Asking for some advice debugging BB Preamp please - solved

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Asking for some advice debugging BB Preamp please - solved

mdtcarroll
This post was updated on .
Hi There,

Happy New Year folks!
I'm hoping you fellow pedal junkies might be able to offer some advice please. I've built the BB Preamp from http://tagboardeffects.blogspot.com.au/2012/10/xotic-bb-preamp.html.

The basics are ok. Bypass works, foot switch toggled gives me huge volume boost. At almost min volume , I get close to unity :-). So it sounds like a fully cranked Marshall, which is to be expected.

The gain pot does not seem to have any effect when I play. At zero gain it's pretty high gain - and maxed it sounds no different. However without playing I can hear a bit of a difference when sweeping from min to max gain. It sounds like the hum increases slightly with gain (not a bad hum, just the kind of mild hum you would expect to hear when turning up an amp from min to max without any playing).

Also, I don't seem to hear any difference when switching between the two diode combos (I'm using an on/on dpdt)

So the issue is around the gain / volume clipping area. I built it with the 47k substituted for a 22k as suggested. Wondering whether I should sub the 151p for a 220p as suggested in the thread? Might help with the high output volume?

I swapped the Drive 1 and Sw1-5 wires at the switch as I wasn't sure if polarity mattered there?

I've scraped along all the gaps between the copper to ensure there are no bridges etc.

Any other suggestions to check before providing voltages and pics to dig deeper?

Regards,
Mark

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Re: Asking for some advice debugging BB Preamp please

mdtcarroll
Adding pics...





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Re: Asking for some advice debugging BB Preamp please - solved

mdtcarroll
In reply to this post by mdtcarroll
Hi There,

Well I managed to sort it out after trying a number different things. It turned out that the 9V jumper on the 3rd bottom row had a bad solder joint. I guess it was powering the transistors but not the IC's. Not sure it explains the behaviour I was getting, but hey, it works now - just like the demos. The volume is still pretty high. Unity is just under nine o'clock!

Thanks all! Even though there were no responses, just the process of having to articulate what's been done and checked certainly helps in the troubleshooting process. I still believe one of the best quotes in troubleshooting is this:

"A problem well defined is a problem half solved"!

Happy New Year!

Mark