Fixed Bias Amps

[charvel750]

Can someone explain how fixed bias amps work? Does the circuitry somehow bias the tubes to itself? Or do you have to select tubes to match the amp? I have a few fixed bias tube heads and have measured the plate voltage at 420 V, but the EL-34's I have are not measuring correctly to match that. Please explain, thanks.

[jackson1]

I can't tell you much apart from: No, a fixed bias amp does not adjust itself. The bias is adjusted manually on a a trim potentiometer (resistor). The tech does that when changing tubes to help them operate correctly in a given amp. I think fixed bias is usually used for power amps above 50W, whereas "cathode bias" is used for lower power amps like preamps.

[Cygnus X1]

I'll wait for DonP this time.
I know, but I use too many words to explain it.
Let's just say the nomenclature is misleading.

[DonP]

Hey i am to save the day (thinking of that Andy Kaufman skit).

First, just like people confuse MIJ Charvels with Ft Worth "USA" Charvels, there is confusion over Fixed Bias vs. Cathode Bias.

True fixed bias means your tubes are connected directly to ground, and a fixed negative voltage is applied to the grid to keep the tube at the correct idle setting. The tube amp manufacturer saves money on the parts that let you adjust the negative voltage on the amp to match the tube's sweet spot.

Cathode bias (sometime incorrectly called fixed bias) is where you don't have a negative voltage on the grids, but you have a resistor between the cathode and ground which makes the cathode positive compared to the grid (at zero volts). The nice thing about this design is the tube "levels itself", but the bad thing is you don't get full power with that resistor, and so other issues as well (good for rock, but not for metal).

Think of a tube as a water valve in your house, and the water is electricity. The water you make sound is by turning the valve on and off really fast. You hand can only turn so far however, so you want to have the water valve in a middle setting so that you don't shut off the water when turning lower, and you don't want to hit the full open point on the upside. You want your hand to be able to turn freely both ways without hitting full open or closed. Setting the valve in a middle position is just what bias is doing in a tube - setting it so that the tube won't bottom out or hit the ceiling.

[Cygnus X1]

The single power tube, single ended SEL is cathode biased.
It does metal better than rock or blues.
All that gain comes from the preamp stages.

But, true...with a push pull class B or A/B, it would turn to mud real fast with a cathode bias. The SEL operates mostly in class A. (But that's a whole other discussion, one which seems to cause a lot of controversy. And which I just walk away from, not learning much).

[DonP]

Class A amps are almost always cathode biased - I don't think I've ever seen any other type of bias for them. It's just real easy to do.

Non-cathode bias is dangerous. You have a negative voltage on the plates to keep them from running away. If something goes wrong with the negative voltage bias circuit - BOOM!

[Cleveland Metal]

I'm thinking you guys still didn't quite answer his question...

That, or I'm still half asleep... haha...

[Cygnus X1]

I'm thinking you guys still didn't quite answer his question...

That, or I'm still half asleep... haha...

DonP gave him an explanation of the two major biasing schemes.
Yes he can have the amp biased to those new tubes.
Always rebias when swapping out power tubes on a fixed bias amp.
(Exception is if the tubes are preselected, like Mesa does).

Cathode biased amps generally have a bit more tolerance, so it isn't always necessary. But I check it regardless.

[charvel750]

OK, maybe it's the words I'm using here. My amp does NOT have an adjustable bias pot. There is no way that I know of to adjust that, it lacks the trim pot inside. So is that a fixed bias amp? I don't care what it's called. What I want to know is do you select tubes that draw the proper current get it where the bias formula works?

[DonP]

Your amp is Fixed Bias.

It was designed to run "safe" on any tube you select, as long as you select the proper tube (like, if it's a EL34 tube, don't be sticking a 6550 in there). You supposed to be able to run any EL34 tubes.

Lets say with a Marshall and an adjustable bias. The Bias can adjust from lets say -25 to -40. With a fixed bias amp, it would be set to maybe -35 to -40, just to make sure the amp doesn't have thermal runaway.

[charvel750]

Thanks, Don. Weird thing is that when I tested the present tubes they are reading around 20. I have a pair of GT's that when installed test at 45. So it seems like you have to choose tubes that fit the range to make the amp run most efficiently. Is there a way on a fixed bias amp to modify it to make it "unfixed?" How so, and is it an easy job?

[emperor_black]

Thanks, Don. Weird thing is that when I tested the present tubes they are reading around 20. I have a pair of GT's that when installed test at 45. So it seems like you have to choose tubes that fit the range to make the amp run most efficiently. Is there a way on a fixed bias amp to modify it to make it "unfixed?" How so, and is it an easy job?

I believe you can do it. the 5150 is a fixed bias amp and I got replies from Peavey itself telling me that the amp's bias cannot be changed in its stock form. However, FJA mods offers a bias pot mod which installs a pot on the circuit which allows you to bias the amp. Also, King amplification in san jose offered that same mod for aroun $100.

[marcus]

Thanks, Don. Weird thing is that when I tested the present tubes they are reading around 20. I have a pair of GT's that when installed test at 45. So it seems like you have to choose tubes that fit the range to make the amp run most efficiently.

This is true. Not all non-adjustable bias amps run well with whatever tube you throw in there. Mesas in particular are built with the philosophy that you shouldn't have to bias your amp, however they just seem to want you to buy their tubes. Incidently, a lot of tube suppliers on the internet will sell you tubes that test to operate in Mesa's specs.

Is there a way on a fixed bias amp to modify it to make it "unfixed?" How so, and is it an easy job?

You can remove the bias resistor and add a pot, although how difficult it will be to perform depends on the amp.

[marcus]

The single power tube, single ended SEL is cathode biased.
It does metal better than rock or blues.
All that gain comes from the preamp stages.

But, true...with a push pull class B or A/B, it would turn to mud real fast with a cathode bias. The SEL operates mostly in class A. (But that's a whole other discussion, one which seems to cause a lot of controversy. And which I just walk away from, not learning much).

If your amp is single ended with a single power tube than it does operate in Class A. Or, it sounds like ass Operating "mostly in class A" is not class A at all, but class AB. As for class B, I don't know about with tubes, but with transistors you would never use this kind of an amp for a musical application as they have boatloads of crossover distortion. It may sound groovy for metal though.