"SHAKA TUBE" - almost working ??

[MartyMart]

So, story so far :
Circuit boards built as per Pauls layout, but on vero.
Thank you very much Aron/Paul BTW  

Subs:
The 50k resistor (?) is a 47k
The input cap is a 0.047
Tone pot is a 50k

The AC PSU was giving me 18.5 volts ( no load )
I used an "in-line" 680 Ohm 1 watt resistor, giving me exactly 12 volts !
For around 10 seconds ( while I got my meter on it ) I had 15.7 volts
going to the board/valve  !!

So:
With the "gain" at 100% I have a nice "medium crunch" overdrive :-)
Tone/gain/vol pots are working, the power board trimmer is set at half
resistance and doesnt seem to make a difference.
With the gain set to anything lower than 3/4, I get a "square wave" very
nasty distortion with signal "cutting in/out" all the time, like a very
"splatty" badly biased transistor circuit would sound, but worse  !!

Ideas ?
Did I "screw" the 12AX7  ??

Thanks, as I said "almost there" !!
Marty.

[petemoore]

I doubt you've damaged the tube, if it is lighting, and relatively 'new' it's almost certainly 'good'.
 The easiest way I know of to test a tube in a known working circuit is to compare it with a known good tube...

[Paul Marossy]

You didn't mix up Pins 6 & 8 on the tube, did you? They're kind of obscured on my layout. Maybe that is the problem?

[MartyMart]

You didn't mix up Pins 6 & 8 on the tube, did you? They're kind of obscured on my layout. Maybe that is the problem?

AHA !!
...... NO, i spotted that one, my connections are right unless I have the
whole thing reversed..... but that would not work at all !!

Just popped in a brand new 12AX7 and its the same .... there's an awful
"cut out" going on along with this very "square wave" sound.... almost
a "digital" sounding thing .. wierd unless its the PSU cutting out, or
not supplying correctly via that 680 Ohm resistor ..  ??
It happens even at max pot rotation, but is worse lower down, seems
like I can't get a "cleaner" gain setting at all ..

Damm, so close !  :x

Marty.

[Paul Marossy]

Well, I guess this is not going to be a simple fix.   :?

OK, well, what power supply are you using? I am using a 13VAC 800mA wall wart with mine.

[markr04]

From what you've described, you have the same problem I do. If so, it's not the tube, but the opamp.

http://www.diystompboxes.com/sboxforum/viewtopic.php?t=35100

[Paul Marossy]

I have an unconfirmed suspicion that this circuit is sensitive to the opamp. The original schematic is using 1/2 of a TL072, and my layout uses a TL071. All I can think of is that this somehow affects the operation of the opamp, I don't know how, but it seems to be the case....

EDIT: Can one of you guys try to wire it up using 1/2 of a TL072 instead and see what happens?

EDIT #2: I etched my PCB per the layout I have on my Shaka Tube page, but when I built mine, I modified it for use with 1/2 of a TL072. Mine works, and you guys seem to have problems getting yours to work. There might be something here.

[MartyMart]

Mark04, thats interesting, it DOES sound like we have the exact same problem......
OK, my vero layout is a little "tight" for re-arranging with a TL072.. so
how about some other "single" op-amps.. NE5534/LM741/TL070 I have.??
One "extra" component I used a 1M pull down resistor right at the input
before my 0.047 cap

Marty.

[Paul Marossy]

MartyMart-

Try all of those single opamps and see what happens. Maybe this is a case where the actual opamp used will make a noticeable difference. That 1M resistor to ground at the input shouldn't mess things up at all.

[MartyMart]

OK:
NE5534ap - just farts a lot
LM741 - farts very quietly !
TL070 - farts very quietly
Another TL071 - Same as original - occasional "Eureka" followed by lots
of "farts"    :cry:

Do we need a resistor "into" pin 3 here ... like a 1k5 or something ??

EDIT : It's such a shame, when I get the minute of "working" sound it's
really wonderful, in particular with the new Marshall ECC83 valve :-)

EDIT2 : I dont understand how 1/2 a TL072 would be any different ??

Marty.

[LoKi6922]

its pretty hard to damage a 12ax7... hmm... maybe try building the 'diode comp. opamp' for this one?

thats what i was planning on doing for my shaka tube. with everybody talking about it, i just had to start building one...  :lol:

too many things on the go for me... heheh

[Paul Marossy]

Do we need a resistor "into" pin 3 here ... like a 1k5 or something ??

Worth a try, I guess.

I dont understand how 1/2 a TL072 would be any different ??

This is my quandry as well. There must be some kind of difference between the two of them, otherwise we wouldn't be having this discussion.  :wink:

[MartyMart]

Hmm, just thinking through some "similar" circuits .. the "Rat" for instance, has a 1k resitor before pin 3 and also some voltage coming
down from a 1M resistor from 4.5 volts to the input  ....
I may try this and see if we can "kick" the input a little
BTW what's the purpose of the 470k to ground right before pin 3 ?

Marty.

[puretube]

http://diystompboxes.com/sboxforum/viewtopic.php?t=35100&postdays=0&postorder=asc&start=0

there has been another PS problem with this circuit here, lately...

[Paul Marossy]

BTW what's the purpose of the 470k to ground right before pin 3 ?

I'm guessing that it's to set the input impedance. Maybe adjusting this resistor value might have some effect on the gating problem as well.

[puretube]

that`s the bias resistor to centre on 0V, while the opamp worx on +/- something...

[Paul Marossy]

that`s the bias resistor to centre on 0V, while the opamp worx on +/- something...

OK, that makes sense. I am a little ignorant about opamps running on a dual power supply.  :oops:

[MartyMart]

that`s the bias resistor to centre on 0V, while the opamp worx on +/- something...

OK, could we have found the problem ?
How about a 500k trimmer here..... perhaps its not 0 volts   ?

Marty.

[markr04]

Marty, take a look at the opamp thread. I played with values here and got no further.

But maybe it needs to be specific as only a trimpot can be?

[aron]

It must be some sort of weird problem with the layout or power supply layout.

There's nothing special about the front end of this circuit.