"SHAKA TUBE" - almost working ??

[transient]

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

So, are you saying that it works *normally* for a few seconds 'till it suddenly becomes quiet and "clean"?

.
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[wui223]

Yeah, as i strum chord it produce sound then the sustain will went off after about 3 seconds, it is a suddenly drop, just like 'pop' the sound gone.
Can i use another transformer which is 12-0-12 center taped and rated 6VA which i believe is 500mA ?

I use audio probe last time but cant solve my problem, maybe i should try
i am thinking of integrate a signal generator and amplifier to audio probe, so that i dont have to plug to guitar amp. What do u think?

[Paul Marossy]

It sounds like you're describing a mis-biased opamp...

[aron]

Yes, sounds like something is misbiased.

[wui223]

Misbiased? How to solve it? My electronic knowledge is far behind urs, i need to work hard.

[aron]

Post your pin voltages first.

[wui223]

Here there are:
V+: 12V
V- : -10V
BIAS: -5V

OPAMP
4: -10V
8: -12V
OTHER PINS ARE AROUND ZERO

12AX7
1: -2.8V
2: -9V
3: -8V
4: 12VAC
5: 0
6: -10V
7: -8V
8: -8V

Something must be wrong coz all tube pin measure negative voltage

[wui223]

Finally got it sound after a recheck, one spot is miswiring :oops:
But the hiss is extremely loud why? and i couldnt turn the gain pot past half rotation, else the hiss is more and more and the volume will drop significantly. Anyone know solution?

[Paul Marossy]

Finally got it sound after a recheck, one spot is miswiring :oops:
But the hiss is extremely loud why? and i couldnt turn the gain pot past half rotation, else the hiss is more and more and the volume will drop significantly. Anyone know solution?

Look for questionable solder joints and connections to ground. Also, verify that the pins of the preamp tube are making good contact with the socket.

[wui223]

Paul could u take a look at ur PCB layout? I was confused bout the connection of gain pot, in schematic centre lug is connected to pin1 (output) while in PCB centre lug is connected to pin2. Which one is correct?

[Paul Marossy]

It doesn't matter, it's wired as a variable resistor. If your gain pot works backwards from what you expect, just switch the outside lug connections.

[wui223]

Yeah i got it. Why in my built, the pin 1,2,3 of TL072/RC4558 didnt show any voltage? Do u think it is normal? I use strip board for this project. I am sure i have all the ground tied up. But the hiss/hum is extremely loud when i turn the gain pot up. the only usable range is just 1/4 of the full rotation. Do u think i can swap the transformer with a 500mA 12V output transformer?

[puretube]

Hmm, this thread is becoming a little "annoying"
.... EVERY week  !

grrrrr ....

MM.

http://www.diystompboxes.com/smfforum/index.php?topic=39797.msg284871#msg284871

[MartyMart]

Sorry Ton, as John Cleese said in a fish called wanda  " I apologise unreservedly"  !!
Yup,  I DID reverse the tube pinout .... 
Now I'm going to go away and "hide" for a week  !!

MM.