"flipper"

I'm not alien to electronics but I don't have any tube experience and
I'd like to design and tinker with a small SE amp. Since it's to
tinker with I'm looking at junk parts and cheap tubes. E.g. 6L6 (6W6).

** Hang on - the 6L6 and 6W6 are not equivalents.


Before I plunge too deep into trouble I figure verifying the obvious,
like the characteristics section of the data sheet, is a good place to
start and I'm already confused.

From the 6L6 data sheet for class A and 200V plate


** You mean the 6W6 data sheet.


Rp 28K, load 4k, Rk 180 and Gm 8000
So, do I assume the 180 is bypassed, or not?


** Yes.


Anyway, if I understand the equations correctly that comes to a stage
gain of 28 if Rk is bypassed and 17 if not.


** Hang on - those are small signal numbers.

You cannot go applying them to the full output case.


Now, the data sheet says 3.8W out with 8.5V peak AF on the grid.


** That is for large signal operation - with lots of 2nd harmonic.

3.8 watts into 4000 ohms implies 123 volts rms - or 174 peak.

But even this is simplistic as the *real* voltage waveform will have
significant non-linearity.





............. Phil

OP snipped

If you want to gain realistic understanding, ignore the data sheet -
which only gives examples of typical operation - and instead read about
the basics of how tubes are operated as power amplifers and how these
circuit parameters are designed or selected, and why, which often is
well-covered in only a few pages of many tube manuals. Then, the
only-for-example data and many other things will make some sense to
you, and you may avoid endless misconceptions before any of them take
root.

Phil Allison wrote:

"flipper"

I'm not alien to electronics but I don't have any tube experience and
I'd like to design and tinker with a small SE amp. Since it's to
tinker with I'm looking at junk parts and cheap tubes. E.g. 6L6 (6W6).


** Hang on - the 6L6 and 6W6 are not equivalents.


True, to add to the confusion, even ignoring the heater specs,
50L6's and 6L6's are not equivalents. And the 1L6 is way
different than a 6L6. 50L6 and 6W6 are equivalent except
for the heaters, though. Usually, the pattern of American
tube designations #XX# is that the first number is the
heater voltage, and that the XX# portion, if the same, would
mean an equivalent tube aside from the heater. BUT NOT
ALWAYS. Always look in the data books to check this for
any pair of dissimilar heater rated tubes.

Flipper, don't ignore the data sheet, it's all you've got.

Phil has made the key point, that the Va can swing higher than HT
because of the transformer. This is not often stated in books I've
seen.

Next you might wonder if there is any limit to how high Va can go.
In principle, the limit would be imposed by insulation breakdown, ie
very high. In practice you can see by the loadline that Va can't go
up much further than you can go down. Up more than down (or vice
versa) is distortion.

Which is where Phil's twice comes from.

cheers, Ian

"flipper" wrote in message
...
I hope this is a proper place to ask for some help, so here goes.

I'm not alien to electronics but I don't have any tube experience
and
I'd like to design and tinker with a small SE amp. Since it's to
tinker with I'm looking at junk parts and cheap tubes. E.g. 6L6
(6W6).

Before I plunge too deep into trouble I figure verifying the
obvious,
like the characteristics section of the data sheet, is a good
place to
start and I'm already confused.

From the 6L6 data sheet for class A and 200V plate

Rp 28K, load 4k, Rk 180 and Gm 8000

So, do I assume the 180 is bypassed, or not?

Anyway, if I understand the equations correctly that comes to a
stage
gain of 28 if Rk is bypassed and 17 if not.

Now, the data sheet says 3.8W out with 8.5V peak AF on the grid.

8.5 times 28 is 238V. How do we get 238V from a 200V plate?
Ok. Maybe Rk isn't bypassed.
That's 8.5 times 17 for 144.5V

Now I don't understand their meaning of 'peak'. 144.5 'peak' into
4K
doesn't give me 3.8 Watts, assuming they mean 3.8 Watts RMS.

Neither does the 238V for that matter.

I'm sure it's something incredibly simple but what is it I'm not
getting?

Ian Iveson wrote:

Flipper, don't ignore the data sheet, it's all you've got.

Right. Besides, the poor bloke who wrote it would probably feel better if
someone read it


Phil has made the key point, that the Va can swing higher than HT
because of the transformer. This is not often stated in books I've
seen.

Right. Considering how many people have mentioned that I gather it comes as
a surprise to some.

Next you might wonder if there is any limit to how high Va can go.
In principle, the limit would be imposed by insulation breakdown, ie
very high. In practice you can see by the loadline that Va can't go
up much further than you can go down. Up more than down (or vice
versa) is distortion.

Which is where Phil's twice comes from.

cheers, Ian

Thank you. I appreciate the help.