Biasing a Fender Blues Junior Question

[jar0ahr]

Is there an adjustable bias on the blues junior or is it fixed bias? I have biased many of my bigger amps with 6l6's and el34's but not an el84 based amp yet. I just ordered an el84 bias test lead.

Thanks. John

[52 Bill]

Is there an adjustable bias on the blues junior or is it fixed bias?

The original circuit is non-adjustable, but you can either change resistor values or find a way to add a trimpot to make it adjustable.

[stokes]

The schem I have is very fuzzy,but it looks like you could change R31 to a 50k cermet pot to make it adjustable.

[ctcpete]

I just stumbled on this thread. If you're still looking, Bill M sells a mod for this purpose. I installed it recently in my own Blues Junior and it was pretty simple. It is a good idea as from what I understand the power tubes run pretty hot at stock set up. Here is his site comcast.net/~machrone/bluesjunior.htm

[buffalono]

I have a cathode bias EL 84 amp too, a Trace Elliot Velocette. I need to check the bias on it because the only tubes that don't redplate are an old pair of rattling JJs. I've tried 2 new pairs of JJs and they both redpalted as well as NOS Slyvannias and some pull that checked out ok on a emmision tester. I've checked the bias, the cathode resistor, used contact cleaner and touched up the solder joints. Could this be a too hot bias problems too?

[Enzo]

Since it is cathode biased, the grids should sit at zero volts DC. Is that the case here?

[buffalono]

Yes 0 volts is what I measured. I'm going to check the plate dissapation next. Won't do it today because I have certain Valetines Day obligations I had better attend to.

[ctcpete]

Confused abiut redplating

Just how much glow is normal for these power tubes? Is it normal if component near the tip (the plate I believe) is glowing red?

[izzybell2810]

Just how much glow is normal for these power tubes? Is it normal if component near the tip (the plate I believe) is glowing red?

The plate is the solid square piece of metal from the base almost to the top of the tube. If it is glowing red then it's said to be "red plating" this shouldn't be confused with the orange glow coming from inside the plate which is the heater fillaments.

Dave.

[Enzo]

There is the little orange glowing heater in the center of every tube. This has to be there. SOme of them stick out more than others and are more visible. That is also normal. The glow we don't want to see is the actual metal structure inside getting red hot. if you see the whole length of the metal thing in the center of the tube getting red hot, that is bad.

[tubeswell]

I know this is

1) an old ghost thread, and

2) I'm probably going to be frowned on because it has nothing about plate dissipation in it (but Fender obviously didn't care about that when they designed the BJ, so...)

Here's my quick'n'dirty 1 resistor bias mod for the BJ.

No need to take the boards out. Just solder tack 150k in parallel to R51. That's it.

Heres the math. 150k||33k = 27k. 22k/(22k + 27k) = 45% of Vin. (Vin is about 26V)

Shifts the bias voltage to about -11.5V (with the plates sitting at about 320V), which is better than the -8.0V it comes with from the factory (where the plates are sitting at 300). Makes your EL84s last longer.

(Although, strictly speaking, one should measure the tube current, and plate voltage etc and install an adjustable bias voltage mechanism)

This has been field tested and approved, by me.

[Randall]

It is always good to hear from you kiwi.