biasing to 100% with cathode bias...your opinion?

[iknowjohnny]

Is this considered normal or is it a very subjective thing? I often hear you should be at or close to 100% to avoid crossover distortion. Others say no because the tubes will not last. Whats the lowdown? Is there just no answer other than to say go 100% if you want tone over lifespan and visa versa? Or is it that not biasing to 100% will cause a lot of nastiness when cranked. The guys at 18 watt don't generally bias at 100% do they? And they are known for using thier amps cranked. Discuss....

[Jana]

I am sure others are going to disagree with me on this.

If you are using cathode bias you must bias to 100%. It is futile to try to bias to anything other than class A with cathode bias. Think it through and you will see why... both tubes are at 100% at idle, when one is at 125% the other is at 75%, the total current draw remains constant, the bias voltage doesn't change.

With any other class, while they may be operating in the class A region at lower levels, as the amp is pushed harder and it moves into the AB region, the total current draw will change, thus changing the bias voltage, which changes the current, which changes the operating point, which changes, blah blah blah. See what I mean?

Cathode bias... Bias to 100%. The questions then are, is your power transformer able to handle the current? Are your voltages in the range for safe Class A operation? Is the impedance of the OT suitable for this? Can the OT handle the current?

[iknowjohnny]

Well, i can't determine whether the amp can handle it, but i can tell you what the specs are....the OT is a hammond jcm 50 watt replacement at 3.2k, the plates are seeing around 388v,the tubes WERE EL34's but i'm now trying 6550's. So can i assume the OT's primary is going to prohibit going to 100% being that it's not optimal for 6550's? (actually i don't even know what 6550's require in that rgard because i can't find the data) Or would a different secondary tap work to optimize it wwith the 8 ohm speaker load?

[Jana]

do a search in google and you should find all the data you need for 6550's. If memory serves me right, 42 watts is 100% for a 6550.

I'm not going to do the calcs on this. If you want to learn how to do it yourself, get Dave Funk's Tube Amp Workbook. He explains how to use the tube data and bias for Class A, determine the impedance you need, etc.

[iknowjohnny]

I found the data and it shows at 3k with the approximate voltage i'm running, the max for class A is 105 MA. So i'm way safe where i am and the OT is just about right at 3.2k. I will try the max current just to see how it sounds running in class A. But if no big improvement i'll put it back for tube life. But i mainly just wanted to see whether the primary was ok for this tube and happily it's right there.

[gahult]

6550 tube data:

http://www.nj7p.org/Tube4.php?tube=6550

Gary

[iknowjohnny]

yeah, that shows the same specs as the data i found on the drtube site. drtube is a great site BTW

[Alexo]

I am sure others are going to disagree with me on this.

If you are using cathode bias you must bias to 100%. It is futile to try to bias to anything other than class A with cathode bias. Think it through and you will see why... both tubes are at 100% at idle, when one is at 125% the other is at 75%, the total current draw remains constant, the bias voltage doesn't change.

With any other class, while they may be operating in the class A region at lower levels, as the amp is pushed harder and it moves into the AB region, the total current draw will change, thus changing the bias voltage, which changes the current, which changes the operating point, which changes, blah blah blah. See what I mean?

Cathode bias... Bias to 100%. The questions then are, is your power transformer able to handle the current? Are your voltages in the range for safe Class A operation? Is the impedance of the OT suitable for this? Can the OT handle the current?

I guess I'm an other...


Some might argue that the changing operating point under heavy signal is an attractive feature of cathode biased amps, and it contributes to the feel of the amp, but that's subjective.

But, just because you're biased up to 100% does not mean that your power stage is operating in class A. Odds are, unless you're running a relatively low rail voltage, that you're still in AB operation, albeit a little closer to the class A side.

To answer the OP's question, I'll refer to the adage that as long as your tubes aren't redplating, and you like the sound, you're biased correctly. Theoretically, 100% dissipation is within the tube's limits, so it should be fine.

I would be surprised if a PT that was bought for a pair of EL34's would be very happy running 6550's at 100% dissipation, that's a big jump in current demand...

[iknowjohnny]

But the primary is 3.2K which is close or perfect according to 6550 specs, and it seems i am not even at the 50 watts it's made for. Why would this be a problem?

[Alexo]

But the primary is 3.2K which is close or perfect according to 6550 specs, and it seems i am not even at the 50 watts it's made for. Why would this be a problem?

Sounds like you're taking about the output transformer, I'm talking about the power transformer: Say you're running 2 EL34's at 19 watts each at 400 volts, that's 95ma idle current. Now say you stick in a pair of 6550's, biased for 42 watts each at 400 volts, that's pulling 210 ma from your pt on the plates alone. If your PT is beefy enough, then it'll be fine. Just sayin' the PT you'd need for 6550's would be bigger than what you'd need for EL34's.

[Andy Le Blanc]

100% is fine, its the max for the given conditions.. You should avoid going over because the tubes life will shorten and wont reflect the published data.
Class is reflected by the difference between max and zero signal current draw.

[iknowjohnny]

Alexo.... my PT is 250ma if i recall. Is that enough?

[Alexo]

Probably enough to idle the 6550's, but that's not leaving much headroom. But if it sounds good... I'd just keep an eye on it and make sure it doesn't get too hot.

[iknowjohnny]

Well, then i suppose leaving it at 70 something ma is probably ok, no? i mean, it sounds great like that so theres really no need to bias it at max as far as i'm concerned. I wanted to try just to see, but if it would be that close i'll just leave it as is.

[j-po]

I am sure others are going to disagree with me on this.

If you are using cathode bias you must bias to 100%. It is futile to try to bias to anything other than class A with cathode bias. Think it through and you will see why... both tubes are at 100% at idle, when one is at 125% the other is at 75%, the total current draw remains constant, the bias voltage doesn't change. ...

What happens when the other tube goes temporarily beyond 200%? Does the the other tube go to negative current flow to keep the bias voltage constant? Or are you saying the tube is not capable of exceeding 200% just because you biased it at 100% heat dissipation.

If one wanted to keep the cathode bias voltage constant a zener diode could be used in parallel with the bias resistor ensuring voltage does not exceed that of what it is at idle. I do not recommend this though as the fluctuating bias is giving some protection from excess current.