My 14 year old nephew has also been "blowing away" others at his school with his version of the amp he built.
However I have noticed every now and then a strange feedback issue that occurs in the reverb circuit. In fact is is only in the reverb recovery part of the circuit, I can remove the 12AT7 from the first part of the reverb circuit and the problem still occurs.
The FB is somewhere in the range 600 - 800Hz and once it starts , usually after playing a single note in a run, the only way I have been able to stop it is turn off the amp or bump/move the tank. Not the perfect solution if in front of an audience. As my nephew had experienced similar problems (all be them less frequent than I had) I started to think about this from an problem solving perspective.
So after some experiments which included connecting a tank outside of the cabinet, connecting to another reverb tank located in another amp, building a "Faraday cage" to put the tank in, fitting a smaller 25 watt speaker into the cab, changing the orientation of the reverb tank, I came to the conclusion that the problem was magnetic feedback, not the normal electrical or acoustic/vibrational feedback.
Because the amp is a "pocket rocket" and uses a 50 watt speaker, the magnet is far closer to the reverb tank than what would normally occur in a standard Princeton Reverb set up. So the feedback starts and is magnetically inducted by the coils in the reverb tank. Move the tank 50mm back away from the speaker ... no issue. Move it closer to the speaker...even more feedback.
This shows how close the speaker magnet is to the reverb tank
So what to do ? what to do indeed? I didn't really want to get another cabinet slightly taller as i really like the compact style of the PR size cabs. So I looked in a totally different direction.
Rather than try and fix the distance issue between the tank and the speaker, I decided to look into magnetic shielding
and sourced some Mu-Metal Foil, which is designed for shielding low-frequency electro magnetic fields.
MU Medtal is an alloy of nickel, iron, molybdenum (80% molybdenum)
Order some sheets and it arrived two nights back.
My idea was to make a (origami style) box for the tank to sit inside, which in turn the box and tank would be put into the reverb tank bag as per normal
I had visions of having to strengthen the foil with a backing of some sort , so the "box" would not get crushed inside the reverb back. However when the foil arrived it was obvious this would not be required . 0.2mm thick steel sheet is pretty stiff and hard to bend !!! So I cut out my origami box
Then folded it, with the aid of a hammer and a hunk or steel I had handy.
By this stage it was 10pm and I had not realised that I had not even had dinner yet... decided to just put the tank in the box, then everything into the tank bag and see if it solved the issue. Turned on the amp... played a few runs and .. and .. shit ... the feedback was still there.
Annoyed, I went off to make dinner and think about the issue.
Came back an hour later with the notion that I should cover up / fit the final end of the box, just for completion sake. So I made the final end piece, fitted it to the box and repeated everything. Turned on the amp, played away and YES.. no more magnetic feedback !!!! I had solved the problem. I sort of remembered by second year electronics classes that talked about when shielding magnetic field you have to ensure all 6 sides are covered otherwise you are wasting your time... well those old professors were correct.
While I had the back of the amp off, I decided to fit a Weber 10F150-O smooth cone speaker that I had had for a couple of months. I had been running it in to see if it would loosen up a bit. Well it has definately loosened up and sounds great... seems it needed the 20 or so hours of break in to start to sound good.
put everything back and the amp is perfect again.
)
