Shaka Tube Transformer Question

[rody82]

Hi,

I've just built the Shaka Tube. It already sounds nice at some
points, but at high gain settings it becomes a bit ugly... I mean
a more harsh type of distortion.
I'm using a transformer i found in a lamp. It says: 12V 20W and 20VA
Is it okay for the Shaka Tube? I measured a bit more than 13V at
the heater filament, and V+ is around 14V V- is something like -11V.
If needed i can measure them exactly.
Used 100K 100K 10K pots. (10K log for volume)
Changed 10K bias trim to 100k, but that doesnt change too much.

[Paul Marossy]

It already sounds nice at some
points, but at high gain settings it becomes a bit ugly... I mean
a more harsh type of distortion.

Your voltages sound right where they are supposed to be. What opamp are you using? The BJT type opamps (like a JRC4558) sound harsher to my ears than does a JFET type (like a TL072). Anyhow, I think that ugly distortion is due to the opamp - that's really where the distortion is generated.

[rody82]

Well, measured everything:

V+: 15,7V
V-: -13,8V
BIAS: -13V

TUBE:
1: 4,7V
2: -13,9V
3: -13,5V
4-5: 13,3V AC
6:  2,1V
7: -14V
8: -13,5V

OPAMP:
pin4: -13,8V
pin5: 15,1V

There must be some problem somewhere...

[Paul Marossy]

I didn't measure everything in mine at the time that I built it, but  I start out with 12.8VAC. My V+ is 11.8V, V- is -14.6V and bias is -13.6V.

You're maybe a little high on your voltages. I used a 10ohm resistor in series with my transformer to knock down the incoming voltage to 12.8VAC. Maybe you could try that and see how that works for you.

EDIT: Just thought of something. You are sure that your tube is wired correctly, right? A lot of people wire the tube socket wrong and then it doesn't work right...

[rody82]

I didn't measure everything in mine at the time that I built it, but  I start out with 12.8VAC. My V+ is 11.8V, V- is -14.6V and bias is -13.6V.

You're maybe a little high on your voltages. I used a 10ohm resistor in series with my transformer to knock down the incoming voltage to 12.8VAC. Maybe you could try that and see how that works for you.

EDIT: Just thought of something. You are sure that your tube is wired correctly, right? A lot of people wire the tube socket wrong and then it doesn't work right...

If the layout shows the bottom of the tube, then i'm sure its wired correctly.
I'll try lowering down the voltages a bit.

[rody82]

Probed the output of the opamp, and i dont like that type of distortion.
With the gain at max (or past 12 o clock) i can hear an unpleasant type
of (hard to describe) "limiting" ? (Not that gating, farting sound some noticed
at previous topics, i've already solved that one)

Used a 270pf ceramic at the feedback loop. (Thoug the DMM measured
it at about 250) Does this make the sound harsh?

By the way, i was wondering why that assymetry in v+ and v- is necessary.
(Tubes?) I mean why should V+ be much higher (for example 14V) than V- (around -11V)
Can this cause that harshness at the opamp?

Or do i want too much and this circuit is best at doing light overdrive tones?

[Paul Marossy]

Probed the output of the opamp, and i dont like that type of distortion.
With the gain at max (or past 12 o clock) i can hear an unpleasant type
of (hard to describe) "limiting" ? (Not that gating, farting sound some noticed
at previous topics, i've already solved that one)

I think that's just the opamp clipping the signal. (literally limiting the signal)

By the way, i was wondering why that assymetry in v+ and v- is necessary.
(Tubes?) I mean why should V+ be much higher (for example 14V) than V- (around -11V)

I think it's to create asymmetrical waveforms.

Can this cause that harshness at the opamp?

I think you're just hearing the nature of solid state clipping. What do you mean by harsh?

Or do i want too much and this circuit is best at doing light overdrive tones?

Probably. What tube are you using? You could try a 12AT7 or a 12AU7, too. Some people like it better with those tubes.

[rody82]

What tube are you using? You could try a 12AT7 or a 12AU7, too. Some people like it better with those tubes.

I have only 12AX7, but several different types. Sovtek, Peavey, JJ. Not too much improvement on the distortion type. Its
generated before the tube.

I think you're just hearing the nature of solid state clipping. What do you mean by harsh?

I mean the sound is not very well defined (especially i can hear this when playing chords), and it
has a hint of fuzziness too... (a bit like BigMuff distortion, but that sounds better)
I like my BSIAB2,ThunderChief much better, so i think i have a misbiasing problem here,
or maybe i have a wrong type of transformer or some other problem

When playing the neck pickup on my strat with gain low it does have a nice bluesy sound though...

[Paul Marossy]

Well, the Shaka Tube circuit is a "starved plate" design. It means that the preamp tube is operating out of its linear region. It's not going to sound like the BSIAB II or similar circuits - it's a different animal.

[rody82]

Maybe I'll make some sound samples to decide
if its a problem, or just my taste   

[Paul Marossy]

Maybe I'll make some sound samples to decide
if its a problem, or just my taste   

That might be a good idea. 

[rody82]

Well, here is the link i hope it works:

www.hszk.bme.hu/~sr428/Rody_ShakaTube_BridgeHB.mp3

Recorded with an Ibanez S470, and a Roland Cube 30. Used the
recording out on the amp.
There are short passages, gain increasing. The first was recorded
with very low gain, and the last with gain on max.

Opinions?

[Paul Marossy]

That sounds pretty much normal to me. Like I said, it's a starved plate design, so it's going to sound different than other types of circuits.