Your thoughts on biasing like this

[daz]

I often read of people saying on a cathode biased amp you can select a cathode resistor(s) that is just large enough to keep the tubes from redplating and you'll be fine other than needing tubes more often. But when i've had my cathode biased amps just on the edge of redplating before, i'd check the voltages and using a calculator like weber's come to find the tube are running way over spec. So that being the case, who's right? Is it fine to run close to redplating as long as you're willing to buy tubes every 5 or 6 months or whatever, and if so why is that ok when you're running them so far over spec?

[tubeswell]

Heat is the byproduct of energy transfer, and when you run a thing (be it tube or whatever) for a while and it produces heat, the heat has an entrophic effect on the durability of the thing you are running. The more energy transfer, the more heat. So yes, if its hotter, chances are it will wear out faster. How much faster - who can say?

[black_labb]

you can run your tubes at redplating if you want to replace them every day too. the specs are there for reccomended use, but the numbers are just reccomendations, not hard facts. theres no switch in the tube that will cause the glass to shatter and the plate to fly out the front of the amp into your eye when your el84 goes over 14 or whatever watts of dissipation at idle.
its like a car, you can run it close to the red line, and it will work with some results that you may like, but the engine will wear quicker. or you can run it at a more moderate RPM and get more use out of it. its all a tradeoff. on some cars with more conservative specs will let you go over the red line a bit, but again, more wear.

[daz]

Well, i understand all that. But i asked because what i find odd is that the amp i'm talking about requires literally double the resistance to run at the max recommended specs than it does to redplate. in other words, i can run cathode resistors about 225R and it will be on the verge of redplating. actually , it may have been less than that...possibly 170R. Been a while so my memory may be off tho i recall trying both those values. anyways, the point i'm getting at is that i finally ended up at 450R to get it to run at max specs ! Just seems to be a huge difference in resistance between where it redplates and what i end up with resistance wise when it's showing max recommended specs. So i've been wondering how much tone i may be sacrificing and have been considering halfing the resistance and see how she sounds and if it redplates.

[Chuck H]

It's pretty common to run cathode biased amps hotter at idle than fixed bias amps. You probably know that but I mention it because it sets up the next observation.

In cathode biased amps the bias shifts cooler (more negative, or actually less more positive) as current increases. So if you don't run cathode biased amps hotter at idle than you would for fixed bias you end up on the cold side when the amp is working, which is the only time you hear it of course. Also, if you really drive the output with cathode bias there is a greater chance of crossover distortion due to this bias shift or grid conduction. And the standard fix for crossover distortion is to heat up the bias.

How hot is too hot... Well, if your amp has voltages, transformers and a PI arrangement such that it should be class AB1 then you really DON'T want to run the tubes as hot as you can. Even if you don't have red plates it can result in reduced output efficiency, a lack of dynamics and even damage to the amp or tubes. There really is no advantage to running a class AB1 amp in class A.

IIRC the Weber calculator does differentiate between cathode and fixed bias. I remember thinking that for me personally on the amp I was doing at the time that the Weber calculator seemed a little cold. No problem. You've already seen how far you CAN push it if you want to.

Set the amp up with the "correct" bias and then heat it up a little and see if you like the tone better. If you do the try a little more. Pay attention to how the tone is changing. For instance, if you really dig the heavy compressed tone of a really hot bias consider that it may still be nice to have more dynamics in the clean tone or more crunch in the in between tones. You'll be able to hear where things start trading off. It's not unusual in AB1 cathode bias to have el34's, 6v6's and 6l6's at 80 to 90% dissapation at idle if the plate volts are low. 100% is common for el84's if the plate volts aren't too high. IME beyond that you reach a point of diminishing returns and below that is perfectly acceptible as long as your not getting to much crossover distortion.

Chuck

[tbryanh]

"Redplating" is the maximum possible the tube can be driven.

"Running at the max recommended spec" is the maximum the manufactuer recommends.

So there is a gray area between the two.

Rock and roll can be on the edge. Speaker coils burn up, output transformers burn up, tubes burn up, etc.

Why do these things happen? Because rock musicians do not operate their amps the way manufactures intend them to.

[txstrat]

I believe nobody intends to ruin or at least damage an amp. That said it's sort of a matter of how much you are willing to spend on new tubes.
Have a look on the old Fender amps. Commonly they are biased quite on the cold side and most of them sound just great.
When I biased my last build (6g3 with 2 JJ 6V6) on the voltages given in the schematic it ran at 41ma (way too hot I know). I biased it to 30ma (still hot) and can hear NO difference. I might not have played it loud enough to hear the difference but what I'm trying to say is, I like the tone as it is now and the tubes probably last longer.

[defaced]

Just seems to be a huge difference in resistance between where it redplates and what i end up with resistance wise when it's showing max recommended specs. So i've been wondering how much tone i may be sacrificing and have been considering halfing the resistance and see how she sounds and if it redplates.

That's called the factor of safety. This is usually seen in mechanical design, but it can be applied many places including this one. Basically you never want to run a part on the edge, so you over design and run in a safe area.

For example., if you've got a chain that will break at 1000lb then to get a factor of safety of 2, you'd rate it as being able to support 500 lb, or more likely a FoS of 3 and rate it to support 330 lbs. There's a big gap there between 330lb and 1000lbs and that's what is keeping the system safe. It doesn't mean you can't lift 999lbs with it, it just means you risk premature failure if you do.

[Five_E35]

What tubes? The specs, recomendations, and various calculators are generally based on standard spec (U.S. and Euro NOS) tubes. There are a number of imported tubes that are really non-spec tubes and should be given new numbers or suffixes IMO.

Russian tubes in particular are based on generally equivalent designs, but don't always operate the same. JJ in particular has been importing a number of power tubes lately that are way out of spec such as their EL84's and 6V6GT's. Put one of their EL84's in an amp set up for normal EL84's and it will run hot because it's not really an EL84. Put one of their 6V6GT's in a 5E3 with a 5AR4 rectifier and you can get 18 watts out of it. Whether this is good or 'make it louder' marketing is open to debate, but people should be aware of it.

When biasing on the verge of destruction, one should also consider how AC line voltage variations will affect the amp.

[daz]

Thanks all. Chuck, that was especially helpful. I think i will try this because once i turn it up to a certain point i don't like the tone as much. So i think i may be on the cold side when approaching stage volumes. They are winged C EL34's by the way with a 3.2k hammond OT and running around 400V plates. Each tube has 450R on it with 22uf bypass caps that are switchable in and out. (no caps=low output/more squish) The 450R is actually two 225R in series, so i will try cutting one out then adding maybe 50R in increments if needed till i find the optimal resistance.

[Chuck H]

Daz

Here's a thread you may find interesting. On page two I explain how I built my last amp with a bias arrangement that is cathode biased for clean tones but fixed bias under overdrive. This arrangement makes the amp soft when you want it soft and tight when you want it tight. I like it. Read the whole thread, there's some good stuff in it. I explain the circuit on page two.

http://music-electronics-forum.com/s...ighlight=chuck

You'll see me mention the "Paul Ruby mod". This applies mostly to el84 amps like the 18 watters but you could use it for any tube. Here's a page on it.

http://www.18watt.com/storage/18-wat...z_info_311.pdf

Chuck

[daz]

Thanks Chuck. I'll read up on that later, as i've had a few coors at this point and will need a clearer head to understand it all.

[Steve Conner]

Here's the deal on redplating vs. life, as I understand it at least.

The materials used to make tubes (metal, mica, etc) all contain a certain amount of gas. In the factory, the guts of the tube are cooked at red heat for a long time, with a vacuum pump applied, to drive the gas out. The tube is then sealed and the getter fired.

However, this process doesn't remove all of the occluded gas. The remaining gas will slowly come out over the life of the tube, and the getter will absorb it. As it does this it will turn from silver to brown and ultimately a kind of pale semi-see-thru grey, at which point it can't absorb any more and the tube goes gassy and quits working.

The hotter you bias your power tubes, the quicker they will go gassy. If you run at the manufacturer's recommended plate dissipation, you should get the rated life, too, which used to be something like 2000 hours back in the days when it was specified. By the time you see red on the plate, you're typically well over the manufacturer's rating, and so you'll get much less than the rated life.

On the other hand, some large transmitting tubes are designed to run with red plates, and go gassy if you don't run them hot enough...