Explain Cathode vs 'normal' bias

[Tubebender]

From the pea-sized knowledge molecule in my brain, I've been wondering the difference between the two on a technical and physical standpoint....
For instance, how come Cathode runs hotter, and is generally not bias adjustable? I have a switchable class A and B amp, and was told to set the bias on the class B mode..... When I test the bias in class A, it skyrockets. In my opinion, it's too damn hot in A when set at a comfortable level in B....
Any input TGP gurus?

[Tubebender]

anyone?

[WesKuhnley]

Cathode bias, also known as "self bias" is a resistor (sometimes bypassed by a capacitor) from the cathode to ground. As the tube begins to draw current, a voltage begins to form across that resistor, raising the voltage of the cathode. The Grid in this configuration is usually held at ground, and the value of the cathode resistor sets the amount of current through the tube. Because this resistor isn't a pot, you cannot adjust it without changing the resistor. Quiesent current is usually higher when compared to a fixed bias amplifier because as the tube draws more current, the bias voltage rises, causing the current to drop, this is the compressed feeling many describe in Cathode biased amps.

"Fixed Bias" is really a misnomer. IMHO, this should be called Negative Grid Bias, as the cathode in this configuration is grounded, and a negative voltage is developed in the PS, then fed to the input grids of the tubes. This manner of biasing is usually adjustable, though some amps have a "fixed" supply that requires a resistor change, or modification to the amp to make it more easily adjustable.

[Tubebender]

Cathode bias, also known as "self bias" is a resistor (sometimes bypassed by a capacitor) from the cathode to ground. As the tube begins to draw current, a voltage begins to form across that resistor, raising the voltage of the cathode. The Grid in this configuration is usually held at ground, and the value of the cathode resistor sets the amount of current through the tube. Because this resistor isn't a pot, you cannot adjust it without changing the resistor. Quiesent current is usually higher when compared to a fixed bias amplifier because as the tube draws more current, the bias voltage rises, causing the current to drop, this is the compressed feeling many describe in Cathode biased amps.

"Fixed Bias" is really a misnomer. IMHO, this should be called Negative Grid Bias, as the cathode in this configuration is grounded, and a negative voltage is developed in the PS, then fed to the input grids of the tubes. This manner of biasing is usually adjustable, though some amps have a "fixed" supply that requires a resistor change, or modification to the amp to make it more easily adjustable.

wow. I think i need a "bias for dummys" response....

[harryjmic]

I should be good for a bias for dummies response. :D

Here you go, for what it means to you -

Cathode Bias - this is a self biasing circuit. What this means you buy some matched tubes and put them in your amp. The amp regulates itself so there is no bias variablity on your part, you get what you get. In order to really change the tone you need to ask for cool or hot running tubes, this is about all you can do to change it. Common tubes for cathode biasing are - 6V6's and el-84's generally smaller amps but not always.

Fixed or Variable bias - this type of circuit has a potentiometer which can be adjusted. With this type of circuit you can take hot running tubes and force them to run cooler by changing the resistance. You can also set your amp up to run hot or cool depending on what you want to hear. Common tubes, 6L6's and other large bottle tubes, but not always. For example Matchless uses el34's in the Chieftain and Clubman and both these amps are cathode biased.

The thing with cathode biased amps is since the signal goes through the resistor at higher volumes you get voltage drops which makes for a natural compression which is more noticable then with fixed biased amps. Not sure of my explaination is dead on but that's what happens.

[Austinrocks]

both answers so far are very good, have a great explaination here

http://www.lynx.bc.ca/~jc/bias.html

[Phil Harmoneeek]

Can you guys name some quintessential amps under both categories ...

Thanks Randall

[harryjmic]

Can you guys name some quintessential amps under both categories ...

Thanks Randall

Cathode - Dr.Z, Matchless, Vox, Carr (I believe), think el84 stuff and go from there

Fixed - Fender (is adjustable) Mesa (is not adjustable), Marshall (should be adjustable), think el34 and 6L6 in general.

[Tubebender]

I'm concerned with my Fender Prosonic that is Class a and b switchable. Like in my original post, when I set my bias in a/b at roughly 35mA, when testing it in class A, the mA reading almost doubles....

From what I've read here, that effect is due to they type of tube rating in my amp for cathode bias.... I must have a hot set in there.
It seems freakish since I'm suppose to be within a certain bias range in class A/B and B, but when in Cathode bias mode or class A, my mA reading is off the charts.

Which also leads me to believe my tube life will be heavily diminished running super hot in class A........? I find the tone more complex in A....

[rockon1]

I'm concerned with my Fender Prosonic that is Class a and b switchable. Like in my original post, when I set my bias in a/b at roughly 35mA, when testing it in class A, the mA reading almost doubles....

From what I've read here, that effect is due to they type of tube rating in my amp for cathode bias.... I must have a hot set in there.
It seems freakish since I'm suppose to be within a certain bias range in class A/B and B, but when in Cathode bias mode or class A, my mA reading is off the charts.

Which also leads me to believe my tube life will be heavily diminished running super hot in class A........? I find the tone more complex in A....

Cathode bias idles at 100% tube dissapation(sometimes hotter) whare as fixed bias usually 50 to 70 %. So depending on the plate voltage it might be pretty close to where its supposed to be. Bob

[big mike]

I'm concerned with my Fender Prosonic that is Class a and b switchable. Like in my original post, when I set my bias in a/b at roughly 35mA, when testing it in class A, the mA reading almost doubles....

From what I've read here, that effect is due to they type of tube rating in my amp for cathode bias.... I must have a hot set in there.
It seems freakish since I'm suppose to be within a certain bias range in class A/B and B, but when in Cathode bias mode or class A, my mA reading is off the charts.

Which also leads me to believe my tube life will be heavily diminished running super hot in class A........? I find the tone more complex in A....

I'm wondering if that's really just a pentode/triode switch.

[WesKuhnley]

Cathode bias idles at 100% tube dissapation(sometimes hotter) whare as fixed bias usually 50 to 70 %. So depending on the plate voltage it might be pretty close to where its supposed to be. Bob

That's really the kicker isn't it? Simply measuring the current tells you NOTHING. You must multiply this by the plate voltage to get a reading that means ANYTHING.

[rockon1]

That's really the kicker isn't it? Simply measuring the current tells you NOTHING. You must multiply this by the plate voltage to get a reading that means ANYTHING.

Yep but if you know generally what the pv is you can get "close enough" IMO.

30/400pv= .075 x 60% =.045

30/450pv= .066 x 60% =.040

30/500pv= .060 x 60% =.036

So between a 100 volt change the ma settings only change 9 ma. You could probably get a rough estimate thats way less than 100 volts off. Bob

[WesKuhnley]

Yep but if you know generally what the pv is you can get "close enough" IMO.

30/400pv= .075 x 60% =.045

30/450pv= .066 x 60% =.040

30/500pv= .060 x 60% =.036

So between a 100 volt change the ma settings only change 9 ma. You could probably get a rough estimate thats way less than 100 volts off. Bob

I guess it depends on what you call "close enough". 9mA is quite a bit IMHO, do you subtract screen voltage from your reading as well?

[I-IV-V]

I'm no amp guru to say the least, but I looked up the features for the amp and it didn't say anything about a switch to go between cathode and fixed bias or a switch to go from class A to class A/B. Go figure.

Does it have both options, you can pick cathode or fixed bias AND pick between class A or class A/B?

Just curious.

[Tubebender]

I'm no amp guru to say the least, but I looked up the features for the amp and it didn't say anything about a switch to go between cathode and fixed bias or a switch to go from class A to class A/B. Go figure.

Does it have both options, you can pick cathode or fixed bias AND pick between class A or class A/B?

Just curious.

Yeah, I'll can post some pics later, but it says: 30watt class A, 50 watt Class a/b, 60 watt Class B, SS (solid state) .

Bob at eurotubes along with others have told me that class A on the prosonic is Cathode biased.....
should I drop my bias down is my real question if running on A....

[WesKuhnley]

Yeah, I'll can post some pics later, but it says: 30watt class A, 50 watt Class a/b, 60 watt Class B, SS (solid state) .

Bob at eurotubes along with others have told me that class A on the prosonic is Cathode biased.....
should I drop my bias down is my real question if running on A....


You won't be able to change the current in cathode bias mode. The "Fixed Bias" pot will not affect the bias in cathode biased mode.

If you're really worried about it, bring it to a local tech to make sure everything is OK. There is only so much that can be "diagnosed" over the 'net.

[I-IV-V]

Man, I don't know. Like I said, I'm no amp guru.

If you could read the plate voltage like they mentioned, maybe you could figure if it's running at 50% to 70% like rockon1 said, which is normal.

It will run hotter in cathode, so what your seeing (or the heat your feeling) might be normal.

Might be a good idea to have a tech look at it when you get a chance.

Wish I could help more, but that's all I can say.

[billyguitar]

The early Tweed Fenders were all cathode biased. They compress nicely at higher volumes. Oddly my Allen Brown Sugar doesn't compress much until it's really cranked with the Raw way up. I assume it has a lot more filter capacitors.

[rockon1]

I guess it depends on what you call "close enough". 9mA is quite a bit IMHO

My point was that ,yes, 9 ma is a large but thats between the difference between 400pv and 500pv! Its fairly easy to find out approximately what the plate voltage is in any given amp and be within a couple ma IMO. Do I measure my PV-yes but if some one is sqeemish an approximation will get them pretty close. Bob

[drbob1]

You could buy a set of Weber Bias-Rite probes, which measure plate voltage and current, allowing you to calculate plate dissippation (you're missing grid current which can kind of act like a safety factor). Measuring plate voltage with a multimeter is a bit dangerous because you're poking around 450-500v voltages. I can pretty much guarantee that the amp is not truly going from "class A" to "class B"with that dial. Class B sounds bad with guitar amps, and class A is rare as hen's teeth in normal amps (excepting guys like George Alessandro). It's probably changing from cathode to fixed bias, maybe tying the grid and screen? together to simulate triode mode, and maybe switching in a SS rectifier: all enough changes to go from 30w to 70w easily...