My voltage readings

 

I think I've found the problem.

Logic

My thought process is:

• OT is fine
• Speakers have been making noise (that big hum I got when I first connected it with the can cap ungrounded)

Therefore, the power stage is probably working correctly.

So the problem is probably somewhere in the preamp stage.  Let's hope I can trace it with the multimeter when I measure my voltages to the ones I'd expect.

Testing the voltages

So now it was time to start methodically testing the voltages in the amp with a multimeter to see which bit I'd cocked up.

Based somewhat on the testing plans over at Amp Maker and Tube Depot I came up with a diagram and some typical voltages that I should expect at each test point.

With the black multimeter lead grounded (either at the ground bus or on the chassis), with all valves installed and both switches on, the expected voltages are:

F - 345v
H - 320v

A - 1.4v
B - 83v
C - 3.4v
D - 11v

And removing the black from the ground:

E-D - 10.5v

Testing the output transformer

In order to get rid of the nagging paranoia that I'd blown the OT, I thought it was a good idea to test it.

There are some excellent instructions here.

I only performed the simple tests (resistance between winds) but it all looked OK still.  Major relief.

Power up

There's only so much testing you can do before you have to bite the bullet and plug it in. 

There are some detailed test plans but frankly, I couldn't be bothered with all that.  I was keen to see if my amp worked.  I did at least take the precaution of powering up for the first time with no valves.

All OK.

Added all valves, powered main switch.

All valves glowing nicely.  Waited a couple of minutes.

Moved standby switch to "on".

HUGE HUM.

After powering down and unplugging the amp I put it back on the bench and looked for any wiring problems.  After about two minutes of staring I saw what I'd missed - the ground wire for the cap can wasn't connected to the star ground, it was just curled under the turret board.  Whoops!  Stripped, tinned and soldered then time for a retest.

NO HUM.

Right.  Time for the signal test.  Plugged guitar into normal channel, edged up volume carefully...

NO OUTPUT.

Bollocks.  No time for any further investigation so powered down amp.  Paranoia sets in.  Could be a dodgy valve (can be eliminated by swapping them round) or could be something more serious - like the most expensive component of the amp, the output transformer, which can't even be replaced as the boutique maker doesn't make them anymore...

Testing - Continuity point-to-point

This is a useful test to do with a print out of the layout.  Work from one side of the amp to the other and trace the wire runs.  Test for continuity from the furthest possible points, like the valve side of the valve socket right back to the leg of the component on the board, shown here:

Testing - Continuity / Non-continuity

There are some vital tests that you can do with your multimeter on the continuity / diode check setting.  It helps if it has an audible beep to indicate continuity.

This is fairly crucial:

  

If you don't have continuity between the earth prong of the plug and the chassis (and also the shield connection of a guitar lead plugged into any of the jacks) stop now and fix it!

It's also wise to test all adjacent turrets and adjacent valve pin connections for continuity, just in case of "whiskers" shorting out the connections (you shouldn't get these if you've properly tinned your wires, but it's worth checking).  It's worth disconnecting the positive valve heater wire from the transformer for this test otherwise the valve heater pins on the power valves will show as shorted, due to the low impedance of this wind of the power transformer.