15 years ago - Vulcan

[soggybag]

Help me understand the bias situation here. If I understand correctly we need enough current at the base to get 4.5v at the collector? Since the stages are AC coupled there need to bias each stage?

[rankot]

Finally put it into the box - with a little mod, of course. I have added this switch, so I can switch it between two and three stages, as Andy proposed. But I noticed that this switch produce oscillations (or some kind of radio receiving, not sure), so adding 100p capacitor between base and collector of Q3 fixed it. Another 100p at the same place, but on Q2 removed lot's of noise, so I left both in place.

[Fancy Lime]

Help me understand the bias situation here. If I understand correctly we need enough current at the base to get 4.5v at the collector? Since the stages are AC coupled there need to bias each stage?

Yes, that is the situation exactly.

Finally put it into the box - with a little mod, of course. I have added this switch, so I can switch it between two and three stages, as Andy proposed. But I noticed that this switch produce oscillations (or some kind of radio receiving, not sure), so adding 100p capacitor between base and collector of Q3 fixed it. Another 100p at the same place, but on Q2 removed lot's of noise, so I left both in place.

Nice!


Andy

[vigilante397]

Finally got my boards in and got one stuffed. I knew it was going to be heavy, but I didn't realize it was this heavy Great sounding circuit, thanks for sharing, Aron!

[Fancy Lime]

Ranko, Nathan,

how did you guys bias the stages? You did not just use the values from Joe's schematic and hoped for the best, right? On my breadboard I get wildly different voltages at the collector depending on the individual transistor I use. They all "work" in the sense that they all produce a distorted sound but changing the bias point has a HUGE influence on the sound. Just by changing the bias you can go from muffled drive through nice overdrive/distortion to splatty fuzz. I'd love to have that be less of a hit or miss situation without having to individually select transistors or resistors or add trim pots. Tighter specced trasistor types would be a solution but ideally, I would like to have the freedom to use a range of different BJTs and experiment with the sound changes.

Andy

[rankot]

Honestly, I simply used default values and some generic 2N5088 transistors, all parked around 350 hFE. Maybe a lucky hit, but they seem to work fine and I like the sound. It could be interesting to add some trim pots for bias adjustment, but it would be too big for my intended 1590A enclosure.

[vigilante397]

I plan to play with it a little, because it basically goes from heavy to heavier to absurdly heavy, but I just went with default values from Joe's schematic. I may try 5088 instead of 5089 like Ranko, the 5089s are a bit over the top

[anotherjim]

I'm not sure if a traditional bias for 1/2 way collector voltage is all that relevant. What's the collector swing versus base swing? You could rig up a pot across the supply (with maybe 10k series from the wiper) to vary a voltage into the DC side of the input cap and see what limits the collector can swing between?
It already has emitter degen resistors, but there are large value base resistors suggesting it's aimed at high hfe transistors.

[soggybag]

Another question about the bias. The 4m7 and 1m resistors formdivider that sits at about 1.5v. The diode doesn’t change that until the voltage at the anode is less than 0.7v?

[anotherjim]

The transistor doesn't really "see" the Diode Vf in the steady-state. That voltage drop is on the 1M side. For DC, the voltage drop across the emitter resistor (raising the base voltage) should ensure there is always enough voltage at the anode to keep the diode forward biased.

[rankot]

Am I the only one hearing some radio-like noise with both volume and gain pots maxed? The pedal is complete, I also put some HF bypass capacitors in parallel with B/C pins of Q2 and Q3, but I still have that kind of noise. Not much on lower vol/gain settings, but it increase with amplification. Maybe my layout is not good???

There are few changes between this layout and my final pedal - I removed that 100n cap which was mounted below the volume pot and connected it with a switch, as show in this schematic, so I can switch between two or three stages. Maybe this switch is causing the circuit to be more prone to noise?

[anotherjim]

Makes me think of a point made by PRR in another thread recently, a PN junction in series is a radio detector. A pF value cap across the junction stops RF getting demodulated there without affecting audio frequencies. Try 22pF to 47pF across those diodes.

[rankot]

Across diodes? Good idea, I will. Tried 5, 10, 22, 33, 47 and 100p across B/C pins, but it doesn't help as much as I expected, and it seems I that with bigger values I lost some of the shimmer too.

[niektb]

Does the position of the 2-stage/3-stage make any difference?

Another option would be that you're experiencing some crosstalk from the third stage when it's not being used. i've seen this in HiFi Preamplifiers where outputs that were left open would cause noise. That was fixed by adding a (fairly small) resistor (to ground) to the third stage output in case it's not connected.

[Gus]

With high gain effects you need to be careful with wiring and even how the switch is wired

Think about it
Two gain stages is non inverting overall
Three gain stages is inverting overall
High gain
feedback, one adds one subtracts

IIRC years ago Doug H posted about placing the input and output connections to a 3PDT switch to the outsides and the led to the middle for more spacing between the in and out.

Try to cross signals at 90 degrees if you need to cross them
don't run traces with signals that can effect the circuit stability parallel to each other(unless you want the circuit to be at the edge of oscillating)
You can couple signals with a double sided PCB if they align top to bottom (sometimes you can make a few pf cap)

[dennism]

Can anyone compare this (or even better the JFET version of this) to the BSIAB soundwise?    I've built BSIABs in the past, but given them all away.   Thinking about building another, but might do this instead if it compares favorably.   

I've got a small bag of 20 year old J201 from Mouser and I know I'm not ever getting anymore of those, so I want to choose my projects wisely.

[davent]

Can anyone compare this (or even better the JFET version of this) to the BSIAB soundwise?    I've built BSIABs in the past, but given them all away.   Thinking about building another, but might do this instead if it compares favorably.   

I've got a small bag of 20 year old J201 from Mouser and I know I'm not ever getting anymore of those, so I want to choose my projects wisely.

Socket them so if you decide they'd be better served in another project you can easily remove them with no worries of damage.
dave

[rankot]

With high gain effects you need to be careful with wiring and even how the switch is wired

Think about it
Two gain stages is non inverting overall
Three gain stages is inverting overall
High gain
feedback, one adds one subtracts

IIRC years ago Doug H posted about placing the input and output connections to a 3PDT switch to the outsides and the led to the middle for more spacing between the in and out.

Try to cross signals at 90 degrees if you need to cross them
don't run traces with signals that can effect the circuit stability parallel to each other(unless you want the circuit to be at the edge of oscillating)
You can couple signals with a double sided PCB if they align top to bottom (sometimes you can make a few pf cap)

Does the position of the 2-stage/3-stage make any difference?

Another option would be that you're experiencing some crosstalk from the third stage when it's not being used. i've seen this in HiFi Preamplifiers where outputs that were left open would cause noise. That was fixed by adding a (fairly small) resistor (to ground) to the third stage output in case it's not connected.

Yes, I know all that, but I had not much options when wiring my switch - it came as an add-on after the PCB was already made and it was very tight build (1590A). So I presume I will try to bypass diodes with some capacitance.

There is some difference in noise when I switch between 2 or 3 stages, but it maybe also due to different amplification factor. I will see if I have enough space to use DPDT instead of SPDT switch, so I can ground stage 3 input or output if it is not connected. Also, it was noisy even before this addition, but I'm not sure if it was the same or less.

[Fancy Lime]

I played around with the biasing scheme a bit. Specifically, I tested the original Vulcan's voltage divider bias with high gain (MPSA18, 2N5088, 2N5089) and low(er) gain (2N3904, 2N5551) transistors. As expected, the bias point needs to be adjusted between types to get near half supply at the collector. Without readjusting, there is about 4V difference at the collector betweeen a 2N5551 and an MPSA18. Interestingly, I get very tight groupings within the same type. I tested all the MPSA18 and 2N5089 I have (about 15 each) and for each of them, the variation of collector voltage without readjusting any of the resistors, was within half a volt. The MPSA18 are from three different batches, purchased from different suppliers. I don't remember where I got the 2N5089 but some are straight and some are bent legged. So it seems that dispite the huge span of hFE in the data sheets, in practice we can probably use the voltage divider bias without adjusting it to individual transistors, as long as we have set it right for the transistor type and the individuals don't scatter too much.

I also tried the dual feedback with AC shunting to ground. This works much better in terms of accomodating different transistors without readjusting the resistors; only about 2V difference at the collector betweeen a 2N5551 and an MPSA18. However, this configuration is much noisier (on my breadboard). Can anyone tell me why? The sound is pretty much identical to the voltage divider version.

I also mucked about with the switching to get rid of the clicking. Mostly I tried different values for C18 and C23. No luck there yet, still the same clicking. I am aware that worse thing happen to better people every day but this is driving me nuts...

Cheers,
Andy

[Marcos - Munky]

Can anyone compare this (or even better the JFET version of this) to the BSIAB soundwise?    I've built BSIABs in the past, but given them all away.   Thinking about building another, but might do this instead if it compares favorably.   

I've got a small bag of 20 year old J201 from Mouser and I know I'm not ever getting anymore of those, so I want to choose my projects wisely.

Two different beasts. BSIAB gives you that marshall mid boosted mid to hi gain sound (like 80's and beginning of 90's hard rock guitar sounds). Vulcan is more modern sounding, more aggressive and have more gain (way more gain).

IIRC, the NPN Vulcan sounds better than the jfet one.