Load resistance/power/distortion graphs

[dwinstonwood]

Hi all,
I'm looking at this 6V6 graph, and I can see why 6,600 primary Ohms is a typical choice (for low 2nd order distortion). I also see this graph represents 250 volts on the plate.

So, a couple of questions. Would the graph look hugely different with 350 plate volts? Also, if one wanted a lot of 2nd order harmonics could you safely use a 12,000 primary OPT?

Thanks, and sorry if I'm missing obvious and critical knowledge.

[thetragichero]

i'd think a little bit lower (like 5k?) would be preferable because the third harmonic content is lower

i'm betting 6k6 is used because that's about the peak of the power curve and any distortion products are just secondary. remember the guitarists' axiom: more louder is more gooder

[PRR]

> different with 350 plate volts?

Well, if voltage is higher and dissipation is already near a limit, then current must be lower. Impedance is about the V/I ratio. So you shift to a higher-Z load.

Voltage up 1.4 times, current better go down 1.4 times, impedance is doubled. 13.2k.

> if one wanted a lot of 2nd order harmonics

Then don't you want a lower B+ and Rl?

[pdf64]

Note that the chart applies to single ended operation.
Push pull may be different, due to it having a nulling effect on even harmonics.

[dwinstonwood]

Thanks everyone!

In the post linked to below, HBP said, "I had a mental-block about transformer loads for a lotta years, on facts that should have been 'obvious' after a week of reading.  Like many on forums, I didn't see the tube/transformer relationship as being very-simple "Volts, Resistance, Current" and tended to think in the terms I now see others repeat:  'the EL84 wants 8kΩ.'"

This mental block is what I'm trying to overcome and see past.

https://el34world.com/Forum/index.php?topic=26684.msg292896#msg292896

Further down in that same post, HBP said, "'A good OT load' is whatever seems to work as a balance between the tube's (plate current) capabilities and the supply voltage to be used."

PRR, you went straight to the voltage/current/impedance ratio.

It's starting to make more sense. I'm not quite there, yet.

[dwinstonwood]

I'm sure I'm missing something here, but using Ohm's law and a 6V6GT to get 12.25W with different loads:

350V
35mA
12.25W
10,000R

284V
43mA
12.25W
6,600R

328V
37mA
12.25W
8,800R

[pdf64]

SE or PP?
Cathode or fixed bias?
Class A or AB?

[dwinstonwood]

That's the question, isn't it?

And, I have no idea how to factor that in.

Here are the voltages needed (and the power) if the current is kept constant with different loads:

350V
35mA
12.25W
10,000R

231V
35mA
8.08W
6,600R

308V
35mA
10.78W
8,800R

I'll keep reading. Maybe I can get a handle on it. 

[pdf64]

That's the question, isn't it?

And, I have no idea how to factor that in.
...

I suggest to pick a scenario as an example, a starting point, and then work through things.
That list of scenarios is kinda meaningless as it is, without any context of the things I mentioned.

[dwinstonwood]

I think, if I'm really being honest, what I want to know is how, or if, I can use an OPT as a tone shaping component in an amp.
We use coupling capacitors, voltage dividers, various tubes and their plate voltages, bias resistors and bypass caps to "shape" an amp's tone.
Can we think of the OPT as a "tonal component," too? And, if so, how?
That's really where all of this is headed.
Thanks.

[HotBluePlates]

... if I'm really being honest, what I want to know is how, or if, I can use an OPT as a tone shaping component in an amp. ...

DON'T.  Focus on "how do I make this amp function?"


Where guitarists screw up everything (me included) is we start the entire electronics conversation with, "What does ____ do for 'tone'?"  That makes it more complicated than it is, and causes us to overlook the obvious right in front of us.

I'm sure I'm missing something here, but using Ohm's law and a 6V6GT to get 12.25W with different loads:

350V
35mA
12.25W
10,000R

284V
43mA
12.25W
6,600R

328V
37mA
12.25W
8,800R

Power, Voltage Current, Impedance.  These are all related to each other in 2 equations:
Volts = Current x Resistance
Power = Volts x Current

Then we throw in another variable:  the Tube.
We have to stay within its limits of operation.

Then we throw on another Complication:  AC conditions vs DC conditions
We are trying to make Audio Power Output, which is Alternating Current, or AC.
Our tube uses a DC power supply, and idles at DC conditions of volts, current & power (dissipated as waste-heat).


DC conditions are not audio power output, but they tell us what we're working with.
The tube's limits guide us in how far we can push the tube to alternately deviate from the DC condition, and yield AC Power Output.


You had a 250v supply and a 6.6kΩ load in your opening post.  Other things are stated on the graph, but aren't slapping you in the face (though they should):  250v screen, -12.5v bias, 8.8v RMS of applied signal.

     Top graph of Page 3 of the 6V6 data sheet shows 250v screen and conveniently has a grid-line at -12.5v:  ~45mA idle
     
     We can see from the power output (if nothing else) this is a single-ended 6V6:  ~4.5-4.7 watts

     We can cheat because they tell us the Power Output; combining the formulas above we can find the AC Volts across the OT Primary as Volts (RMS) = √(Power Output x Impedance) = √(4.5 watts x 6600Ω) = ~172v RMS

        We should see this 172v RMS will occur when the 6V6 pulls Current = Volts / Impedance = 172v RMS / 6600Ω = 26mA RMS through the primary.

        Check:  172v RMS x 0.026A RMS = 4.47 watts

        But the tube's limits are about Peak values:  172v RMS x 1.414 = 243v Peak, 26mA RMS x 1.414 = 36.8mA Peak

        We should see the 6V6's idle current of 45mA allows us the room to swing upward by ~37mA to ~82mA.  We then check the upper graph on Page 4 of the data sheet to see whether our 250v on the screen will allow this plate current:  it does, as the tube could manage around 100mA.

        We should see the plate voltage swing from 250v down by 243v Peak to only 7v on the plate is not-possible.  The curves converge below the knee to show we need ~20v minimum on the plate, but the error is fairly small & gets ignored.

        We should notice the "applied signal" is given as 8.8v RMS, which is 8.8v RMS x 1.414 = 12.44v Peak, and almost as-large as our bias voltage of -12.5v (meaning the tube is fully-driven, short of gross distortion).

[HotBluePlates]

All of that stuff above is just one relative-balance of Volts, Current, Impedance and Power Output.

We could pick a different supply voltage, work around different limits of the tube, use a different load impedance to get a different balance of Volts & Current, and wind up with a same or different power output.



"Tone" doesn't matter, only getting a "balance that works."  From there, people mainly notice "people like the way this balance-of-factors sounds."

[dwinstonwood]

Thanks HotBluePlates. I really appreciate your time to dive into this.

I'm following your math and explanations well enough. I can see how it's a matter of designing within the limitations of the tube, and how the tube can allow different combinations of voltage, current, and load to arrive at a similar (or, different) power output.

I imagine that tubes were originally designed to work well with predetermined sets of values, and what I'm trying to do is work in the opposite direction: I'm trying to see what various sets of values will work safely within the tube's ratings.

And, I understand that the datasheet is the guide for doing this. So, where I need to do more work is in teaching myself how to read the tube datasheets.

In the Tung-Sol datasheet (attached below) there are three graphs:

They seem to be set up to correspond to the center column under "Single Tube Class A1."

I'm assuming that the vertical dashed line in the third graph labelled "Rated Load" represents the 5,000 Ohm "Load Resistance" in that same center column.

The other two dashed lines: "Ib" and "Ic2" apparently represent the maximum plate current - 47mA, and maximum screen current - 7mA, respectively, since that's where they intersect the vertical load line of 5,000 Ohms.

Anyway, it takes a bit of studying to figure out what the graphs are representing. And every manufacturer presented their data somewhat differently (or, even presented different data).

There are no graphs for the Push-Pull AB1 table. But, the first column that uses 250V on the plates and screens recommends a "Load Resistance" of 10,000 Ohms plate-to-plate. So, still 5,000 Ohms per tube.

Again, thanks HBP for your help. I'll keep working on it, and referring to your math.

https://frank.pocnet.net/sheets/127/6/6V6.pdf

[dwinstonwood]

A light bulb is starting to flicker.

45mA DC is our idle current without an AC signal.

You're saying that the Peak AC current is 36.8mA.

So, the tube must be able to allow the current to swing up 36.8mA ABOVE our 45mA "floor" which results in 81.8mA.

The part I can't extract from the graphs is how and where are you determining "whether our 250v on the screen will allow this plate current:  it does, as the tube could manage around 100mA."

Are you looking at the "Ec1=0" line (the top line) in the first graph?

Of course, this is with the screen at 250V. I imagine everything changes if the screen was at 350V?

Thanks.

[shooter]

A light bulb is starting to flicker.

always fun when that happens


look back in the datasheet, there will be AC signal conditions, typically it will also indicate a max current for that condition.
I think of "bias" as one end of the "swing", zero as the other end.
in SE, say the cathode is 30vdc (-30vdc to G1), G1 can "swing" to 0 with an AC signal.
an AC signal larger that 30Vac will on paper swing the tube into compression or cut-off (distortion)
adjusting the AC to make happy sounds without tube meltdown gives you "tone"

[dwinstonwood]

...look back in the datasheet, there will be AC signal conditions, typically it will also indicate a max current for that condition.

Thanks shooter. Somehow I'm missing that, I just can't seem to see it. I guess I'm blind. 

The only thing I see in the entire datasheet that shows 100mA plate current is this graph. But, I don't really know what I'm supposed to see.

[shooter]

it's in the written part

[dwinstonwood]

Thanks shooter! I was looking at the wrong datasheet. 

The one I was using is the Tung-Sol. The one you just posted is the GE.

Totally different sets of data. 

http://www.radiostation.ru/tubes/6V6GT-GE.pdf

[shooter]

 
when I was at your stage and I couldn't play with high volts for medicinal reasons  , I'd surf datasheets for the better one's, could spend hours with good music surfing through then 


this is a great place to find every variant out there
Frank's electron Tube Data sheets (pocnet.net)

[PRR]

The one I was using is the Tung-Sol. The one you just posted is the GE. Totally different sets of data.   

You are messing with us?

[dwinstonwood]

You are messing with us?

Well, not intentionally. 
But, staring at that datasheet for too long without a break was messing with my head.

And, just a stab in the dark... does "AF" pertain to the AC audio signal? I have no idea what the letters stand for. Thanks.

edited: Never mind:

"AF (audio frequency) (also abbreviated af or a.f.) refers to alternating current ( AC ) having a frequency such that, if applied to a transducer such as a loudspeaker or headset, will produce acoustic waves within the range of human hearing. The AF range is generally considered to be from 20 Hz to 20,000 Hz."

[Willabe]

 

[dwinstonwood]

Thanks Willabe, I'm getting a lot of great info from this thread, so a genuine thanks to all who've replied.

I think I sort of understood this when I first started in this hobby in early 2020, but one thing that's become more obvious in this thread is:

While tube manufactures published "Design Center Values," amp manufacturers were typically trying to wring every last useable watt out of the tubes they were using. I think the Deluxe Reverb is maybe one good example of this.

For someone new to tube amp building like me, this can be confusing. The datasheets recommend one set of operating values, while Leo, et.al., were selling amps that sometimes ran beyond those recommendations. Obviously, louder & cleaner were/are competitive selling points.

At any rate, I'm planning a single-ended 5881 build using the Class A1, center column "Typical Operation" values, to see if I like it, and to hopefully learn from it. Even though I know that knowledgeable guitar amp designers might suggest, "if you want 4.5 SE watts, use a 6V6 pushed harder, not a 5881 at 250V."