Determining Output Stage Loadline

[Lost'n Found]

Hello.

Assuming that I'll be using the 6550A tubes in triode configuration at the
output stage:

I am having troubles in choosing the loadline, and here is why:

In the preamp stage, I would look for the proper quiscent point on the
curves, and then find RL (Plate resistance) from the slope of the loadline,
then find Rc. . .

BUT, the output stage is connected to an output transformer, and there is no
RL.

Once I decide to operate the tube on a certain loadline (I want for example
a loadline with (0v, 100ma) - (400V, 0ma), If this was in a pervious stage,
it would yield a resistor of 400v/100mA = 4k)

how can I operate on this loadline if I can't use RL = 4k? How can I let the
tube see the transformer as 4k load?

Or, maybe I should ask this: Can I choose any loadline at the output stage?
Or I am doomed to the specs of the tube in use? If doomed, what parameters
determine the loadline?

Thank you very much for your help.

[Ian Iveson]

Lost'n Found wrote

> Assuming that I'll be using the 6550A tubes in triode configuration
> at the output stage:
>
> I am having troubles in choosing the loadline, and here is why:
>
> In the preamp stage, I would look for the proper quiscent point on
> the curves, and then find RL (Plate resistance) from the slope of
> the loadline, then find Rc.

>
> BUT, the output stage is connected to an output transformer, and
> there is no RL.
>
> Once I decide to operate the tube on a certain loadline (I want for
> example a loadline with (0v, 100ma) - (400V, 0ma), If this was in a
> pervious stage, it would yield a resistor of 400v/100mA = 4k)
>
> how can I operate on this loadline if I can't use RL = 4k? How can I
> let the tube see the transformer as 4k load?
>
> Or, maybe I should ask this: Can I choose any loadline at the output
> stage? Or I am doomed to the specs of the tube in use? If doomed,
> what parameters determine the loadline?

How many questions is that?

Anyway, consider the transformer primary to be a resistive load for AC
only. That is, you can choose the DC operating point, and then draw
through it a line whose slope represents the transformer load.

Note that in consequence the line will extend to voltages beyond the
HT supply.

In reality the effective load is not constant, and the line is really
an ellipse or similar, but you can ignore that for the moment.

cheers, Ian

[Lost'n Found]

Ok, so If I am using a 4 ohm speaker, and assuming the transformer has 30:1
turn ratio, then would the load be 4*30*30 = 3.6k?

Thank you for your reply

"Ian Iveson" > wrote in message
. uk...
> Lost'n Found wrote
>
>> Assuming that I'll be using the 6550A tubes in triode configuration at
>> the output stage:
>>
>> I am having troubles in choosing the loadline, and here is why:
>>
>> In the preamp stage, I would look for the proper quiscent point on the
>> curves, and then find RL (Plate resistance) from the slope of the
>> loadline, then find Rc.
>
>>
>> BUT, the output stage is connected to an output transformer, and there is
>> no RL.
>>
>> Once I decide to operate the tube on a certain loadline (I want for
>> example a loadline with (0v, 100ma) - (400V, 0ma), If this was in a
>> pervious stage, it would yield a resistor of 400v/100mA = 4k)
>>
>> how can I operate on this loadline if I can't use RL = 4k? How can I let
>> the tube see the transformer as 4k load?
>>
>> Or, maybe I should ask this: Can I choose any loadline at the output
>> stage? Or I am doomed to the specs of the tube in use? If doomed, what
>> parameters determine the loadline?
>
> How many questions is that?
>
> Anyway, consider the transformer primary to be a resistive load for AC
> only. That is, you can choose the DC operating point, and then draw
> through it a line whose slope represents the transformer load.
>
> Note that in consequence the line will extend to voltages beyond the HT
> supply.
>
> In reality the effective load is not constant, and the line is really an
> ellipse or similar, but you can ignore that for the moment.
>
> cheers, Ian
>
>

[Patrick Turner]

Lost'n Found wrote:

> Hello.
>
> Assuming that I'll be using the 6550A tubes in triode configuration at the
> output stage:
>
> I am having troubles in choosing the loadline, and here is why:
>
> In the preamp stage, I would look for the proper quiscent point on the
> curves, and then find RL (Plate resistance) from the slope of the loadline,
> then find Rc. . .
>
> BUT, the output stage is connected to an output transformer, and there is no
> RL.
>
> Once I decide to operate the tube on a certain loadline (I want for example
> a loadline with (0v, 100ma) - (400V, 0ma), If this was in a pervious stage,
> it would yield a resistor of 400v/100mA = 4k)
>
> how can I operate on this loadline if I can't use RL = 4k? How can I let the
> tube see the transformer as 4k load?
>
> Or, maybe I should ask this: Can I choose any loadline at the output stage?
> Or I am doomed to the specs of the tube in use? If doomed, what parameters
> determine the loadline?
>
> Thank you very much for your help.

Perhaps you should travel on your keyboard and screen waves to
http://www.turneraudio.com.au/education+diy.html
Here you will see many pages listed on tubes and how to use them,
and one page listed there specifically is focused on load matching SE triodes,
and has a lot to say about trioded 6550.

I suggest you dally long enough to read all of what I have there, maybe it
all starts to make sense....

Patrick Turner.

[Ian Iveson]

Lost'n Found wrote

> Ok, so If I am using a 4 ohm speaker, and assuming the transformer
> has 30:1 turn ratio, then would the load be 4*30*30 = 3.6k?

Simply, yes. For the purpose of drawing a load line.

More strictly, it only takes into account the resistive part of the
load. OK mid band with well-behaved speakers, but not at the extremes
of bandwidth, where the inductance and capacitance of the transformer
become significant.

cheers, Ian

[Eeyore]

Lost'n Found wrote:

> Hello.
>
> Assuming that I'll be using the 6550A tubes in triode configuration at the
> output stage:
>
> I am having troubles in choosing the loadline, and here is why:
>
> In the preamp stage, I would look for the proper quiscent point on the
> curves, and then find RL (Plate resistance) from the slope of the loadline,
> then find Rc. . .
>
> BUT, the output stage is connected to an output transformer, and there is no
> RL.
>
> Once I decide to operate the tube on a certain loadline (I want for example
> a loadline with (0v, 100ma) - (400V, 0ma), If this was in a pervious stage,
> it would yield a resistor of 400v/100mA = 4k)
>
> how can I operate on this loadline if I can't use RL = 4k? How can I let the
> tube see the transformer as 4k load?
>
> Or, maybe I should ask this: Can I choose any loadline at the output stage?
> Or I am doomed to the specs of the tube in use? If doomed, what parameters
> determine the loadline?

Because the output transformer presents a near short-circuit to DC, the
quiescent anode / plate voltage will be near as dammit your B+.

The AC load will be the reflected impedance of your speaker through the output
transformer.

In operation the anode / plate voltage will go higher than B+ as well as lower.
Say for a 200V pk-pk signal and a B+ of 300V the anode voltage will go between
200V and 400V.

Graham

[Lost'n Found]

Thank you all for your input. Things are getting much clearer to me now, and
hence I know better what to ask.


The DC operating point at the output stage is ideally equal to that of HT.
Then I choose a proper value for Eg to get the desired swing/power.
The loadline or load curve should pass through that point (Where vetical
line at Ea intersects Eg)
The ratio N of the output transformer determines the slope of the line or
tangent to the curve at that quiescent point.

If the design does not meet requirements, I do the following:
To get a different loadline, I need a different transformer with different
N.
I can change HT to get a different point on the graph, and hence different
swing. . .
Change tubes.

I am designing a guitar amplifier, and I was trying to make the signal clip
and maximum output power of the amp. However, I am using high HT, and the
signal will be able to swing both sides at maximum power the output
transformer can handle. Thus, I can either choose a different HT, get a
differnt output transformer, look for different tube, or make the signal
clip in the drive stage (which wont be actually overdriving the amp).

Again, thank you for your help.





"Lost'n Found" > wrote in message
...
> Hello.
>
> Assuming that I'll be using the 6550A tubes in triode configuration at the
> output stage:
>
> I am having troubles in choosing the loadline, and here is why:
>
> In the preamp stage, I would look for the proper quiscent point on the
> curves, and then find RL (Plate resistance) from the slope of the
> loadline, then find Rc. . .
>
> BUT, the output stage is connected to an output transformer, and there is
> no RL.
>
> Once I decide to operate the tube on a certain loadline (I want for
> example a loadline with (0v, 100ma) - (400V, 0ma), If this was in a
> pervious stage, it would yield a resistor of 400v/100mA = 4k)
>
> how can I operate on this loadline if I can't use RL = 4k? How can I let
> the tube see the transformer as 4k load?
>
> Or, maybe I should ask this: Can I choose any loadline at the output
> stage? Or I am doomed to the specs of the tube in use? If doomed, what
> parameters determine the loadline?
>
> Thank you very much for your help.
>
>
>