Why is my amp oscillating ?

Hi. I try to build a 100 W guitar amp. I purchased the Project 27 board from Rod Elliot which works fine when I use a frequency generator or an iPod as input. But when I connect the preamp to it, it starts oscillating. First I thougth the preamp layout is bad, but the funny thing is that the preamp works fine on its own. So when no load is connected to the preamp it works fine, no oscillation. I measured it with an oscilloscope and it does exactly what it should. But as I said the whole thing starts oscillating when the pre- and the poweramp are connected. I used shieled leads as input, as connection between pre and power, and also as output to the speaker jack.

Its my first DIY amp and I really dont know what to do. Could you help me ? Should I post pictures from the whole thing ?
I wired the jacks and the pots as shown in figure 1B, except i connected the preamp direct to the poweramp without the effects loop. I did not have the connect the circuit ground to the chassis. I know that this is dangerous and also illegal I guess, but for testing purposes I deciced to try it without.

I dont know what exactly the frequency is, because the scope is in my school and I did not write it down. But it was around 3-4 kHz
The frequency was about 3-4kHz, I cant tell you exactly because the scope is in my school. I wired the pots and the jacks exactly like shown i figure 1B, except I did not use an FX-Loop, so i went directly into the poweramp from the pre-out. To prevent hum I did not connect the circuit ground to chassis in the test phase. I know that this is dangerous and maybe illegal but I thought i would better when testing.

Edit: Sry for the double post, but I got a lag or something, somehow I did not see my previous post.
Last edited:
Since I dont have anything to measure at home I can tell you tommorow what exactly affects the oscillation. Here are some pics, maybe there is an obvious reason which I just dont know/see or whatever. First I tried it like this (with shorter cables of course)

Then I tried it like this: