English Channel - oscillations

[stringsthings]

i'm getting oscillations when the volume and treble are full up ... is there any place where i can put a small cap to tame the squeals?

schematic: http://www.runoffgroove.com/englishchannel.html

i shielded the input wire and the wires to the volume pot and that helped somewhat ... the circuit is on vero and i'm hoping the problem isn't built-in to the layout ...

[crane]

Did you try adjusting trims?

[stringsthings]

yes.  the voltages on the transistors are pretty close to where they're supposed to be.  the circuit sounds fine most of the time. would increasing the cap across the volume pot help with the oscillations?

[PRR]

First: move some wires around. There is a LOT of gain here so layout is _critical_.

100pFd gate of Q4 to ground.

[stringsthings]

got it working!

the 100pF on Q4 helped a bit ... moved some of the wires on the treble pot and noticed an improvement ... went ahead and shielded those wires ... much better  .... i'm surprised at how sensitive this circuit is  ... i guess there's more gain involved than i first thought ... i may go ahead and shield the other wires tomorrow and see what it does ...

[PRR]

> i guess there's more gain involved than i first thought

Q1 = about 25
Q2 = about 25
Q3 = about 1
tone = about 0.1
Q4 = about 10
Q5 = about 15

25*25*1*0.1*10*15 = about 9,000

If 1/9000th of Q5 Drain signal sneaks back to Q1 Gate, it may oscillate.

If 1/350th of Q5 Drain signal sneaks back to Q4 Gate (a hi-impedance point with lots of sneakage potential), it may oscillate.

Assuming 30pFd impedance at Q4 Gate, we need only 30pFd/350= 0.1pFd of stray coupling from Q5 Drain to Q4 Gate area.

Twisted-pair has 30pFd per foot. 0.03 inches of TP would be too much.

We don't run Q5 Drain and Q4 Gate in the same TP, but if the wires are an inch long and an inch apart we may be in trouble.