"SHAKA TUBE" - almost working ??

[aron]

It sounds better but I still think that the tubes are still not biased correctly yet.

You might try changing out the bias trim pot....

Anyone else have any ideas???

[puretube]

Peter: you`re the 1st,
that doesn`t instantly reply: Futterman!

[Paul Marossy]

Probably not the case, but are you sure that you have the bias trimpot connected up properly? The wiper is what would be connected to the bias network. Are you absolutely sure that your bias network is connected to the wiper of that trimpot?

In this circuit, how the bias is adjusted will influence how it sounds by making the signal on the grids (Pins 2&7) of the 12AX7 more or less negatively biased. It almost sounds like you have a wire crossed somewhere, and the plates (Pins 1&6) are becoming a negative DC voltage instead of a positive DC voltage as a result. The voltages that you measure on the cathodes (Pins 3&8 ) will be dependent on what is going on at the plates (Pins 1&6).

Basically, what is happening in your tube is that it is in a "cutoff" mode, and it can't conduct much current. As a result, you get that blatty, gated sound.

The fact that those negative voltages on the plates don't really change much with the tube pulled out seems to point to a wiring error...

[markr04]

Thank you all for your help! I put a 25K trimpot in there and cranked it CCW. I have:

V+: 14.7
V-: -13.2
bias: -13.3

1: 9.0
2: -13
3: -13
6: 9.0
7: -13
8: -13

How are those? I don't hear any more gating at this point.

I made new recordings. These are 4mb .wav (for quality) files of different tubes in it.

Chinese 12AX7 (I think it's Chinese. Pulled from a 1995 Peavey 5150):
http://www.siteofmark.com/Chinese_12AX7_(from_5150).wav

Electro-Harmonix 12AX7EH:
http://www.siteofmark.com/Electro-Harmonix_12AX7EH.wav

Sovtek 12AX7WA:
http://www.siteofmark.com/Sovtek_12AX7WA.wav

[Paul Marossy]

Well, now those voltages are looking better! And it sounds a whole heck of a lot better, too. You look a little low on the V+ voltage at the tube, though. You'll have a little more headroom if you can get that to around 14V. What transformer did you use (V, mA)?

[markr04]

Paul,

I used a 12vac 800mA. I have a 1w 10ohm in series before it ever hits the PS board. Maybe I ought to remove it?

[Paul Marossy]

Paul,

I used a 12vac 800mA. I have a 1w 10ohm in series before it ever hits the PS board. Maybe I ought to remove it?

You know, I was just thinking this morning... I forgot about those 50K resistors on the plates of the 12AX7. A measurement of around 9VDC on those sounds about right. I never actually measured mine.  :oops:

What is your voltage on Pin 4? If it's within 10% + or - of 12.6VAC, you're fine. If not, you may want to adjust things a little bit. It's not really that critical if you're a little higher than 10%, your tube just won't last quite as long - not really too much too worry about since preamp tubes last a very long time compared to power tubes.  

[MartyMart]

You know, I was just thinking this morning... I forgot about those 50K resistors on the plates of the 12AX7. A measurement of around 9VDC on those sounds about right. I never actually measured mine.  :oops:

Mine are all 47k's - I've never even seen any 50k resistors !!

I just tried some decoupling caps to ground from the op amp pins 4 & 7
( TL071 remember ) and also from the trimpot + & - to ground, plus a
10uf from "bias" to - on trim pot.
I got a slightly "smoother" sounding OD, but still have a serious "level"
drop on full gain.
At clean gain pot setting, I have a volume lift above unity and no more
"farty" stuff going on.
If I used an output boost on this, it would be sounding fine, though at
full gain, its still only a "mild overdrive" tone .... ??

BTW - it's not re-built yet and is starting to look like "Frankensteins baby"

.. !!!!

Marty.

[Paul Marossy]

I can get a pretty high gain sound out of mine. But, I do have a FET preamp in my guitar, too. It gives some boost, but it's not a huge amount...

[transient]

Mine was high-gain too, i had to do some modding to lower the gain. But i do have that FET pre in my guitar as well.

...
emre

[wui223]

Can somebody tell me why the distortion will loss after some seconds( low sustain) ? I am really stressed

[Paul Marossy]

Can somebody tell me why the distortion will loss after some seconds( low sustain) ? I am really stressed

The Shaka Tube isn't a real high gain circuit, and it's working in a "starved plate" mode, but you should be able to push your amp into a fairly heavy overdrive with it. Mine doesn't have tons of sustain, either.

What kind of tube are you using? Are you sure that it's in good condition?

[wui223]

Mine is a RSD 12AX7, it is a brand new one. the tube light up as usual. But just i could not get overdriven sound from it. maybe the opamp is not working right. I dont see any changes when i turn the biasing pot. WHY?

[Paul Marossy]

What opamp did you use?

[wui223]

i try both TL072 and 4558. i use 1000uF caps in the power board, does it matter? and i use jumper instead of 10 ohm/1W resistor, coz the transformer output is 12+ only. i change the cap parallel to gain pot to 220pF and 47k resistors to replace the 50k used in series with tube. basically thats is what i did

[Paul Marossy]

What voltage are you seeing at V+?

[wui223]

around 13~14+ and V- is -10~v, i use analog coz my DMM blow fuse already. Is the voltage too low?

[Paul Marossy]

Maybe just a little low, but I'm not sure it's enough to cause your problem. In my last Shaka Tube build, this is what I measured at the settings I liked best:

Incoming AC voltage: 12.8VAC
V+:  11.8V
V-: -14.6V
Bias: -13.6V

It sounds like you are in the ball park, but something still seems off. What power supply are you using? (voltage/current)

[wui223]

12-0-12 centre taped transformer, current i think is 1000mA coz i dunno how to measure it. the shopkeeper just tell me it is 1000mA rating.  Did i screw anything up? i can hear very low sound volume but is clean also

[Paul Marossy]

Those specs on the transformer sound OK. You must have something killing your signal somewhere - it should be quite loud and you should have some distortion, too. Time to build an audio probe!  :wink: