Output transformer matching to an output Tube

[anthony101]

Hi everyone,

I need y'all help, where can i find information on Plate to Place load resistance on output tubes? Or if someone knows the answer please chime in.

Here is what i need;
I have what seams to me an output transformer from a vintage aircraft repeater that i took apart. After testing the transformer here is what i read, 16:1 with a Z ratio of 256 and a load of about 2k at @ 8ohms,

My question, what output tube can i use with this output transformer? i was hoping to look into a book of some type and get a plate to plate load resistance and find 2k and cross reference a tube, and build a simple amp.

Thank you !

 

[Hi-FiGuy]

That looks like a low level transformer. Probably for input or interstage use. If the primary is rated for it, you could use it in a cathode of a tube as a low level output stage (Line Stage output).

 

[anthony101]

Thanks, Hi-FiGUy,

Line Stage works for me, any particular schematics that you can recommend or reading i should focus on?

thanks

 

[Hi-FiGuy]

The specs seem a bit odd. Do you have a spec sheet for this unit?

 

[anthony101]

No sheets or specs, I was able to perform initial testing as per the, "How to test output transformers" on Youtube. These are the numbers that i got, unless i'm way off?

 

[Hi-FiGuy]

This may be a 10k:600 transformer or some other standard 16:1 ratio of impedance. The other thing to find out is what frequency response this unit has.

 

[anthony101]

Here is the method i use to calculate the impedance of this OPT, however i used 19 volts AC.


How to calculate the output transformer impedance of an OPT with unknown values

Working with tube amps you may need to calculate the output transformer impedance especially when we have an old transformer or a transformer with unknown values.
All we have to do is to calculate the turns ratio of the transformer. To do this we need an AC voltage source and an AC voltmeter.
So we apply an AC voltage to the primary of the transformer and we measure the output voltage to the secondary.
Turns Ratio = V in (to the primary) / V out ( measured in the secondary)
e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is :
5 / 0.21 = 23.8 That means a turns ratio 24:1
Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary.
The impedance Ratio is the square of the turns ratio:24 X 24 = 576 that means an impedance ratio 576:1
So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)

 

[anthony101]

Found the info i was looking for!

FYI just in case some of you also need this info.

Single Ended Class A

Tubes ok to use @ 2k Plate to Plate Load Resistance
2A3
6Y6
7A5
25L6
35A5
35B5
35L6
48
50A5
50B5
50L6

 

[nashvillebill]

I doubt that little OT would handle more than a watt or so of output power though, it looks rather tiny to me.

 

[anthony101]

Yes its about 2 inches tall by 2 1/2 wide, maybe i'll get 1.5 watts?

 

[mpholland]

I'm guessing if that is an output transformer it won't have crap for frequency response no matter what tube you use. It will probably sound like an old CB radio.

 

[gadget73]

I'm thinking that is more useful as an input transformer for high impedance mics or something. Might even work as a step-up transformer for a moving coil cartridge if you've got a twin to it. I don't expect that can handle any sort of DC current through it like an output transformer would need. Physically, a single ended output transformer has to be sizable just to carry current. Even the wee little guys used in AA5 type radios with their 1.5 watts of audio output have more of a core than that one likely has. That looks similar in size to the interstage transformers used in 1930s radios.

 

[anthony101]

Yeah, i think you guys are right on about the frequency response and low wattage, and as far as CB radios goes them was good old days....Roger Out!