Tube to Output Trannie Ratio?

[nickt]

Hi Guys,

Anyone know of a good link showing a chart of tube type to output trannie ratio?
I've found some info but not all in one place.

6v6 8k
el84 8k
kt66 4k

Yeah I'm a lazy dumbass who keeps misplacing links and forgetting numbers...

[drhulsey]

... Yeah I'm a lazy dumbass...

Well, I'm not sure about the lazy part
Don't know if this will help or not:
http://radioremembered.org/outimp.htm

[Andy Le Blanc]

Use your RCA manual.....

You can cross reference also when shopping for OPT, CEdist and few others
gives tranny info for the products they carry. Hammond transformers has
good ballpark data for its product too.

[drhulsey]

I also found this in Dan Torres Inside Tube Amps

[nickt]

Hey fantastic - see lazyness works

Thanks guys...

[tonestack]

The quick and dirty way of determining primary impedance is: (Va * Va) / W Regardless of what the data sheets say, there is no way on God's green earth that a 2 x EL84 amp operating with 300VDC on the plates is going to deliver 17W to the load with a 1000-to-1 impedance ratio (8K primary into an 8 Ohm load), as (300 x 300) / 8K ~=11W (Granted, impedance rises and falls with respect to frequency, but it is not going to fall that much on average). The proper primary impedance for a 17W 2xEL84 amp with 300V on the plates is ~=5.3K into and 8 Ohm load (or an impedance ratio of ~663 to 1) . That's why the primary impedance on vintage VOX AC15s is in the low 6K range.

[jjman]

The quick and dirty way of determining primary impedance is: (Va * Va) / W ..................

That's the most useful thing I've ever seen written on the subject.

I always read that "the voltage changes the appropriate impedance for the OPT" but never knew what to do with that info.

Can I use this equation for SE outputs too?


Thanx!

[M Fowler]

The chart the Tino Zolotta provides notes the intersection of the tube load and speaker impedances is the turns ratio.

For example

5000K primary into 8 ohm speaker secondary equals 25.00
5500k primary into 8 ohm speaker secondary equals 26.22

[tonestack]

It's not rocket science. Think about the problem in terms of power.

Power (W) = Voltage (E) x Current (I)

As I = E / R, W = E x E / R (or E^2 / R, where "^" is used to denote raised to the power of).


In the case of an output transformer, we are dealing with Z instead of R; therefore, W = E^2 / Z. We can pump as much current as we like into the output stage; however, the amount delivered to the load is bounded by the reflected impedance of the load and the efficiency of the output transformer.

Now, let's look at the problem from the secondary end of the output transformer. An 8K-to-8 ohm output transformer has an impedance ratio of 1000-to-1. Impedance ratio is the square of the voltage ratio. We have a voltage ratio of ~31.6-to-1; therefore, with three hundred volts on the plates, we have a secondary voltage of 300 / 31.6 ~= 9.486833 volts. Plugging this voltage level into the power equation shown above gives us 9.486833 x 9.486833 / 8 ~= 11.25W.

The equation that I have outlined above sets the upper limit for the amount of power that can be delivered to a load. It does not set the optimal primary impedance for a particular tube or set of tubes. The selected operating class and point must be capable of delivering the desired amount of power. A Champ has a 7K-to-4 Ohm output transformer. However, single-ended amps are only 25% efficient; thus, 7K is more than low enough to the deliver 3-4 Watts that these amps produce.