Do you have to bias pre-amp tubes?

You do not have to bias pre-amp tubes, period.

You normally do have to bias power tubes.

BUT ...

Many/most amps that run EL84 power tubes do not have to be biased, because of their electronic design.

Kiwi

 

I'd like to hear more about this. Do tell.

 

Well. not all EL-84 amps are cathode biased so check before making assumpions.

Fixed bias means that the bias doesn't change during the normal operating process. Most fixed bias amp can be adjusted. I like to adjust the bias when I replace the tubes, and again in about 6 months. Other than that I leave it alone.

Cathode biased means that the bias comes off of the tube itself. Often it's called self biased. With this method the bias shifts constantly as the signal shifts. You'll get more "sag" and "bloom" on what you play, but it's less aggressive. Since EL-84 tubes are not terribly agressive, most amps with EL-84's are self biased to get max bloom and sag, plus make the ckt less expensive and tube replacement easier.

Preamp tubes are almost always self biased, although some vintage and re-issues are "grid leak biased" which is another form of self biased.

I hope that helps.

 

Incorrect. You absolutely have to bias preamp tubes. Try grounding the cathode and see if it works. (Assuming standard Marshall or Fender type topology).

Preamp tubes are self biased. That's what the cathode resistor is for. Go ahead and measure voltage on pin 3 or pin 8 to the chassis. You should read -1V to perhaps -2.5V. Lowering Rk (cathode resistor) brings the voltage closer to zero (closer to saturation), where increasing the resistor lowers the voltage (closer to cutoff). This of course assumes your grid is at 0V and referenced to ground through a grid resistor. The designer of the amp chooses Rk to bias the stage in the operating range he is looking for. People who modify amps frequenty re-bias the stage for a different operating characteristic (by changing the resistor). It is generally not something the average user messes with.


BK

 

KIWI- actually MOST modern EL84 designs overbias the EL84's.

Max plate dissipation for an EL84 is 12W. Designs such as the Fender Pro Jr, Blues Jr., Vox AC 30 and other have them biased at 15W to 19W!!!!

Fender designs are Fixed Bias, so a bias control trimmer must be added. Vox appears to realise they've overbiased it as there are 2 X 100 Ohms in parallel for the common cathode resistor. Generally you can just clip one out. If they like it hot I experiment with various values from 240 Ohms to 470 Ohms for the second resistor to make it run at between 11 and 12W IPD.

The only designs I've seen that run the EL84 at correct levels are
from Orange and Bad Cat.

You MUST take screen current into account. Ik-Is=Ip.
Ip X Vp = IPD. (EL84's have a significant Screen current in relation to the cathode current, in all makes of EL84.)

Generally speaking, if the plate voltage is below 400V and the cathode resistor is 820 Ohms or above you can assume a pre-amp tube is biased at a sane level.

-Jon

 

I figured I might get jumped on by the tech people here.

The original question was asked by a relative newbie, the kind of amp owner we all once were, asking about those glowing things in the back of his amp that are swathed in such electronic mystery.

I'll stand by my original reply, which was intended for the majority of amps and amp owners.

The rest is a bunch of overcaffeinated tech heads eager to split hairs and electrons.

You know perfectly well that you can swap out 12a_7 pre-amp tubes all day long without harm, and that a lot of EL84 amps are self-biasing.

Especially the ones where the freakin' amp designer himself (Dr Z, in this case) says you can swap EL84s without rebiasing.

Kiwi

 

You don't need to worry about biasing preamp tubes if/when you change them... which I believe this addresses the original question asked.

As for power tubes, regardless of biasing scheme, if you don't check them you are risking either a grossly over biased or grossly underbiased condition... both of which will make an amp sound less than optimal and in the case of an underbiased condition will shorten the life of the tubes.

Self-biasing would require some sort of on-board computer to monitor current draw and then some sort of servo controlled pot to maintain it.

 

What he said!

Only the last sentence I don't know about. I'm would think there is some kind of current sensing/limiting circuit that could be built without the use of computers and servo controlled pots.

But I think the point is that cathode biased amps do need to be checked and adjusted just as fixed biased amps do. If you've spent the money for an amp like a Dr Z, you did so because you wanted top line tone and excellent reliability. So spend a little money to have it biased properly, or learn how to do it yourself. If this isn't important to you maybe a good solid state Peavey would be just fine too; afterall you don't have to replace tubes or have them biased. Simplifies things even more.