First project - runoffgroove Thor. Help needed...

[38eighteen]

Hi,

You may have seen my other posts explaining that I am in the process of building a Guitar amp for my son. The 1st step is building runoffgroove.com Thor to use as a pre-amp.

I found some software called VeroDes that is supposed for help with VeroBoard layout but I just found it "clunky". I did however use it to print out a blank board of the size I was using and roughly draw on the components - not perfect, just good enough that I knew I could fit them all on.

All the bits arrived yesterday and I managed to get it running, but need some help "tuning" it.

First some pics...

Ready to solder:


Soldered top:


And bottom:


I used the "Ready to solder" pic to help double check everything - I printed it and the schematic full page and then traced the circuit and labelled each component on both. It was worth the effort, because Thor thundered first go!

Now for the problems...

I have 2 different 9V plug packs and they both produce about 13V. With this input voltage I cannot get Q2 bias down to 4.5V - I can't get it much below 7. Running like this it has no bass response at all and the "Bottom" switch seems to have no effect. When I run it off a 9V battery (that measures as 11V??) I can get both FETs biased as suggested and it sounds much better. However the Bottom switch still seems to have no effect.

Can I just reduce the output from my plug packs with resistor?

Could I put in a different biasing trimpot for Q2? I wouldn't know where to begin figuring out an appropriate value.

Do I need a good power amp and speaker to hear the effect of the Bottom switch?

If there anything in the pictures above that looks "bad"? There is a mix of different caps because I got my original order wrong and had to get some more from a different supplier. Should I try to find some more 100n ceramic discs and get rid of the green ones??

Thanks,

Brian

[Minion]

Hi, If your Source voltage is 13v then you would bias the transistors to 6.5v ,The transistors get biased to half supply what ever that may be.....

Also You might want to socket the fets because with fets there can be huge tolerances so some fets just will not work in some configurations so you sometimes have to try a few fets to find ones that will Bias and work properly, this is what has allways pissed me off the most about fets and which makes some fet curcuits a bitch to work with......

The caps should work fine as long as they are the correct value and you should also notice the "Bottom" effect when switched on...

Hope this helps....


Cheers

[fdeck]

The high voltage reading from a 9-V battery suggests that maybe the battery in your voltmeter is low.

[38eighteen]

Originally posted by Minion
Hi, If your Source voltage is 13v then you would bias the transistors to 6.5v ,The transistors get biased to half supply what ever that may be.....

Ah, yes - I remember reading that somewhere.

Originally posted by Minion
Also You might want to socket the fets because with fets there can be huge tolerances so some fets just will not work in some configurations so you sometimes have to try a few fets to find ones that will Bias and work properly

More stuff clicking into place... someone else (thanks Brett) told me in another thread that I would need to be prepared to experiment with different FETs and I didn't really understand what he meant - now I do. I'll grab some sockets and a bunch more FETs.

Do you happen to know if "huge tolerances" issue applies to both the 2N5457 and J201? That is, do I need to buy a batch of both types?

Originally posted by Minion
The caps should work fine as long as they are the correct value and you should also notice the "Bottom" effect when switched on...

So, if I can't notice any Bottom effect does that suggest that I have done something wrong or simply that I need to experiment with different FETs?

Originally posted by Minion
Hope this helps....

Sure does!

Originally posted by fdeck
The high voltage reading from a 9-V battery suggests that maybe the battery in your voltmeter is low.

Thanks for the tip - I was still running the cheap nasty one that came with the DMM. I replaced it with a good one and the 9V battery now reads 9.7V.

Thanks,

Brian

[Minion]

Do you happen to know if "huge tolerances" issue applies to both the 2N5457 and J201? That is, do I need to buy a batch of both types?

Yes, all fets have a Huge Tolerance ,you might look into building a simple Fet tester ,This will let you test fets so that all of the fets are within a certain tolerance range...there should be a Fet testing rig schematic over at runoffgroove.....

Cheers