anyone know the KT45 output primary impedance?

[imaradiostar]

I don't wanna hurt Dr. Z's business, I'm just curious for comparison sake. Feel free to message me privately if you know but don't want to admit it.

thanks,

jamie

[Jazz P Bass]

That is really interesting.
Nowhere on the Dr's web page is it stated what the output load of the KT-45 should be. (can be?)

[imaradiostar]

I'm looking for the primary impedance but it is interesting that the secondary impedance isn't listed. Other places say it has 4, 8 and 16 ohm taps.

jamie

[big_teee]

It sounds like a ef-86 version of a marshall JTM45, or perhaps a version of the 2204.
It has 2 EL34s.
I notice that a JTM45 uses a 8K primary, and the 2204 has a 3.6k primary.
So not real clear on this, what determines one needs the 8k, and the other needs a 3.6k?
Both give you the 4,8,& 16 ohms output?
Terry

[octal]

I'm pretty sure the early jtm45s used KT66 output tubes into an 8K load. That's kind of high for 6L6 type tubes by modern standards, but it will work. Once they switched over to EL34s, the load was adjusted to somewhere around 3,5K.

[TransLucid]

Don't forget that the primary impedance depends on the supply voltage. Lower supply voltages require lower primary impedances for the same power output.

[big_teee]

Don't forget that the primary impedance depends on the supply voltage. Lower supply voltages require lower primary impedances for the same power output.

This is a question?
Is there a formula used to derive at the primary impedance, or is determined by the Class of amplification or what?
Thanks in advance.
Terry

[imaradiostar]

Thanks for all that have contributed. I'm aware of the common primary impedances for EL34's. I was just wondering what impedance the KT-45 used specifically.

thanks.

jamie

[TransLucid]

Terry -

Here's how I think of it: Once you know the voltage supplied to the OT primary, you then choose the primary impedance based on the desired power output using Ohm's law. For PP output stages, where the primary is center-tapped, each tube sees half of the total windings, or 1/4 of the total primary impedance, since the impedance is proportional to the square of the windings ratio. So, if the total impedance of the primary is, say, 8k ohms, each tube will see 2k ohms.

For example, if the supply voltage is 400V and you want 25WRMS (50W peak) of output power, P=V2/R, or 50=(400)2/R, or R=(400)2/50 = 3200 ohms. Now that's for half of the primary, so the total primary impedance would be 12.8k ohms.

There are three variables (voltage, power output, and primary impedance). You can choose any two and calculate the third.

Once you know the power output, you choose the output tubes so that they can handle the specified output. You can find out by plotting the load line.

Bob