I have an '87 4104 (2204) that I have modified to have an extra gain stage. Amp sounds decent so far and was playing quite extensively last night with no issues. After shutting it down I noticed the power transformer and the output transformer were quite hot to the touch. I got concerned as I have never noticed any of my Marshall amps getting that hot, well the older JCM800 era amps anyways. This amp still has its original transformers but it has new filter caps installed. I checked and it is running the proper rated fuses for safety. I started thinking the heat could be the tubes, maybe the EL34's. I do need to load in a new set of EL34's as this has an oddball mix of one JJ 6CA7 and one Svetlana EL34. They are biased close with 2mA of each other while the 6CA7 is running a bit hotter and the 6CA7 is drawing 6mA through the screens while the EL34 4mA through the screens. Plate is 480V and I have the bias set at aprox 31mA/29mA shunt method measured across OT windings. Heaters are running 6.9VAC due to my high wall voltage. I do have a bit of hum which is audible if i turn the master volume up 8 to 10, which is normal for a high gain amp anyway. Last night I started rolling some different 12AX7's through it tp see if the hum would clear up a bit more. That is when I had some oscillations happen. The oscillations only happened when I put an RFT ECC83 in V1. I initially had Chinese 12AX7's in it. I put an RFT in V2. Then when I added the second RFT in V1, that is when the helicopter noises kicked in. This happened with no guitar plugged in and master volume off. When I turned the gain down to 0 the oscillations stopped. Tried a different RFT in V1 and same problem. So I am now wondering if the oscillations are due to my high gain circuit and the RFT's possibly having much higher gain than the previous tubes I had in it? As for hot transformers, I am going to load in a new matched set of EL34's later and see if that helps. But as for the oscillations, I was already thinking along those lines when I discovered the transformers being hot. I was thinking that there could be oscillations happening that were not audible but causing the transformers to work harder. Then when I stuck the RFT's in there I did here oscillations. So that was a trip for sure. I do not have a scope to check for this. And is there another way to check for oscillations that are not audible? I might go ahead and increase the snubber cap across the phase inverter plate resistors from the stock 47pF to 100pF for the time being. I already have at least one snubber cap in the preamp, on the second gain stage if I am remembering correctly. The RFT's I have were a set of four supposed NOS. All of them did the same noise when I tried each one.