I did some more tweaking and I managed to get rid of the oscillation. I started with openloop and it was stable but a small 50Hz oscillation. I'm assuming this has to do with ground loops. When connecting the feedback loop, it stated oscillating like crazy. It had a low frequency oscillation of 3-400Hz and a 16Vp-p and a higher frequency one (1-1.2Mhz) over. Once I added the RC zobel network on the output, the low frequency oscillation went away but I still had the high frequency one at around 300kHz and around 5Vp-p. I added an RC (200pF+1k) over each half of the primaries and the frequency of the oscillation went to around 400kHz and the amplitude dropped to below 3Vp-p. I then changed R21/R22 (from the images in my previous post) and the oscillation went away. There is still a low 50Hz oscillation but this might be a ground loop. I then applied a 1kHz signal (200mVp-p) on the input and the output looked very nice. The strange thing is that when I used a 10kHz input, the output amplitude went way down (0.2Vrms from 1,3Vrms). So there is definitely a capacitor (or more) that have a wrong value and that affects the HF response. I had a quick look but everything seemed fine. There is the lead capacitor that has a strange value in the original schematic (274) and that I assumed it was a 27nF and I used a 22nF. I tried adding a 1uF in parallel with this but nothing. The stability of the amp is still not 100% as it still oscillates in some cases. I'm now expecting a delivery from Mouser with some more components. I will increase the value of the capacitor from the RC networks placed on the primaries. I will try with 1nF and I will increase/decrease the resistor. At the moment the high HF attenuation is the one concerning me. As this might also be the culprit for the oscillations. Ah, I almost forgot, I also tested with square wave on the input (1kHz) and the output was almost a triangle. So there is definitely some HF roll-off somewhere.
Stop trying with the feedback, first get it stable without(And do test this to into Mhz regions as you already noticed it instable there)
If the base amplifier without feedback get to oscilate there is something wrong somewhere else.
You can't make a instable design stable by adding fb, fb connected wrong wil get you a oscillating amp(wil give you positive feedback).
1kHz signal (200mVp-p) on the input and the output looked very nice. The strange thing is that when I used a 10kHz input, the output amplitude went way down (0.2Vrms from 1,3Vrms)
That is not a wrong cap value! possible caused by the oscilation. The lowest freqency passed to the next stage depands by this value, not the higher frequency's.
The 50Hz oscilation; you checked this with scope, arta?
Can't it be that those are just hum from heater or powersupply?
Rem.