5E3 bia requirements?

[zorro]

Does the 5E3 ever need it's bias adjusted or checked? Should a bias pot be added for adjustment if it does?

[steve]

Does the 5E3 ever need it's bias adjusted or checked? Should a bias pot be added for adjustment if it does?

The 5E3 is a cathode bias amp. I've heard cathode bias as being refered to as "self biasing." On the stock schematic, there is a 250 ohm, 5W resistor that is connected from the cathodes of both 6V6 tubes to ground; this resistor sets the bias. You typically don't add a pot to set the bias, it is a matter of measuring the voltages and replacing the resistor if need be.

It is a good idea to know how to check the bias to see if the tubes are over their dissipation rating. Having said that, I've heard of the tubes being run way over the dissipation ratings for decades and the bias has never been checked.

To contrast, the 6G2 Princeton has a fixed bias supply. The cathodes of the 6V6 tubes are connected directly to ground and a circuit supplies negative voltage to the grids of the 6V6 tubes. I don't believe that Fender used a pot to adjust the negative bias voltage, so in this case this is where you would add a pot to make the bias adjustable.

HTH,

steve

[zorro]

I was worried that if the 6V6 tubes are not matched there might be a problem. But if like you say this is the way the 5E3 is typically operated, why tamper with a proven circuit.
Thanks

[stokes]

zorro,tube matching is a different issue than biasing.For optimum performance,tone and hum reduction your power tubes should be closely matched,there are many opinions as to how closely matched they should be,but many agree that within 5ma's is okay for guitar amps.Like steve says,it is a good idea to check that the tubes you are using are operating in a safe range,as different tubes will draw differently.In my cathode biased amps I use 500ohm wirewound adjustable resistors and tweak them any time the tubes current draw drifts or when I change tubes,its probably not that critical,but I came across a supply of these resistors for free a long time ago.

[bob p]

great minds think alike. i have a mixed bag of vitreous enamel power resistors that i got on eBay, and i've used adjustable ones for cathode bias too. on occasion i've even used an adjustable resistor with two taps to individually bias the tubes. the only real drawback to doing this is that you'll need an extra cathode bypass cap.

[stokes]

I got about 8 or 10 of those 500ohm 25w adjustable Ohmites for free when my supplier in lower Manhattan retired and closed up.He gave me a shopping bag and let me wander thru his stock rooms in the basement,what a score I made that day.I still have tons of parts,but I sure wish I grabbed a few more of those adjustables,still have a bunch of fixed value 10 watters,but those adjustables are great.Wouldnt using the 2 taps make it more of a bias balance?I got a 25watt 500 ohm rheostat that I use to dial in the right value and just use a fixed value as close as I have.