Monthly Archives: April 2014

Killing the Hum – Now it’s really done!

I have done a lot of research and asked a lot of questions over the course of the last three weeks. Unfortunately the Metroamp forum has crashed and no longer a source of information. I turned to another forum called The Amp Garage. A lot of people jumped in to help and many offered suggestions. I was curious about the “Larry” grounding scheme some people have incorporated. I also thought that phase inverter filter cap might be bad as the hum vanished when I pulled the phase inverter tube. A leaky diode could also be the problem as my research hinted. I decided to make some changes, but it would mean pulling the board and redoing a lot of wiring. Since this is my “forever” amp I decided to go for it. I found a JTM version of the Larry grounding which I adapted to my 2204 circuit. Since I was pulling the board anyway I decided to make some other changes I wanted to do. They are as follows:

– Swapped out the top hat diodes for Ultra Fast UF5408
– Swapped all of the 22AWG wire on the board for 20AWG
– Swapped out the skinny shielded input wires for Lava Mini ELC
– Added a 1000pf bright cap (mustard)
– Incorporated the “Larry” star ground scheme

These changes took a couple of days but I took my time and did them carefully. I ran the same voltage tests and fired up the amp. I thought I had mis-wired something because it didn’t seem to be making any sound at all. But low and behold, plugging in my Tele proved differently. The amp blasted to life. The hum was gone and it sounded even better than the first test! I jammed on the amp for a while. I felt proud. This thing took damn near a year but I eventually got it done.

Am I really done? No way. I want to record some sound clips. I also want to change the JJ tubes and try some N.O.S. glass. Mike from KCA tubes and I have been discussing which tubes fit into my budget. That’s a good one – budget! I have also acquired a 1990 Marshall 1960a 4×12 slant cabinet as I believe this amp deserves some serious paper to push the sound through. I have ordered a separate head shell cabinet from Mojotone so I can stack it with the 4×12. This will give me the option on putting the amp in the combo cabinet for smaller gigs, or swapping it to the head cabinet for bigger venues.

What a great day.