Thoughts on the Shaka Tube

[Thomas P.]

It was almost over a year ago when I finished it and since the past days there were some discussions about it I rewoked mine. I was never that happy with its sound – there were some strange harmonics and overall it seemed to be a bit shrill (what forced me to have the tone-pot alsmost at minimum). So I resoldered and rerouted some of the wires. After I found the inside tidy enough (for me of course) I gave it a test but the only thing that improved was the backround noise – which is good. But if you looking for a really overdrive not quite satisfying. So I left the box and used a software to look what to do (sadly I don’t own a scope). After a few simulations I came to the conclution that a 10nF cap at the end of the circuit was quite a good idea (someone mentioned it but I don’t know his name) which gives a much soother waveshape in the end.
With that in mind I went on with the real pedal and got a cap on the output jack. Well, it was an improvement in terms of the pedal sounding shrill befor but not in terms of these strange harmonics – which bothered me most.
Finally I found out what the problem was – and that might be interesting to some of you. For practice (and testing pedals) I have some really old SS-amp not a really good one but clean (which makes it a good candidate for testing). I always have to keep the volume low or some of the people around me can get really angry. So I did this time during the test. But after a while playing I turned the volume up to see how the noise is about. Very silent though. But I can’t withstand playing some notes at higher volume (which I’m sure you all can understand). So I did and imagine what I heard??? A really cool and extremly good sounding overdrive without any of those bothering  harmonics. Though it seemed that they were still there (hardly audible) when I played really (really) silent but surpressed while playing normally.

Strange, isn’t it? They don’t seem to be amplified very much

[Paul Marossy]

From your description, it sounds like you are hearing some odd order harmonics in the upper octave range or something. Maybe 7th, 9th, 11th or even 13th - they could be present in the signal, but would be at low levels because the voltages for each of those octaves decreases as you go up the octave scale. You may not hear them when the amp is loud and the gain is set high because the 3rd order harmonic will predominate because of the opamp, which is a solid state device. The second most dominant harmonic would ordinarily be the 2nd, which would happen via the preamp tube. Just a theory based on everything I have read thus far...

I know that on my first build, it sounded kind of strange - it always sounded just a little off, almost like the opamp was mis-biased or something. And certain opamps wouldn't work at all!? What type of opamp did you use? Maybe trying a different type might work a little better? I used an LF353 because it has a wide bandwidth, and they seem to sound good in most everything I have built that uses dual opamps. Also, 12AX7 tubes can produce gain-bandwidth of up to 8mHz, so they are quite capable of amplifying higher register harmonics.

Have you checked all of the voltages in the circuit? Maybe something is a little lower or higher than it should be.

I agree that it kind of sounds shrill with the tone control up towards the treble end. I used a 50K for my tone control, it's all that I had lying around. It sounds best to me at 25% from the bottom of the low side. This cap on the output jack, is it wired so that it is bypassed when you bypass the effect circuit? (BTW, it was aron that mentioned that output cap thing)

[Thomas P.]

That’s it Paul – the harmonics that appear during the amp is at low volume are sounding like the ones you hear in a misbiased Si-FF.
I used to test it with a TL072 and afterwards with a NE4558 – both sounding really good at higher volumes (of the amp). I used then a combination of a TL072 together with a LF353 which sounded good in a Bluesbreaker but this time failed completely.
I’m going to check the voltages today and posting them later on.
I also set the tone control a ~25% but with the cap at the end it’s no problem going towards the treble end. Which brings me to a good point you mentioned – the cap at the end isn’t bypassed. I will route the Level wiper back to the board and place the cap there in the new layout I’m working on.
How did you set the bias of the tubes? I dumped the bias control and connected it to V- (like the Chandler Tubedriver has it).

[Thomas P.]

I have measured the voltages and here they are:

I found that my transformer has (instead of 12VAC) constant 15VAC which gives:

V+   : 19.6V
V-   : -18.5V

Tube:
1   : 0.77V
2   : -21.6V
3   : -17.8V
6   : 5.05V
7   : -19.6V
8   : -18.5V

Opamp:
between 4 and 8 I’ve got 38V (which seems a bit high to me)
1   : 53mV
2   : 41mV
3   : 24mV

[Paul Marossy]

Thanks for the info, tomboy.  

Keep in mind that I am not exactly a guru in these sorts of things, but a couple of things stand out to me here.

Your supply voltage to the opamp may be a little bit too high. The data sheet for the TL072 chip indicates a "absolute maximum" supply voltage for Vcc+ & Vcc- as being +/-18V. It appears that you are exceeding that voltage, so if the chip isn't being obviously damaged, and can take the higher voltage, then it might make things sound funny because it's operating outside of its normal parameters.

So, to remedy that, you may want to consider a voltage dropping resistor in series with your power supply somewhere. Maybe you could put them in at the power supplies to the opamp, and leave the tube at whatever it gets. I put one in right at the point of entry, at the "AC jack". I used whatever value it took to get my power supply down to ~12.6V, I don't remember what it was exactly. It ended up not being a very large value, though. My wall wart also puts out 15V with this circuit, and it is labeled as a 13VAC wall wart.

I haven't measured the bias voltage on the tubes, but the trimpot is pretty much all the way to one side, at the place where it sounds the fullest. I guess I should measure it sometime...

As far as that cap on the end is concerned, I took your suggestion on the 0.01uF cap on the end of the circuit and put a switch in that will act like a bright switch/treble cut switch. I haven't tested it out yet, it was too late when I was done with my mods. But, I will test it out when I get home from work today.

Thinking a little bit more on the harmonics thing, maybe in this case, an opamp with a high bandwidth is not a good thing, because whatever weird stuff the opamp produces the tube will definitely be able to reproduce it. The one thing I have always wondered about is the bias control - doesn't it affect the tube and the opamp? In my mind, it seems like you would want the opamp voltage rails to be symmetrical, and the bias to affect the tube only. I don't know if some opamps respond better in this type of scenario than others, or if it matters at all...

[Thomas P.]

I already put a resistor in front of the 10R resistor which took the voltage down to good values. I have to change the IC to see if it was already damaged.
I tried to put the resistor at the supplys AC input but that took all the voltage down (and it was only a 470R) - :?

I also thought about the supply being asymetrical. I wonder what this does to the bias of the IC - but the chandler pedal has the same supply so I think this is some kind of compromise. Maybe it would be better to have a supply of +/- 15V. I think the bias affects only the tubes since it's connected to the 10k dividers at the grids.

[Paul Marossy]

"I think the bias affects only the tubes since it's connected to the 10k dividers at the grids."

I think you are correct about that. +/-15V might be better, it's a little more conservative. It might make your output kind of lower than before, though. Maybe that is a good thing?

Anyone else have anything to add on what appears to be the asymmetrical IC power supply?

OK, Here's an update. I tried out the 0.01uF cap at the output, and that makes things a little more smooth, but it is still rather subtle, at least at low volume levels. Maybe even a 0.02uF cap might work better for a treble cut cap. I also tried using a JRC4558D opamp in place of the LF353 opamp, and it seems to sound a little bit smoother. I also readjusted the bias a little bit. It really sounds pretty good now. I'm still debating if I am going to use a 0.01uF or 0.02uF cap. This circuit also seems to be rather sensitive to even what brand tube you pop in the socket. I tried a different 12AX7 tube in there tonight, and it seems to sound better. But maybe the one I had one in there previously is a little bit worn out... I wonder if different brand preamp tubes react differently in a starved plate configuration?