Output Transformer Primary Questions

[kward]

I think the slope is so flat that it'll only take +4/-4 of signal to swing 300v on the plate.

Oh wait. I did the class A calculations assuming a class B load line. duh!

let me revisit my calculations...please stand by.

 

[FlaCharlie]

At 15.6k primary impedance, 260V B+, 150V screen, PP class A operation, you would idle each tube right at the limit of 30 mA. The stage would deliver approximately 3 wats output. This output stage configuration is highly NON-optimized for class A operation because of non-even current swing-ability. The plate voltage is too high so that the tubes will be run into cutoff on the negative going input signal long before the tubes hit zero grid volts on the positive going input signal. The stage is really operating as an AB1 stage under those conditions. But it isn't even optimized for that class of operation either...so it's a lose-lose proposition.

To optimize the stage for class A operation (where optimization in this context means max output power the stage can deliver) you would want 180V plate, 150V screen, and bias each tube at 40 mA (this assuming 15.6k OPTs). That will position the bias point so that there is roughly equal current swing-ability on either side of the bias point. In this situation, the stage will deliver right around 5 watts output.

As I said, I don't care what class the amp runs in.

But when I asked earlier about using an OT whose primary was 2x the value of what the data sheet suggests for SE operation, Douglas @PakProtector commented that "this works as long as you STAY CLASS A". Which implies that it won't work, or perhaps not as well, in AB.

That's why I asked how to ensure the amp stays in Class A. Because I would like to use the stock OTs, which measure 15.5k. If it's easier to use the stock PT and OTs in Class AB, that's perfectly fine too.

I haven't tried to figure out how much drive these tubes need or if the stock input / phase inverter scheme would work. I'm just focussed on the question of can I use these tubes with the stock PT and OTs.

My initial interest in these stems from a comment on another forum that they were used in PP Pentode in a radio in the old Soviet Union. The poster thought they put out ~10w. After some digging I found the schematic but it doesn't have any OT specs or voltages and I think it used an interstage transformer.

 

[kward]

Right...so after fixing my calculation error: (Thanks 6DZ7 for pointing that out)

Class A PP amp: 260V plate, 150V screen, bias current at 30 mA, will deliver about 5 watts output. (ignore my previous post which used the wrong load line for class A operation)
Class AB1 PP amp: (restating from above): 300V plate, 150V screen, bias current 25 mA, will deliver ~7 Watts output.

These "look" quite similar, but remember that class A operation has both sides of the OPT winding energized at all times so sees 2x the impedance of the class B line in AB1 operation...

I haven't tried to figure out how much drive these tubes need or if the stock input / phase inverter scheme would work

Class A PP amp using the above biasing will need approximately 8V peak drive
Class AB1 PP amp using the above biasing will need approximately 11V peak drive

But when I asked earlier about using an OT whose primary was 2x the value of what the data sheet suggests for SE operation, Douglas @PakProtector commented that "this works as long as you STAY CLASS A"

 

[FlaCharlie]

Class A PP amp: 260V plate, 150V screen, bias current at 30 mA, will deliver about 5 watts output.

Class AB1 PP amp: 300V plate, 150V screen, bias current 25 mA, will deliver ~7 Watts output.

I assume that the 30mA per tube / 120mA total in Class A and the 25mA per tube / 100 mA total in Class AB includes both the plate and G2 current. Then there's a bit more for the input tubes.

It looks like AB might be preferable then, because of the lower current demands put on the PT. Getting 300v B+ also seems do-able, especially if I use -9.5v, non-adjustable, fixed bias from a 9v Lithium battery.

These "look" quite similar, but remember that class A operation has both sides of the OPT winding energized at all times so sees 2x the impedance of the class B line in AB1 operation...

Is this is the basis for Douglas' comment about 2x the impedance working as long as the amp is running Class A?

What negative effects might there be running 15.6k primaries using the Class AB operating points you've suggested?

 

[kward]

I assume that the 30mA per tube / 120mA total in Class A and the 25mA per tube / 100 mA total in Class AB includes both the plate and G2 current.

The current numbers quoted did not include screen current.

It looks like AB might be preferable then, because of the lower current demands put on the PT. Getting 300v B+ also seems do-able, especially if I use -9.5v, non-adjustable, fixed bias from a 9v Lithium battery.

And in PP AB1 mode, you have more room to fiddle. You could go a little less plate voltage and make the bias voltage a little less negative, or a little more plate voltage and make the bias voltage a little more negative.

Is this is the basis for Douglas' comment about 2x the impedance working as long as the amp is running Class A?

I think he was referring to going from SE to PP .class A, whereas I was referring to the impedance seen by one tube at full power in PP AB1 vs PP A.

What negative effects might there be running 15.6k primaries using the Class AB operating points you've suggested?

The issue that comes to mind is how well will your output transformers work in doubled impedance mode? Meaning your output transformers were engineered for 4 ohm taps, but you are wanting to run 8 ohm speakers, which doubles the effective primary impedance. I don't know how well these particular output transformers will respond in the frequency extremes in a doubled impedance scenario. Others with more knowledge about running double impedance output transformers will need to respond to that.

 

[FlaCharlie]

The current numbers quoted did not include screen current. . . .

The issue that comes to mind is how well will your output transformers work in doubled impedance mode? Meaning your output transformers were engineered for 4 ohm taps, but you are wanting to run 8 ohm speakers, which doubles the effective primary impedance. I don't know how well these particular output transformers will respond in the frequency extremes in a doubled impedance scenario. Others with more knowledge about running double impedance output transformers will need to respond to that.

The reason I asked if your current figures included screen current is that, when I looked at the load lines I drew last week (using the curves I posted earlier), I noticed that the horizontal axis (X? Y? I forget) is Plate Voltage but the vertical axis includes both Plate and Screen Current.

As a result my lines are not totally accurate but, perhaps, relatively close. Did you use those same curves? If so your calcs may be off a bit too.

As for the OTs running at 2x normal impedance . . . Although I have no way to measure, I did this recently with another project based on a Magnavox 185 and the end result was impressive. The OTs are not the same part number but are similar in size and impedance so hopefully the ones in the 8802 will work OK.

I'll probably do a write up on that project soon.

Thanks for all you help on this!

 

[PakProtector]

The power level is not part of the definition of a class of operation. But if you want to say the bias voltage in the amp is set for AB operation then go ahead an call your amp an AB amp. Both tubes could be conducting for the entire cycle at 95% power. I'd call it a class A amp, but I'm not anal about labels.

You might find it helps to 'be anal about labels'. The language in a technical discussion has to mean something specific, and unequivocal. The Class A definition is clear, and has been covered quite adequately. Picking a different definition, especially one borne out of a Marketing department is decidedly not a good idea, and clouds the discussion, making it imprecise at best. That is not a hallmark of a solid discussion IMO.

Taken to a subtle extreme, what is a driver? some sort of golf club? last stage voltage amplifier, the creature behind a steering wheel/handlebar, a loudspeaker, or the last stage amplifier capable of delivering power( grid current)? As far as tube circuits go these days, the definition has been changed.
cheers,
Dougals

 

[kward]

The reason I asked if your current figures included screen current is that, when I looked at the load lines I drew last week (using the curves I posted earlier), I noticed that the horizontal axis (X? Y? I forget) is Plate Voltage but the vertical axis includes both Plate and Screen Current.

Yes I did use the same curves you posted earlier in this thread. Interpreting the Y (vertical) axis, I believe it is showing two graphs on one. The first graph using solid lines is plate voltage vs. plate current. The second graph using dashed lines is plate voltage vs. screen current.

I basically eyeballed grid voltage on the load lines I drew--didn't get it too precise as these are ballpark discussions anyway.

The dashed lines make sense as far as their shape since at low plate voltage, cathode electrons are very much more strongly drawn to the screen, so you would expect screen current to rise sharply as plate voltage drops, while keeping screen voltage constant.

 

[FlaCharlie]

Yes I did use the same curves you posted earlier in this thread. Interpreting the Y (vertical) axis, I believe it is showing two graphs on one. The first graph using solid lines is plate voltage vs. plate current. The second graph using dashed lines is plate voltage vs. screen current.

I basically eyeballed grid voltage on the load lines I drew--didn't get it too precise as these are ballpark discussions anyway.

The dashed lines make sense as far as their shape since at low plate voltage, cathode electrons are very much more strongly drawn to the screen, so you would expect screen current to rise sharply as plate voltage drops, while keeping screen voltage constant.

You are correct about the lines. It even explains them in the legend at the bottom, which I failed to notice. Doh.

I guess the lines I drew are correct after all.

Any idea what is meant by the notation: plate current, g2 current = f (plate voltage) ?

 

[kward]

I think it means Ig2 is drawn as a function of anode (plate) voltage. In other words, the X axis is common to both graphs.

I don't particularly care for this old school nomenclature, but...it is what it is, so we who were weaned on transistor theory have to deal with it.

 

[PakProtector]

It's more helpful to be practical and realistic.

Scenario: You are so proud to have designed and built the perfectly biased class A PP amp. Perfectly centered you say to all who are in awe of its sound. You've been showing it off for a year, sounds just like day 1. You stick the scope on it to prove it to everyone (2 people at the "party") and WTF! It's going into cutoff... tubes got weak over time, emissions dropped on the low (-) signal swing, and it's running in AB class and you didn't know it.

Or worse yet, God forbid this should ever happen to your amp. One tube is biased for class A and the other one you misread the meter and biased in accidentally to run in AB.... I guess you'd call it AAB to be anal and perfect.

First of all, I have not built an amp in several years. Fixing that now. Second, I don't let stuff like tube wear catch me as you described. I don't misread meters whilst biasing amps. The amps all have 10R cathode sense resistors. Every last one.

A mis-biased amp is just that. It should get labeled as such, that and no more. Well, adjusted properly maybe...LOL
cheers,
Douglas