Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on March 02, 2014, 07:20:29 pm
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Please help me find the correct math formula for Z OUT calculation, according to the chart below. Things are not quite easy to understand here. Thanks ! :worthy1:
Colas
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You can't get it from the data sheet, it depends on how you use the tube.
What is it you need Z out for? That will probably help determine how to figure it
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If you want to use 2 X EL34 in PP class AB with 430 VA ; Z is 6600 ohms . You need a Output transformer with primary close to 6600 ohms
EL34 RATING ( 3rd line )
AB1 P/P UL 43% 430 - - 125 - 130 10.0 - 10.2 470 6,600 20 0.8 Shared Rg2
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Thanks Stratele, I have understood that already. What I want to know, is the math formula that will give me the right z out at a given g2 and plate voltages. Look at the examples, at the beginnig of the PP voltages, g2 voltage comes over the plate voltage and it changes the z out. How do thet calculate that ? I would like being able to find out the matching impedance of a OPT, according to certain parameters, like: type of class, screen and plate voltages, bias, control grid voltage, current etc. etc. What are those parameters that are necessary to take count of, and how do we calculate the f...........Z out ???????
Coca Colas.
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Hi Colas,
Is asking me knowledge I have not :dontknow:
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This will give you a Zout for centre-biased Class A operation
Zout = Va/(Pa/Va)
Where:
Va = Anode voltage (at idle).
Pa = Maximum anode dissipation.
Class AB is the same Z-out, but you bias the tubes so that one side goes into cutoff for less than 50% of the time, causing the load resistance for the other 'on' tube to double, resulting in the steeper 'B' load line.
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... Look at the examples, at the beginnig of the PP voltages, g2 voltage comes over the plate voltage and it changes the z out. ...
Whoever put together that pdf didn't understand what they were copying into the chart. If you look at an actual EL34 data sheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL34.pdf), what your example sheet calls "Zout" is really "Ra" ([load] Resistance, Anode) or "Ra-a" ([load] Resistance, Anode-to-Anode). I'm sorry I didn't notice their error before, as I just glanced to see it was tube data; hence why I asked why you needed the quantity "Zout" and what you thought that meant.
The plate load of an output tube is almost never equal to the tube's internal resistance, or even its output impedance. For pentodes, it is very much less than the tube's actual internal resistance.
... I would like being able to find out the matching impedance of a OPT...
Let's get one important point out of the way: you do not "match" an output transformer to the tube, the way you match a speaker load to an OT tap. So if you're using an OT with a 6.6kΩ primary impedance, you're not doing it because the tube's output impedance ("Zout") is 6.6kΩ... you're doing it because it's the right load to give the power output you want, given the tube's supply voltage (plate and G2), class of operation and bias.
There is an exception to the above when you're talking about triodes; but I'm guessing you're not going to be building many triode output tube amps.
This will give you [edit: Load Impedance] for centre-biased Class A operation
Load Z = Va/(Pa/Va)
This is the one formula you might be able to use, if you're operating class A. Notice there is no reference to the tube, except plate dissipation. This formula might give an poor choice of load and operating condition if you're mis-using the tube (KT88 at 100v, or EL84 at 800v).
In the end, I think you're asking how do you determine the "right" or "best" OT primary impedance to use with a given output tube. That is mostly Ohm's Law, loadlines and some simplifying assumptions. You'll get an answer faster if you simply copy an amp using the same tubes at the same/similar plate and G2 voltage.
If you still want to know how to do it on your own, we probably ought to point you to some good books. I can show the basic process here, but you'll run into situations where it isn't easy to apply the process.
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Yes I assume that the tube is not being misused, and 'maximum anode dissipation' is the tube manufacturer's recommended maximum plate dissipation (which for most EL34s is 25W, hence in Stratele52's example the result would be 430/(25/430) = 7,396R)