5E3 bias

[Randall]

I just built two Triode 5E3 kits with Magnetic Componets trafos. I'm getting the following:

plate - cathode V = 354v
cathode resistor measures 269 ohms
voltage across cathode resistor = 22.7v
current measured with bias probe = 43mA

My math says this is 15 watts. I am running JJ 6V6s. Is this too high, and what should I do if so?

[JazzGuitarGimp]

I get 13.98 watts. Remember that when you calculate power dissipation, you multiply the current by the voltage from the cathode to the plate. In this case, that would be 354 - 22.7 = 331.3V time the current: 22.7 / 269 = 84.4mA / 2 = 42.2mA

0.422 x 331.3 = 13.98 watts, which is pretty much where you want it.

[martin manning]

If I'm reading Randall's post correctly, the 354V he quotes is Va-k. The listed Vk and Rk put Ik at 42.2 mA, which includes screen current. On that basis dissipation is 14.9W, right at the upper limit, IMO.

[Randall]

That's right, plate voltage read from ground is 377v.

I've read from people I trust that this circuit fares well at around 350 volts on the plates with a 250 ohm cathode resistor. Could that mean 350v from ground and not from cathode?

[telenut62]

You're all ok if you keep using JJ's

[martin manning]

In the other recent 5E3 bias threat I wrote:

"A quick look at Fender's '57 Deluxe schematic shows they are running at 14.8W (250R shared Rk, 349V Va-k), which works, I guess."

If Fender is selling a production amp biased like this it must work well enough. As long as you don't see any red-plating you are ok, but I would expect to replace tubes more often than if it were running a bit cooler.

[Gordie]

If you wanted to cool the bias a little, you could try a 300ohm resistor next. What I've done to "split the difference" is run two 560ohm resistors in parallel. I ended up with a 283ohm resistance.

[Aaron Smith]

I've been working on a 5b3, which is the same output section. I bumped the cathode resistor up to 270, then to 300... the dissipation didn't change significantly. Hence the term "auto-biasing" in reference to cathode bias. You can raise the resistance, but you're just going to see a bigger voltage drop across the resistors.

I think the only real solution (assuming a solution is needed) is to drop the plate voltage.

[Randall]

I just realized that the NOS Black plate RCA rectifier I have is a 5Y3, and not a 5Y3GT. I put a GT in there just to test, and the voltages came down about 10 volts, putting me at about 13.5 watts with no other changes.

Mystery solved. I just ordered a couple of 5Y3GTs.