Crate BV120H bias?

[rockon1]

JPB - Thank you. I stand corrected. Pin 3 is the Plate. Pin 5 is the Grid (Control).

I spent the day reading several sources on Biasing and the different techniques. And here is what I did.

(1) Using a clean signal generator fed into the clean channel, I used a scope and adjusted the Bias control to minimize any notches that appeared on the rising and falling edges of the sine wave (remove the crossover distortion) that appeared in the output. By the way... output was a precision 8 ohm resistor. You increase the gain just before clipping and recheck.

(2) DC voltage measurement at Pin 5: -45.5 volts (for the heck of it)

(3) DC voltage measurement at Pins 3 and 4, 456 Volts

(3) Use the Bias Probe kit to measure the current coming out of the Cathode: 45ma

(4) Calculate Power Dissipation of 6L6GC tube: 456vdc x .045a = 20.52 watts

Max Power Dissipation for the 6L6GC is 30 watts - I am good.

Kill-A-Watt Meter reads 1.32 amps

The Clean channel sounds nice. The Dirty channel? Well, that is another problem. Something is wrong there. But that is for another night and another thread.

Thanks for the education and pushing me to learn this stuff!!!!

They are your tubes

I still think that is set way too hot (for longevity)

Set the K_a_W to read watts.

My guess is you are pully 150 some watts at idle.

Again, thai's fine for EL34 tubes.

Not fine for 6L6 tubes.

Hmmm ...sound right to me.

30/456PV= .066ma (100% dissapation) X .7 = .046ma for 70% idle disaapation. Bob

[g1]

The 70% idle dissipation figure is a mythical catch all. For a lot of amps with higher plate voltages it is way too hot.
There is no single % number that is good for all amps. Single ended amps usually run about 100%. Many newer high gain amps run the power tubes as low as 25%.
Although the 70% figure is probably ok in this case, the factory setting (see post #5) is lower and will give longer tube life.

[rockon1]

The 70% idle dissipation figure is a mythical catch all. For a lot of amps with higher plate voltages it is way too hot.
There is no single % number that is good for all amps. Single ended amps usually run about 100%. Many newer high gain amps run the power tubes as low as 25%.
Although the 70% figure is probably ok in this case, the factory setting (see post #5) is lower and will give longer tube life.

I agree. Was referring to the comment that it was "way too hot" which at slightly below 70% it is not. Cathode biased amps run typically around 100% .The tubes are always running "full bore" so to speak. Fixed bias typically run up to a generally accepted max dissipation of 70%. Cooler is better for tube life and I set my fix biased amps to sound best but not exceeding 70%. I know of no amp spec'd to run its tubes at 25% though.Thats way cold imo. Then again who knows maybe there's an amp out there that does sound good at 25%. Kind of like having a muscle car V8 with a tiny single barrel carb on it though. At any rate it wont hurt it. Anywhere between 50-70 % for a fixed bias amp is usually best sounding. Sometimes to cold will sound poor ,sometimes too hot will sound poor. Basically I'll set by ear not exceeding safe operating current-hence the 70% max.Bob

[TomCarlos]

Thanks again guys... I will revisit your replies here and go back and reset the bias. At this point, it is a learning curve for me!! Tom

[Enzo]

The Peavey 5150 is a good example of cool power tubes. At 480-500v B+, the tubes in a stock amp are conducting in the 11ma to maybe 20ma tops depending on tubes. That power amp is clean and strong, they get the tone in the preamp.

[rockon1]

The Peavey 5150 is a good example of cool power tubes. At 480-500v B+, the tubes in a stock amp are conducting in the 11ma to maybe 20ma tops depending on tubes. That power amp is clean and strong, they get the tone in the preamp.

Really! Never owned a 5150. Do own two Peavey Ultra's (Ultra 112 and Ultra plus)and they run them much hotter. Ive never correlated plate voltage (or heard of doing so ) with way cooler bias. I would think 30ma would be a more acceptable minimum for the 5150. Bob

[rockon1]

Thanks again guys... I will revisit your replies here and go back and reset the bias. At this point, it is a learning curve for me!! Tom

According to your calculations your at 68%.Yeah probably warmer than need be but within operating parameters for the tube. 50-60% will run cooler and most likely sound just as good. That said if it sounds better closer to 70% it shouldn't be an issue. My son unwittingly ran his Crate Stealth at 110% for over a year before I checked it for him! Not recommending that of course but the amp survived. I'm surprised the sovtek 6V6s did though! :O Bob

[Jazz P Bass]

The Peavey 5150 amps are an exception to the rule of biasing.
There is an article on the net, by Peavey's own Roger Grimm, where he explains the why of the fact that the 5150 bias is set very low.

In a nut shell, all of the distortion is in the preamp.

The Resonance control (bass response at different volumes) depends on the low bias.
Roger's articles: Tech: Ninety Nine Days-Roger Crimm
Tech: Ninety Nine Days-Roger Crimm

[Enzo]

Ive never correlated plate voltage (or heard of doing so ) with way cooler bias.

I didn't correlate the two, but we were talking low percentage bias dissipation, and without that voltage those currents won;t tell you that. So I included the plate voltage typical in those amps so you could see that 14ma - for example - at 500v is 7 watts, or 23%.

Yes, the 5150 is at the end of the scale, but it demonstrates that bias can be a very wide range and still perfectly fine. 70% is nothing more than a place to hang your hat.

[rockon1]

I didn't correlate the two, but we were talking low percentage bias dissipation, and without that voltage those currents won;t tell you that. So I included the plate voltage typical in those amps so you could see that 14ma - for example - at 500v is 7 watts, or 23%.

Yes, the 5150 is at the end of the scale, but it demonstrates that bias can be a very wide range and still perfectly fine. 70% is nothing more than a place to hang your hat.

Agreed. The hat sits on 70% as the end of the line go no further not this is where it has to be imo. I'd like to see that article thiugh. 23% seems silly. Not after power tube or pi distortion but some modicum of efficiency. From all I've read 25% would be deep into crossover distortion territory. Bob

[rockon1]

Ok read it but he doesn't really explain much about how much crossover distortion is "severe" or why it's a good thing for the 5150. He does go on to say that to cold is not good either.
Honestly its hard to put rhyme or reason to some things. For instance my Mesa DC3 is non adjustable fixed bias with 400pv and -11 grid voltage running EL84s. Basically that means by design it ran any tubes crazy hot.(130+%) I modified the bias and cooled it down to 80 % . Who's right? Sometimes I just don't know.lol Bob

[Jazz P Bass]

Agreed. The hat sits on 70% as the end of the line go no further not this is where it has to be imo. I'd like to see that article thiugh. 23% seems silly. Not after power tube or pi distortion but some modicum of efficiency. From all I've read 25% would be deep into crossover distortion territory. Bob

Have you used a scope to actually observe 'crossover distortion'?

And then listen to it.

A Class B amp would sound raspy & bad. (severe x over)

But add a small amount of bias to slightly turn the tubes on & wallah.
Class AB.

My question is: 'why go any further with the bias'?
What is the benefit?

[rockon1]

Have you used a scope to actually observe 'crossover distortion'?

And then listen to it.

A Class B amp would sound raspy & bad. (severe x over)

But add a small amount of bias to slightly turn the tubes on & wallah.
Class AB.

My question is: 'why go any further with the bias'?
What is the benefit?

Well Roger Crimm says crossover distortion is a good thing with the 5150. That's a first I've heard of it being a good thing. At any rate a set of tubes will last year's running cool at 50%. Without a scope I bias between 50 and 70 by ear. I can't see going lower than 50% though. None of my amps sound good below that to my ears. Bob

[g1]

Ive never correlated plate voltage (or heard of doing so ) with way cooler bias.

I have read on this forum and elsewhere warnings about cooling bias down when plate voltages are higher. Also the idea that it is not high plate voltages that kill tubes, but high plate dissipations.
Many manufacturers exceeded max plate voltages by wide margins. They got away with it by running cool bias, like 50%. A couple examples, Musicman and Ampeg V4. Often the amps did not have adjustable bias. Now everyone takes those amps, installs adjustable bias, set them to 70% without even checking what the stock value was. Then when the tubes are short lived, they blame the high plate voltage and ask why they don't build tubes like they used to . How about "we don't bias tubes like they used to" instead?
Remember Aiken's Wombat bias tool? Instructions were to not exceed 70% with plate voltage lower than 420V, do not exceed 50% with plate voltage over 420V. Here's what you said about it
"rockon1
12-15-2010, 07:41 PM
Well all I can say is that the Wombat appears to be a well thought out,superior product! Bob" (Compu-Bias vs. Aiken Wombat [Archive] - The Gear Page).

[rockon1]

I have read on this forum and elsewhere warnings about cooling bias down when plate voltages are higher. Also the idea that it is not high plate voltages that kill tubes, but high plate dissipations.
Many manufacturers exceeded max plate voltages by wide margins. They got away with it by running cool bias, like 50%. A couple examples, Musicman and Ampeg V4. Often the amps did not have adjustable bias. Now everyone takes those amps, installs adjustable bias, set them to 70% without even checking what the stock value was. Then when the tubes are short lived, they blame the high plate voltage and ask why they don't build tubes like they used to . How about "we don't bias tubes like they used to" instead?
Remember Aiken's Wombat bias tool? Instructions were to not exceed 70% with plate voltage lower than 420V, do not exceed 50% with plate voltage over 420V. Here's what you said about it
"rockon1
12-15-2010, 07:41 PM
Well all I can say is that the Wombat appears to be a well thought out,superior product! Bob" (Compu-Bias vs. Aiken Wombat [Archive] - The Gear Page).
[ATTACH=CONFIG]27304[/ATTACH]

Ok I'll stand by that. Aiken is a very well respected amp builder. As time goes on however Ive seen a lot of crazy contradictory stuff too. My aforementioned Mesa. 100 volts over an EL84 specs at 400 volts and fixed bias hotter than the sun. Yes it ate tubes but was designed that way. No tube low current draw or otherwise would bias anything under 130%. Crazy. what were they thinking? Interestingly I settled on 80% @ 426 volts. Lower it lost its mojo but at least it wasnt at 130+....
I didn't read the instructions for the Wombat but Aiken is OK with high plate voltages and higher dissipation as long as the primary impedance is taken into account. He also states basically anywhere from 50-70% which I concur with.From his biasing page.

"A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube (to account for the higher dissipation that occurs under signal conditions), while true class A amplifiers generally run right at the maximum plate dissipation (the dissipation at full power is lower than the dissipation at idle in a true class A amplifier). For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 400V class AB operation, it should be biased no higher than (0.7 * 25/400) = 44mA. This doesn't mean you should automatically bias all tubes to 70% of max dissipation! They can be biased at any lower current if desired, and many people prefer a point of around 50% to 60% of the max plate dissipation, which contributes to longer tube life. In addition, the "70% rule" falls apart as you use very high plate voltages, unless the primary impedance is increased accordingly. In some cases, if the voltage is high enough, there is no bias setting that will result in safe operation without exceeding the maximum plate dissipation of the tube. For example, in a 100W EL34 amp with 480V on the plates and a 1.7K primay impedance, there is no bias setting that will keep the output tubes from redplating at some point in the power curve, even if you bias the amp at 0mA! If you double the primary impedance to 3.4K, however, the amplifier will operate fine at all power levels, and you can bias it all the way up to 52mA without ever exceeding the plate dissipation at any point in the operating curves. This may sound odd, because the bias point is right at 100%, but it works because of the very high plate impedance load, which never allows the tube dissipation to increase above the idle level. It also may not sound as good, because the screen voltage should ideally be decreased so the loadline intersects the "knee" of the curves if not, the nonlinearity increases drastically. The bottom line is that you have to take into account not only the plate voltage and plate current, but also the primary impedance to find out the safe bias area."

So yes there is more to take into account than PV and current. One size doesn't fit all but I didn't say it did. That said if the "proper" primary impedance is used then according to Aiken 70% is ok with high PV. Now that said -if an amp sounds good at 50% then leave it at 50%!

Bob