Ignorant Question #637 - Cathode Resistors

[Andy Le Blanc]

I smell toast.

[tonestack]

I have a 2xEL84 amp that I built thirteen years ago that has a 3W cathode resistor. I have torture tested this amp, and the cathode resistor still shows no sign of heat fatigue. The cathode resistor on a Champ dissipates approximately the same power as the cathode resistor on a 2xEL84 amp (half as much current, but at almost twice the bias voltage), and there are tons of forty-plus-year-old Champs out there with their original 2W bias resistors.

[tubeswell]

Yep 2W should be fine for 9.15 Volts on 100 Ohms, because like I said before, at that voltage, the resistor is dissipating (91.5mA x 9.15V =) 0.837W (you might as well say 0.84W). However stick a 5Wer in if you're concerned about it. I prefer build my amps for lots of tolerance anyhow, so I'd probably use 5W there anyway.

[Andy Le Blanc]

A champ may not be the best example, it is certainly a demonstration of a
2W resistor being used as a bias resistor, but in a SE design the max current
condition occurs with no signal, and I believe a champ PT develops only
around 70ma. Similar current is developed across the bias resistor but
conditions are not the same at max signal nor the potential for a runaway
failure as a push pull circuit.

[solderstain]

This is an interesting discussion. Moreso than I expected it to be. Since this is my personal project and not a customer's amp, I think I might try 2w resistors just for a giggle. If it smokes in my living room (this is my 'living room amp'), only my cat will be upset.

[tonestack]

A champ may not be the best example, it is certainly a demonstration of a
2W resistor being used as a bias resistor, but in a SE design the max current
condition occurs with no signal, and I believe a champ PT develops only
around 70ma. Similar current is developed across the bias resistor but
conditions are not the same at max signal nor the potential for a runaway
failure as a push pull circuit.

What condition for runaway? Short of a dead short across the cathode resistor, a cathode-biased amp is self-limiting. We would have to pull ~152ma through a 130 Ohm resistor to dissipate 3W. At 152ma, we would develop a negative bias voltage of ~ -20 VDC (not to mention pull the B+ down on the average 2xEL84 power transformer). With a typical plate voltage of around 320VDC (and a power transformer capable of delivering at least 152ma of current without the B+ sagging), that bias voltage would leave us with a cathode to plate voltage of ~300 VDC. Both of the EL84s would be shut off with a cathode to plate voltage of 300 VDC and -20 VDC on the control grids.

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