Redplating anomaly?

[dgta]

Not an expert on redplating, so I'm a little baffled by an observation.

I have a couple of old 6L6GBs that I plugged into an amp. The amp is PP UL, AB1, fixed bias, 470V B+. Biased the 6L6s to 55mA each and they barely glow a bit at the crease where the metal is bent at 90 degrees (inside corner). OK, I figure that's normal, metal is probably just a bit thinner there and running close to max dissipation.

But then I apply signal and as I get to max power, the redness goes away. Tubes work fine, produce clean power, looks perfect on the scope. Why are the tubes dissipating LESS at max power? Supply is not regulated and sag is negligible under full power (maybe 10V) and both PT and OPT run cold at full power.

What am I forgetting?

[DF96]

For pure Class A the power drawn from the supply does not change with signal. All that happens is that up to half of it gets sent to the load, so there is less power left to heat up the valve.

Class AB complicates things, but the general idea that power is diverted to the load still applies.

[Defiant]

You're forgetting that 6L6GB's have a max dissipation of 22 watts. 470Vx0.055A=25.85W. Assuming you measured the current at the cathode, then some of this current is coming from the screen, but you are still over-dissipating the plate. 470V is also well in excess of the max plate voltage rating of a 6L6GB (400V max), and WAY in excess of the max screen rating (300V max). This is why you are getting some plate color.

As to why plate dissipation drops as you approach max power, that is a common characteristic of class AB and class B operation. Depending on where the quiescent point is set, the max drive, and the load, the dissipation of the output devices (tube or transistor) will increase with signal up to some percentage of max power (for class B operation) and then decrease as you approach max power. Since you are biased quite hot you will see the dissipation drop below quiescent at some percentage of max power and continue dropping up to 100% power.

[dgta]

Thanks guys.

Defiant, I'm not forgetting that part. Actually these are RCA tubes and I believe the data sheet says 19W plate dissipation. The amp is designed for 6L6GC, I was doing an unrelated experiment when I noticed the redplating behavior.

Dissipation did NOT increase with signal at all, it stayed constant from idle to about 60% power and decreased gently from there to full output.

[Defiant]

Originally Posted by dgta

Thanks guys.

Defiant, I'm not forgetting that part. Actually these are RCA tubes and I believe the data sheet says 19W plate dissipation. The amp is designed for 6L6GC, I was doing an unrelated experiment when I noticed the redplating behavior.

The plates are glowing due to excessive dissipation. 6L6GC's and related tubes (7027A, 7581, 7581A) have a higher dissipation rating due to the use of different plate material (explosive-clad tri-metal alloy in the GC vs plain nickel in the GB and earlier) which spreads and radiates heat more effectively. The plate is also larger in the GC. In other words, there are very real differences, and very real limits on the earlier 6L6 types.

Dissipation did NOT increase with signal at all, it stayed constant from idle to about 60% power and decreased gently from there to full output.

That is due to the high bias current you are running. Class B operation will show the increase-then-decrease of dissipation as you approach full power, while class A will slowly decrease as you approach full power. Class AB1, which you are running) falls somewhere in-between depending on bias current. The behavior you are seeing is completely normal for your operating conditions.

[tubelab.com]

Why are the tubes dissipating LESS at max power?

As defiant stated this is a totally normal thing.

A class A amp dissipates the most power at Idle and the least at full power. This is because ALL of the energy from the plate supply is dissipated in the tubes and none goes to the speaker at idle. As you turn it up the current stays constant, but some of the power supplies energy goes to the speaker.

A true class B amplifier dissipates NO power at idle since neither output tube is conducting. As you apply signal the tubes will start to work and the dissipation will increase reaching maximum at full power output.

A class AB amplifier falls in between these two extremes. It dissipates power at idle since there is an idle current and no power output. It will also dissipate power at full output. A well designed class AB mmplifier might get 60% efficiency at full power output since considerable power is being transferred to the speaker and the tubes are swinging from fully saturated to fully cutoff. Each tube is cut off for almost 1/2 of the time. As the power output is reduced the time in conduction for each output tube increases to 100% at idle, and the tubes spend no time in saturation so they dissipate more.

All class AB amps exhibit this effect but the power level at which the dissipation reaches maximum depends on the load line and bias conditions.

My hot-rodded red board (125 WPC) will show some redness at about 50 WPC, yet none at full power. It does not show any redness with music no matter how loud it is cranked since music has peaks 10 to 20 db below the average power level.

[famousmockingbird]

new 6l6gb's might have different specs but old 6l6gb's have max ratings for screen 270v and plate 360v. I don't know what datasheet defiant is looking at but I am looking at RCA's datasheet. I had a thread not to long ago about the 6l6gb tube and it's capabilities and the consensus was that arcing can occur with the high voltages because the heater is so close to the plate. And I am no expert on class of amps but I was under the impression that usually to reach class A operation the plate voltage is usually lower than most AB setups. 470v class A for a 6l6gb sounds high and I would expect tube life to be short.

[dgta]

Originally Posted by tubelab.com

As the power output is reduced the time in conduction for each output tube increases

That's the part I was forgetting. Thank you.

Famous, you are correct, but this isn't about tube specs, I know I was abusing the tubes. It was about the power dissipation curve, which hadn't occurred to me because I don't redplate my tubes very often. I need to get out on the wild side of the specs more often

[famousmockingbird]

Cool! I was more curious on where defiant got his specs from because I was trying to find the datasheet for the sovtek 6l6gb.
I might start a thread on class A because my mind is still a little confused.

[Defiant]

Originally Posted by tubelab.com

A true class B amplifier dissipates NO power at idle since neither output tube is conducting. As you apply signal the tubes will start to work and the dissipation will increase reaching maximum at full power output.

That is incorrect. A class B output stage will reach max dissipation at it's 50% efficiency level, which happens at an RMS voltage of 2Vb/π. This equates to an output power level of 40% theoretical max. The reason I say "theoretical" is because no tube or transistor can swing completely to the rail, so a bit more math with the load lines for the tube or transistor along with the actual loading is needed to determine where this point actually lies and what the overall efficiency will actually be. But, for discussions sake we can stick with the statement that a class B or very low biased class AB will reach max dissipation in the 40-ish percent rated power region.

Originally Posted by famousmockingbird

Cool! I was more curious on where defiant got his specs from because I was trying to find the datasheet for the sovtek 6l6gb.
I might start a thread on class A because my mind is still a little confused.

That was off of a Tung-Sol data sheet. The RCA sheet shows a bit more conservative rating. Apparently, Tung-Sol uprated the tube a bit while RCA stuck to the old metal shell ratings. Metal tubes are less efficient at dissipating heat since they have to radiate the heat from the plate to the shell, which then has to dissipate the heat by convection and re-radiation. By contrast, glass tubes can dissipate heat much more effectively since much of the radiant heat from the plate passes through the glass. Likewise, the bases of the metal shell tubes have higher leakage current vs their glass bottle counterparts, so they cannot tolerate as much voltage before they risk bias instability. The RCA sheet is a bit older than the Tung-Sol sheet, so I don't know if they bumped the GB's rating later or if they kept the conservative metal shell rating while Tung-Sol (and perhaps others) bumped the ratings...