Tube Cricket Debugg

[Pushtone]

I built the Tube Cricket from the Beav's website and it needs a little work.
Any help would be appreciated but first, full project details.


The Schematic:
http://www.beavisaudio.com/Projects/TubeCricket/TubeCricket_Final_2008.gif



The Symptom:
It works and has some very good tone but a constant and loud hum is present.
It hums with only the speaker plugged in. The hum is not influenced by the input.
If I plug in a guitar lead and touch the tip I get that familiar hum too underneath the constant hum.



What I did:
The only substitution from the schematic is the chip amp itself.
I used an LM386-N3 instead of the JRC386D as on the schem.
I know the Beav writes the LM386 dosen't work well in the circuit but since its a text-book datasheet application I can't figure why it wouldn't.
Other than that I eliminated D1 from the Beav's power supply section. Eliminating D1 did not effect or cause the constant hum.

So far I have eliminated the 9V reg section with no effect on the hum.
Removed the LEDs, no effect.
Soldered a second decouling cap before the speaker jack. Tried a 1uF film and another 220uf electro in both pol. directions, no effect.
I was going to try a 1000uF cap but not holding any hope it will have an effect.

I've audio-probed the circuit right up to the input of the chip amp on Pin-2 and it all sounds clean without this hum.
Why does the chip amp hum? Is this what the Beav meant by "LM386 does not work well here"?




What I built On:
The tube section I built on a lug board.
For the chip amp section I made a PCB.

On the PCB I included a regulated power supply to produce the 12VDC the Cricket wants and a second reg for a couple of 9V outs to power pedals.
(some Workhorse gave me the idea to include a 9V supply in an amplifier  )
Also on the PCB is the power supply and chip amp sections from the Tube Cricket.

Here is the schematic I made to help me make the PCB.
Component numbers refer to the Beav's schematic only.





Here is the layout I made from the schem above.






The Voltages:

AC power input: 19VAC
After a 10Ohm 5W resistor: 16VAC
Output of BR1: 19.4VDC
Output of BR2: 17.98VDC
Pin-3 of 7809: 8.98
Pin-3 of 7812: 11.78 (I think my meter is calibrated a little low, should read 12V)

Chip Amp
1 1.34V
2 0V
3 0V
4 0V
5 5.43V
6 11.97V
7 5.51V
8 1.34V

12AU7 Tube
1 Plate 5.77V
2 Grid  -0.1V
3 Cathode 0.23V
4 Heater + 11.97V
5 Ground 0V
6 Plate 6.32V
7 Grid -0.1V
8 Cathode 5.71V
9 NC

I also metered the speaker out jack
Speaker Output: 4.1VDC
Thats the hum right there!!


This would sound so good if were not for the constant hum.
Chip amp builders, thanks in advance.

Dave S.

[GREEN FUZ]

I have some interest in this schematic too, as I intend to build it. Unfortunately, all I can offer is that the symptoms sound like a grounding issue. Also, stating the blindingly obvious, why not try the JRC386D and see how that works ?

[ambulancevoice]

ive read before that the LM386 is not as stable and well working as the JRC386

[Pushtone]

I moved C11 farther away from C1 on the PCB layout but that didn't help.

Doubled checked all the grounds. The circuit seems to be working. But I have hum.

AFA getting the other chip... I'll have to wait until I can put together a sizable parts order.

Strange thing is I've looked at both data sheets and the example application is the same.
Same thing for the Ruby amp at ROG. The parts and the way they are connected are the same for both chips.


Dave S.

[dano12]

Have you tried jacking the signal in just before the 386? i.e. bypassing the tube section to see if the hum/noise still exists?

Also, try supplying the 386 at pin 6 with Vcc instead of Vin. Vin isn't filtered, not sure why I did that....

[Bifrost]

Hello,

My first post here on forum. Just want to say that hum on speakers is probably from bad power supply. If u getting hum try other ac/dc adapter...

I have also build tube cricket, it sound beautiful, but have one little problem. Whenever I hit strings on guitar LED on tube cricket goes fade and than i get short buzz on speaker. Like power is not stable. Anyone could guess what is problem? I tried few things, and would like to try connecting dc directly without stabilizer, but don't have exactly 12V power supply, would this circuit survive 16V? Also here is the sound clip http://www.youtube.com/watch?v=8yQgX3nyOFM problem is not noticeable always, only if it louder or hitting more strings at once, and it is only for a moment hardly noticeable but still annoying me.


Edit: I try connecting dc directly without power regulator, same thing... Something drains power somehow, can anyone confirm me that led should be fading a bit while play guitar via tube cricket? Problem could be something else like input signal or bad speakers, or also could be tube itself...

[DavenPaget]

Not enough current .

[Bifrost]

Thank you on reply. Do you have Tube Cricket so u sure, or it is your opinion? Because i try it with 16V without voltage regulator and with voltage regulator and still it had drops in both cases.