi*f1*t + 2*pi*f2*t)  +  cos(2*pi*f1*t - 2*pi*f2*t)]

Ignoring the relative amplitudes, you can see that the frequencies resulting are a sum and difference of the originals.  This holds for signals containing many frequencies, and in fact, you can transfer (heterodyne) the entire spectrum modulated on an AM signal to the audio range by multiplying the AM signal by its carrier.

Now with power supply noise:

Suppose you have a single frequency at 1090 kHz  (a common used AM radio station) and a bunch of audio frequency noise modulated onto that carrier, which results in (from above) in 1090 kHz +/- noise frequencies.

Now, if this noise is in your power supply, it has likely gotten into your signal path as well... so:

You have a BJT amplifier as a square-law multiplier, and you have 1090 kHz +/- fnoise in BOTH the signal path (at the base of the transistor) and this same signal on the power supply.

When you multiply the two, you get audible noise.  Let me demonstrate how using only the carrier in the signal path, but all of the frequencies consisdered in the power supply.  This will give you the highest amplitude noise component in the audio path, but keep in mind that all of these noise frequencies I'm ignoring in the signal path generate cross terms that are audible, but you can see how complicated this can get, so just assuming somehow only the carrier is in the signal path, but the whole set is in the power supply:

cos(2*pi*1090 kHz) * cos(2*pi*1090 kHz +/- fnoise)

which results in signals with frequecies:
0  :  A DC term from 1090 kHz * 1090 kHz

+/- fnoise :   Any spectral components within +/- 20 kHz of 1090 kHz will be audible.

So what happens to the "-" fnoise?  This effect is called "folding".  This "negative" frequency simply becomes a real frequency by being "folded" or "mirrored" back to the positive spectrum, so it simply doubles the amplitude of fnoise.

Now you can see what a noisy static could result if you had a very noisy power supply with not only 1090 kHz AM on it.  That doesn't even consider the cross-terms from the other harmonics and fnoise terms...and the intuitive thought is that the audible frequency noise in the power supply matters.  RF noise itself is not audible, but so many of our stompbox circuits are liable to act as multipliers and "heterodyne" normally indaudible frequencies to an audible frequency range...and not always pretty either.

Fortunately, we don't see this to an extreme extent in guitar effects because we usually have well shielded cases, and low-pass signal paths that don't allow the RF signals into the "multiplier" so there is no effect even if it is in the power supply.


I have had a significant amount of trouble with power supply and the "guitar cable is my antenna" listening to "Fun Lovin' Bob" oldies station through my guitar amp.

So for those of you who do hear radio or excessive static might want to think about filtering your power supply and adding some 47 pF - 100 pF caps in parallel to ground with your signal path.  Usually one on the input is sufficient.  This is the reason that many of the schematics you see (especially distortion) have these small-valued caps that don't appear to be doing anything useful.

You can also see how this works with audible noise, as well.  You have a 180 Hz ( 3rd harmonic or 60Hz) in the power supply, and your guitar in the signal path.  You will hear every harmonic from your guitar +/- 180 Hz, hardly musical...so before you say "Ugh...that is one trashy sounding distiortion, stop playing and listen for a power supply buzz.  If it's there, you're probably hearing more power supply trash intermodulation terms than a poor sounding pedal...this is most common in bread-boarding.  If the power supply trash is not audible you probably just have a crappy sounding distortion.

For those of you that don't know yet, do you have a guess at how a Theremin works?

[Joe Davisson]

I don't think exact transistor gain matters a lot. The 10k resistor might need a little adjusting to get somewhere between 4 and 5 volts on the collector. (This is true of all BJT/JFET/MOSFET stages biased this way.) My 2n5089's are around 500, FWIW.

I think your best bet to reduce overall gain is to replace the 220k's with higher values. The first one could be 2.2M/100pf, and the second one 470k, for example. I went a tad bit low on those, forgetting how weak my pickups are (HS3's).

J201s work fine with some rebiasing. Leave the 4.7M/1M alone (or use 10M/1M), and adjust the drain resistor to reach ~4.5v. Gain is lower, but it might work good for an output stage or something. Thanks for trying the Vulcan, and have fun with the circuit -- there are many possibilities.

-Joe

[Arn C.]

Thanks Joe for popping in and for your input.
I tried mine out last night with my 2n5089's with hfe's of 748, 764, 785 (in this order)  Wow, way too much power!   I couldn't turn up the volume any more than 2 or 3.  The sound was great tho!!

Today I will put some 2n5225's with hfe of 344, 346, 349. and I will try this tonight.  Great pedal Joe!!!!

Peace,
Arn