Saturday, 14 March 2015

Playing electricians roulette

I'm tweaking an amp I built back in '06 to change the tone, hopefully to be a bit crisper and less congested in the mids, which means lots of swapping out components, testing the thing with the chassis open to errant fingers with >300V running through.

Fun times. :-)

Well, I survived..... so far, anyway.

It feels good to be doing something technical again, using a different side of my brain & learning.

This amp was based on a Fenderised version of traditional 12AX7 + EL84 designs with a negative feedback loop to give more headroom & help it stay clean for longer. I tried different values of resistor in the NFL (current value about 116K) but reducing to 48K caused a volume drop and increasing to 220K seemed to produce some kind of resonant overtone and reduce the 'singing' quality of the amp. Removing NFL altogether gave very little clean headroom, and it was just flabby & tubby. In the preamp stage I'm using 15K cathode resistors, and had a coupling cap only on the second triode, so popped a 1uF cap on the cathode of the first triode and added a 470pF cap across the 100K resistor coupling the 2 gain stages together.

Between the 2 tweaks it's popped a bit more sparkle into the amp and reduced the mud somewhat while keeping it cleanish at a reasonable volume level. If I could run it through a 12" speaker it would be quite loud, but it's about to go into a dead Marshall MG30 combo that's been laying around (thanks Ben Morris, if you're reading this). Maybe it will even get an outing to church tomorrow. :D

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