Was looking over the N78 tube info http://www.r-type.org/pdfs/n78.pdf and saw something that confused me. Triode AB with 300v needs a 5k output transformer but at 350v it calls for an 8k transformer.
1) Why did the needs of the transformer change so much? Did they just try an 8k transformer and that combo of B+ and current is what was needed to get the distortion that low?
2) Is there a consistent relationship across tubes that you can use to figure out what operating points to use when you have an 8k transformer instead of a 5k one? Or is that always to be found using the tube curves somehow?
3) Aren’t they missing a lot of info to go along with the distortion measurements? Surely different transformers, topologies, etc. will affect the final number right? What did the manufacturers assume when they post distortion figures like that?
Thanks for any help. Still trying to figure out how to read tube data.
1) Why did the needs of the transformer change so much? Did they just try an 8k transformer and that combo of B+ and current is what was needed to get the distortion that low?
2) Is there a consistent relationship across tubes that you can use to figure out what operating points to use when you have an 8k transformer instead of a 5k one? Or is that always to be found using the tube curves somehow?
3) Aren’t they missing a lot of info to go along with the distortion measurements? Surely different transformers, topologies, etc. will affect the final number right? What did the manufacturers assume when they post distortion figures like that?
Thanks for any help. Still trying to figure out how to read tube data.
You can run tubes "low and hot" (low voltage, high current and low Z load), "high and cold" or somewhere in between with different numbers for distortion, damping factor and output power as as result. Take a look at https://www.vtadiy.com/loadline-calculators/loadline-calculator/ to get a glimpse of what happens when you change U, I and Z.
isaacc7,
Are you wanting to design a vacuum tube amplifier?
From your other recent post, perhaps you are trying to learn the hints and kinks to make a successful and good sounding system.
Much learning is done with doing some simple design, having it critiqued, perhaps modified accordingly, and then building it.
Sometimes a relatively inexpensive and simple amplifier can result in learning a lot, and having something to enjoy for many years, even if you go for more expensive and complex amplifiers, loudspeakers, etc.
Do you have any Hi Fi system now?
Can you list the components.
And, what might you want a new system to do, louder, kind of music, work in a bigger room, etc.
Undefined desires and problems are not solvable.
Perhaps you are just wanting to learn about all kinds of issues, but not purchase, build, etc.
Are you wanting to design a vacuum tube amplifier?
From your other recent post, perhaps you are trying to learn the hints and kinks to make a successful and good sounding system.
Much learning is done with doing some simple design, having it critiqued, perhaps modified accordingly, and then building it.
Sometimes a relatively inexpensive and simple amplifier can result in learning a lot, and having something to enjoy for many years, even if you go for more expensive and complex amplifiers, loudspeakers, etc.
Do you have any Hi Fi system now?
Can you list the components.
And, what might you want a new system to do, louder, kind of music, work in a bigger room, etc.
Undefined desires and problems are not solvable.
Perhaps you are just wanting to learn about all kinds of issues, but not purchase, build, etc.
no, we are here to help each other out, if you look at the tube data for different types of power tubes, this fact is obvious, we do not want our plates running red in any case...
I understand your confusion, the general rule is the higher your HT/B+ the higher the OPT primary Z. To be honest I don't bother much worrying about it and just wing it or chuck any old OPT in as long as it can handle the current and it sound ok. This is because when you draw a load line its for one resistive load, say 8 ohms, but a speakers Z isn't linear with frequency, it changes from anything from 4r to to 40r for instance.
Unless you trying to suck every last oodle of power out it don't matter much, we don't listen to sinewaves, we listen to music which has a complex frequency spectrum.
Unless you trying to suck every last oodle of power out it don't matter much, we don't listen to sinewaves, we listen to music which has a complex frequency spectrum.
What you seem to be missing is the basic P=IV? If V increases then I decreases in the same ratio. Since I=V/R then P=V^2/R. So if V goes up R has to increase by the square of the ratio for the same power. (350/300)^2 takes you from 5k to 6.8k but the characteristics suggested (to the manufacturers at least) that a more optimum load would be 8k rather than say 7k.
Note that the cathode resistor has increased for 350V operation which reduces the gain compared with the 300V configuration so a lower current requires a higher resistance (at the higher voltage) for the same output power. And also that the output power has increased a little for the 350V operation (6W instead of 4W).
As a poster replied above, try drawing the loadlines on the triode characteristics graph.
Note that the cathode resistor has increased for 350V operation which reduces the gain compared with the 300V configuration so a lower current requires a higher resistance (at the higher voltage) for the same output power. And also that the output power has increased a little for the 350V operation (6W instead of 4W).
As a poster replied above, try drawing the loadlines on the triode characteristics graph.
Generalization:
If you run a tube quiescent conditions at a fixed current and fixed plate voltage, for an output transformer with a higher impedance primary . . .
you will get lower distortion, lower wattage, and higher damping factor . . .
Before you even apply negative feedback.
Tradeoffs, tradeoffs, tradeoffs.
All Generalizations Have Exceptions
If you run a tube quiescent conditions at a fixed current and fixed plate voltage, for an output transformer with a higher impedance primary . . .
you will get lower distortion, lower wattage, and higher damping factor . . .
Before you even apply negative feedback.
Tradeoffs, tradeoffs, tradeoffs.
All Generalizations Have Exceptions
I am always interested in learning about how this stuff works. Unfortunately I am not able to do any actual DIY stuff due to medical issues. I joined this board years and years ago with the idea of designing and building my own amps but life decided to make other decisions for me.
Right now I do have a push pull amp that allows me to use a lot of different tubes. It has a regulated, variable B+ ranging from 20v-400v. It also has manual bias for the output tubes. I had the builder use Monolithic Magnetics OT and he picked 8k ones in order to get good sounds from a variety of tubes. I know mist will roll their eyes at this kind of design but I am as much a tube hobbyist as I am an audiophile. I love tubes as objects, their history, and the nostalgia and have a lot of fun trying different ones out. So far I really like how 807/6bg6 and 6aq5 in triode sound with this amp. I am always looking for other tubes that I can try, especially abandoned and somewhat obscure tubes like the n78.
Anyway, I am trying to get better about understanding how amps work so that’s I can imbibe in my tube hobby
OPT's after all are about turns ratios, the reason we draw load lines is so that we stay within the tubes "safe" operating regions, we do not want our output tubes to be red plating all the time...
you came to the right place....seek out the postings of member PRR, smoking-amp, tubelab.com, miles prower and many others, lots of golden nuggets about tubes...Anyway, I am trying to get better about understanding how amps work so that’s I can imbibe in my tube hobby
then Tubelab.com is the guy you want to see...he tortures tubes so that we never have to...
This sounds like a great way to experiment. How do you handle the different tubes bases and pin-outs?
I use adapters and external supplies for the heaters. The amp is set up for the typical 6v6/6l6/el34/kt88 etc. outputs and 6sn7/6sl7/6bx7, etc. input (driver?) tubes. With adapters I have used 807, 6gb6, 7c5, and 6bq5 outputs. I will try 12av5, gu50, and 12v6 tubes eventually. The input tubes have been a lot of fun. Not only are 12sn7 quite a bit less expensive (the only way I could afford GEC) but single triode adapters open up a huge range of possibilities. The regular 6/12j5 and 6c5 put a lot of 6sn7 to shame. I’ve branched out to triode wired pentodes as well. 12j7, c3m, and most outrageously ktz41 tubes all make amazing sounding drivers. The 4 ktz41 are drawing 24w for the heaters by themselves. Not the most efficient way to run 6aq5 in triode to be sure lol.