Amplifer Tune Up

Amplifier Chassis Here the chassis has been removed from the cabinet and set on my bench.
Filter Capacitors A quick peek at the filter caps to view solder joints and any signs of failing caps.  Looks okay.  I suggested the 3 original 40-year-old caps be replaced but the customer was on a tight budget. 
Power Cord Inspection of the power cord.  Looks good.  I also check that the 3rd prong on the cord is in fact electrically connected to the amplifier chassis.  This is important as I often find the 3rd prong to be broken electrically but still attached!  Yikes!
Remove Tubes Here are the pretty li'l tubes all laid out in a row.  I remove them for easier access to bolts and to retension and clean the tube sockets.
Tighten Hardware Tightening transformer bolts.  These very often need tightening, especially on older amps.  You don't want a transformer to fall off the chassis!
Tighten Hardware Tightening each nut on each pot.
Tighten Hardware Tightening each nut on each jack.
Spray Tube Sockets Spraying contact cleaner into the pins of each tube socket.  I then reinsert the tubes a few times to loosen and scrub off any corrosion that may exist within.
Retension Tube Sockets Retension each pin in each socket so the tube fits tight.  Most older amps need this done to rid of intermittent crackles and pops that happen when the tube is physically vibrated by the speaker.  One of MANY reasons I think combo amps are a bad idea.  Don't get me started on that rant!
Clean Jacks Spraying contact cleaner in each jack.
Clean Jacks Cleaning each jack with a phone plug after spraying contact cleaner.
Clean Jacks Spraying contact cleaner into the rear panel RCA jacks.
Clean Pots Spraying contact cleaner in all pots.  When a pot is scratchy I find that 75% of the time this does NOT fix it.  The pot usually needs to be replaced. 
Check Tube Voltage Measuring the plate voltage on the power tubes.  I can then calculate what kind of current reading I'm going to want to get.  You can see that the power tube plates are sitting at 447vDC.
Tube Current Before Here's the current that the tubes are drawing before adjusting the bias.  You can see that they're at 23.72 milliamps, or .02372 amps.
Adjust Tube Bias Adjusting the bias pot.  This Fender amp conveniently has an internal adjustment pot for the bias.  Since the power tubes' cathodes are at ground and their grids have a negative voltage applied to them this is called fixed-adjustable bias.  Getting closer to where we want to be now.
Tube Current After

Voila. A nice bias point for these JJ 6L6 power tubes.  These tubes have a maximum plate dissipation of 30watts.  They are rated 5 watts higher than all other 6L6 brands which are typically 25watts.

Now for reassembly, a quick test drive, and finally a call to the customer.

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