BSIAB II help

[njkmonty]

Ive recently built a Bsiab 2
http://gaussmarkov.net/wordpress/circuits/brown-sound-in-a-box-ii-bsiab2/

Its squealed like buggery on highest gain settings, i used shielded cable , everything, no change,

then I put a 33K resistor from effects input to Ground.

It then removed all squeals and still had all treble available.
Is this ok to do?
is there a logical reason why this happened?

[GibsonGM]

If it works, it's fine   Not causing any odd tonal issues, then I would not worry about it.

What is happening is that you are experiencing oscillation due to (probably) lead dress or insufficient power supply decoupling.  There is a capacitance associated with your first gain stage (all gain stages, in fact, it's called the Miller effect)...with the input impedance, you form a resonant circuit that 'keeps the power going around the loop' - a feedback loop.  Usually you try to get rid of the parasitics (improperly run leads etc) rather than add parts, but...

Your R changed the resonant frequency enough to stop the loop.   

The only odd thing I see is that the 1M already there should do the job of the 33k you added...so, all R's/Z's in parallel must have changed by just enough with your 33k to do the job.   

I expect ALL high gain circuits to squeal at high settings - it's part of the territory.  But if you found a quick hack to limit it that works, and doesn't bother you, power to ya!