ChunkChunk Q3 source resistor

[oldhousescott]

I'm done stuffing the board for my Chunk Chunk and had a question about the source resistor for Q3. It's spec'd at 3k9 (and that's what I put in there), but the original Boogie (and SLO from whence it was derived) has 39k as the cathode resistor for that stage. I've done a cursory search at Gaussmarkov's site and at DIYstompboxes, but couldn't find any pertinent info why the value was reduced by a factor of 10. I was wondering if any has tried using a different value there. Seems like a larger value would help reduce the gain and consequent squealing that often accompanies these builds. I know you can't directly compare FET and tube circuits, but I did read one post where 39k was sub'd for 3k9 and it helped with stability.

[madbean]

I didn't know that, actually. You can certainly increase source resistors to dial it down. The bias trims also give you some flexibility as well as the use of 2n5457 instead of j201.

[madbean]

Also note that the TMB controls are reduced by a factor of ten, although for a different reason. I think that was to reduce the overall impedance of the tone section.

[mgwhit]

The gaussmarkov project PDF (http://gaussmarkov.net/layouts/drboo/drboo-project.pdf) confirms that the tone stack rescaling was to lower the output impedance.

This thread (http://www.diystompboxes.com/smfforum/index.php?topic=76030.0) discusses the 39K source resistor on Q3.  The suggestion is that it's done to facilitate biasing that JFET, but there is no definitive answer there.  The information about the "cold clipper" stage is interesting, though.

[oldhousescott]

This thread .....

Thanks for the link, that's just the kind of thing I was looking for, but not finding for whatever reason at DIYStompboxes. I think I'm going to try changing out R9 for a 39k and tweak the trimmer by ear.

I noticed PaulC contributed on another thread that was linked in the original thread. How cool is that?!

[mgwhit]

Google this: dr boogey 39k site:diystompboxes.com

I've found that Google is frequently better than most internal search engines.  I found more threads than I cared to read last night.

Long story short, that triode stage was designed into the SLO100 to add asymmetrical clipping rather than gain, and made its way into the Dual Rec for the same reason.  No one seems to know why that resistor value was lowered in the design of the Dr Boogey, but since it's difficult to bias Q3 with the 39k source resistor and the 100k trimmer, that probably had something to do with it.  If you want to try the 39k, you'll probably need to increase the value of your trimmer.  Using the stock 3.9k, you're getting more gain and compression out of Q3.  A simpler way to fix this might be to replace the entire JFET stage with a clipping diode of some sort, but that's left as an excercise for the reader.