need help getting crunch

[nickfox]

Hi Guys

I have a question about my fernandez ZO-3 travel guitar. It has a little 3W transistor amp built in. When the battery starts to go dead it develops a wonderful crunch that sounds even better than my 5W laney lionheart. So I have two questions.

1) Is this along the same principle as what Eddy Van Halen did to get his brown sound with a Variac?

2) What would be the best way to alter my little amp to permanently get the crunch? Can I add a pot in there to try and dial it in?

thanks
Nick

[nickfox]

The main reason I joined Amp Garage is because I bought a Trainwreck Express kit which should be here in a couple of days. I apologize if asking about a transistor amp is considered bad form here in the forum.

n

[Malcolm Irving]

When a battery goes flat it gives a lower voltage and has a higher internal resistance. You could simulate that by putting a resistor in series with a good battery. You could also have a switch bypassing the resistor to return to normal. I would aim for about 1 Volt drop (but experiment might be needed). You need to know how much current the amp draws to calculate the value of the resistor, but let's say it draws 100mA you could use a resistor of 1/0.1 = 10 ohms.

[nickfox]

Thanks for your reply. Do you think a 100 ohm pot like this would be ok? It looks like it's rated at 1/2 W. Plus this costs 1 euro for five and says free delivery. Not exactly sure how they're making money off of that...

https://www.amazon.fr/TOOGOO-Potentiome ... e+100+ohms

n

[Phil_S]

If you use a 100Ω pot when the calculation calls for a 10Ω pot, I think it will be a problem to get the right resistance dialed in. The rotation on the shaft is about 270-300 degrees. If it is linear (which it should be), 10Ω on a 100Ω pot represents about 30 degrees of rotation. Given the very small size of the pot, you will have a really hard time controlling it. By contrast, a 10Ω pot with 300 degrees of rotation will give you 1Ω every 30 degrees.

The suggestion of 10Ω may not be correct. It looks like an educated guess to me. See if you can figure out how much current your amp draws so you can look for the correct pot. It's all about whether you can control the beast by dialing in the correct resistance. I see the 100Ω pot as pure frustration.

[Malcolm Irving]

Yes, Phil_S is right, we need to know how much current is being drawn before we can estimate the resistance needed.

[nickfox]

I took it apart and had a look and took some pictures. The amp is a Toshiba TA7313AP. One of the pictures I posted is the data sheet for the amp.

And also, there is a very interesting little pot on the board that has VR written under it. Do you see it? I wonder if this adjusts the voltage to the circuit...

Here is the transistor on the board. It's a C1815 Audio Frequency Amplifier & High Frequency OSC. I'm not sure what it's purpose in the circuit is. It's pretty close to the input from the pickups. It looks like the input goes across the C14 cap and into the transistor. I think the transistor may be the pre amp. Maybe. Someone with more knowledge than me is going to have to answer.

http://www.futurlec.com/Transistors/C1815.shtml

n

[sluckey]

VR usually stands for Variable Resistor.

[Bob Simpson]

<delete>

[Malcolm Irving]

According to the datasheet for the Toshiba chip, the output power is about 0.5W. (Not sure why you thought it was a 3W amp?). It also states that the amp chip is dissipating about 0.75W. Assuming it is running from a 9V battery, these figures imply a current from the battery of (0.5+0.75)/9 = 139mA when the thing is running flat out. So to get a voltage drop of 1V would need a 1/0.139 = 7 ohm resistor.
If you usually run it at less than maximum output, a larger resistor would be needed.

[Phil_S]

http://www.ebay.com/itm/2pcs-BOURNS-336 ... SwJ7RYW5Pt
Cheap but you'll have to wait for delivery.

[nickfox]

I made a mistake about the wattage. I'm diagramming this thing right now. I really want to try and figure out what that trimmer pot is doing. I'll post the schematic as soon as I have it done.

n