OT and understanding

[Willabe]

I would rather have the coal, better tone.

Well, there you go Ed. I tried to tell you.  

Jims big into Deep Purple, Ritchie 200?, Marshall Major?

Me, Rory Gallagher, Irish Tour, double live, 74. Best Fender Strat and tweed Bassman/Twin tone, maybe ever?

Met him, talked with him, had a few beers in the blues clubs with him, over a week or 2, very nice down to earth kind of guy, may he rest in peace.


                        Brad      

[HotBluePlates]

HBP, you are not returning my calls!

Sorry! Power went out Friday during storms, didn't come back on til Monday (not good when the heat index is over 100). Then lost power again briefly this morning. No internet until this morning.

My next question is: As the grid bias gets more negative, what happens to the grid driving voltage?  And what is a knee?

Easy question first. The knee is shown in the attached pic.

Simple answer to "As the grid bias gets more negative, what happens to the grid driving voltage?" Well, you apply a bigger grid driving voltage.

This really has more to do with "what happens as plate/screen voltage gets bigger?"

If you were designing for a lower power output range from your output tubes, you might use a smaller B+ (like 250v) and select your OT load to provide the maximum output power while running class A. Using the previously linked data sheet's number as an example, 250v plate and screen allows the use of -16v bias, resulting in 60mA of plate current at idle. That's 15w plate dissipation at idle (I tend to believe the 6L6GC data sheet used the same "Typical Condition" numbers in some cases as the original metal 6L6 data sheet). A 5k load yields just under 15w of output power at full tilt. If you look hard at that data sheet example, "Peak AF Grid-to-Grid Voltage" is given as 32v. That's 16v * 2, or enough input to drive each grid momentarily to 0v during that grid's positive peak.

But you want more output power than class A will give. You want to raise plate voltage and plate current to get more power, but if you exceed the class A plate current, you have to dial down the current for part of the cycle to keep from redplating. So you jack the B+ up to 450v plate, increase the bias to reduce idle current, and run class AB. Data sheet says run 450v plate, 400v screen, and 58mA idle plate current (that's 26w, so they're still a hot class AB) by using -37v bias. Note this condition's grid-to-grid driving voltage is given as 70v, just below -37v times 2. Sheet says you'll get 55w output with a 5.6kΩ load.

So in general, bigger outputs require larger bias, but also require more driving signal from the phase inverter to reach full (and now higher) power output.

Getting back to the original question, look again at the other two conditions given for class AB in that data sheet (not the 450v plate condition). Voltages, bias, grid driving voltage are all the same. The difference is one has the load impedance almost halved, and the power drops from 26.5w to 18w. From their numbers, distortion stays the same (THD, so the balance of 2nd and 3rd may have changed somewhat), and the only other significant change is plate current drops slightly while screen current rises slightly.

I think way back I said if you change load impedance (up or down) from the ideal, you'll get either less power, more distortion or both.

[Ritchie200]

Wow, so you were hit with the east coast outages?  I hope you didn't lose too much food in the fridge.  We have been over 100 just about every day this week and I have been out in it every day.  It can really wear you down - even if you have AC to come home to.  Be careful!

Sorry for my attempt at twisted humor HBP...  I was expounding on my "Do you expect me to read every post" comment by asking the very thing you explained in the previous post.

However, I do appreciate the additional explanation! I think I have printed out 90% of your posts for reference - unless you were telling LIES about Tele's or the Beatles or something

So...what happens if I change the load impedance?

Jim

[HotBluePlates]

Wow, so you were hit with the east coast outages?  I hope you didn't lose too much food in the fridge.

Yep, all of it. Wife just went grocery shopping too.

But I enjoyed playing my Tele. Right about now, you'll tell me it sounds better when the power's off. 

[PRR]

> what happens if I change the load impedance?

Power goes out, beer cans burst.

> I tend to believe the 6L6GC data sheet used the same "Typical Condition" numbers in some cases as the original metal 6L6 data sheet

There's heavy legacy in those sheets.

The GC added the one AB1 56 Watt condition, illegal on the older 6L6es.

All the in-between models copied the original sheet as close as possible. (Very early on the plate rating was dropped from 21W to 19W which ruled-out some extremes.)

6L6 was introduced when driver transformers were still common. The sheets show AB2 conditions, even though that mode was nearly obsolete a few years later.

For tutorial value (I guess) the sheets show same voltages and loads for both AB1 and AB2 conditions. The higher load works best in AB1, the lower load works best for AB2; this means there are two "not so good" conditions listed.

6L6 was the go-to tube for decades. RCA didn't have to push them, and didn't keep a lock on the market so they didn't have great incentive to brag-up the datasheet. Also RCA was busy copying Farnsworth's invention; TV sold a LOT more tubes than audio. Instead they (or Tung-Sol or GE) introduced other audio tubes with "improved conditions". Like 7027, with 45W-76W ratings AND that extra G2 pin. It's now somewhat clear that 7027 is a high-grade 6L6 in a slender bottle with a dangerous pinout quirk; the real difference is marketing (via datasheets), not guts.

[HotBluePlates]

PPR could you explain briefly what  AB1 and AB2 are, I know its in the RDH4 as well as other places, but some of the readers may not have downloaded, or have copies of some different references on PP operation.

I'm not PRR, but...

AB₁ is operation in which the control grid (G1) is not driven positive and draws no grid current.

AB₂ is operation in which the control grid (G1) is driven positive and draws some grid current. This mode generally allows greater plate voltage and plate current swings from the same tube, resulting in more power output than AB₁. The drawback is the output tube grid consumes power from the driving circuit and very often requires a small output tube and transformer to drive the actual, final output tubes.

The "1" and "2" subscripts are not exclusive to class AB. You can have class A₂ amps. But very often, if you're willing to deal with the problems of positive grid drive, You'll skip class A for the greater power of class AB... or even more likely, skip to class B and a different set of problems.