Calculating the plate dissipation

How hot is your amp running?

The EL84 was designed to be running at about 12w - that's to say that at this output it will last pretty much forever.  Obviously part of the appeal of valve amps is the overdriven sound at various points - clipping preamp stages, power tubes running hot and (to a lesser extent) speaker distortion.  So running the EL84s at beyond their rated dissipation adds to the tone, at the expense of their operational lifespan.  This can be controlled by changing the value of the cathode resistor, but it's probably a good idea to know how to calculate it and make a judgement based on that, rather than just eyeballing the tubes to see if they're glowing red hot.  These are my calculations which I think are correct, based partly on the Amp Maker testing notes.


                                                   Cathode voltage (D)
Cathode current  =                 --------------------------
                                               Value of cathode resistor (120 ohm)


                                          Voltage over Cathode resistor (D-E)
Screen current   =            -----------------------------------------
                                              Value of can cap resistor (1500 ohm)


                                            Cathode current - Screen current         
Plate Current =                   ------------------------------------
                                                                   2


Cathode-to-plate voltage  =  Plate voltage - Cathode Voltage



Plate dissipation   =   Plate Current  x  Cathode-to-plate voltage

Or:

It's alive!

Just a quick check for output from the amp.  The mic on this phone is crap but I don't have a better one.

Troubleshooting

Point B has 0v when it should have 83v.

Tracing the circuit back, the voltage at B is going to be coming from V2 pins 2 & 7 (grid) and pins 8 & 3 (cathode).

Tracing it back further, the anodes on V2 (pins 1 & 6) go back to the 100k resistors (18 & 19).  Which are connected to...  Hang on a minute...

Completely missed that connection on the blue dashed wire (running under the board).

Time for an ugly jumper.

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.