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Hi can someone tell me how to check bias and dissipation etc on the el84's.
I've built a few amps but want to know a bit more.
What pin is what i.e plate voltage pin and which ones are screen resistors etc.
I was looking at ampmakers site and he says check the following
EL84 cathode-to-plate voltage
Combined EL84 cathode current
Combined EL84 screen current
Average EL84 plate current
EL84 cathode-to-plate voltage
Any help welcome and any books you can recommend that aren't too technical.
Thanks.

1 - Measure the voltage across your cathode resistor. Divide that by its resistance value, and then divide the result by two for the cathode current of a single power tube.

2 - Measure the voltage across a screen grid resistor and divide the voltage by the resistance value to get your screen current. If the screen resistor is a shared between two valves, then divide the result by two to get the screen current for a single valve.

3 - Subtract the screen current from the cathode current, to get the true anode current.

4 - Multiply that number by the *difference* between anode and cathode voltage. That is each power tube's dissipation calculated accurately.

HTH

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Which pin goes where on the plexi.
Is the cathode pin3 150r, the plate pin9 100r, are the screen resistors pin2 8k2 for the el84's.
Do you just measure the el84's only and don't worry about the preamp tubes.
These might seem stupid questions to you but I'm still trying to understand this stuff
before i build anymore.
I really appreciate the help.
Thanks.

For the EL84, the cathode looks like pin 3--electrons are boiling off there. The signal comes into the grid -- pin 2. The signal is amplified thanks to these extra electrons and passes through the screen -- Pin 9. It is collected at the plate (pin 7) and sent to the OT. I'm no expert, but that is the way I read it. Hope I'm right since I'm building one for a pal right now!

Lee

In your amp you may find a single 100 ohm resistor shared between the two screen grids. Some 18Ws use 1k instead.

Also the anode is connected to the Output Transformer.

BTW "Plate" is a very old-fashioned term from the days when they had a filament and literally a disc or plate-shaped anode. These days they're almost always cylindrical or sometimes hexagonal. They also used to call capacitors "condensors" - goodness knows why.

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On the trinity plexi it should be something like this.
Measure the voltage across your cathode resistor 150r 5w pin3.
Measure the voltage across a screen grid resistor 100r pin9
That gives me screen current and cathode current.

Thanks for that LeeA, i hope it's right because it made more sense now.
I'm not sure how to quote btw.
can someone let me know if that is right.
Thanks.

If those resistors are shared between two power valves, then they are carrying double the current,. So for a single valve you have to divide each one by two.

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My amp is the standard trinity plexi which does share the resistors.
Just out of interest zaphod which resistor do you increase to 1k which i've seen you mention
somewhere else to do with bias i think and what is the purpose/difference.
I'll have a go at measuring this weekend and see where i am.
Many thanks for the help.

There is more than one way. In the case of your amp you can change the 100 ohm screen resistor to 1k. Your EL84s will thank you for rescuing them from their fiery ordeal, and will therefore live longer. You will also get a touch more compression in the sound.

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Can someone tell me if this is right.
When you measure across the resistors do i go from ct to the other side or do i need to go from both sides as i'm not sure i measured it correctly.

1 - Measure the voltage across your cathode resistor. Divide that by its resistance value, and then divide the result by two for the cathode current of a single power tube.

cathode 12v divided by 148 ohm = 0.081 divided by 2 = 0.040 (is this read as 40ma)

2 - Measure the voltage across a screen grid resistor and divide the voltage by the resistance value to get your screen current. If the screen resistor is a shared between two valves, then divide the result by two to get the screen current for a single valve.

Screen grid 316v divided by 98 ohm = 3.22 divided by 2 = 1.612


3 - Subtract the screen current from the cathode current, to get the true anode current.

40ma - 1.61 = 38.39

difference between anode and cathode = 323
multiply 323 by 0.038 = 12.27 for dissipation.

Not sure if that's right, can someone let me know.
Thanks.

"Measure the voltage across" means exactly what it says. You connect one probe at each end of the resistor or component in question, and literally measure the voltage differential across it.

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Great tones come in small glass jars!