Load Resistance (Ra) question applied to OTs

Captn Dave · 10:00 PM

I have a couple of push pull EL84 transfos with single unknown secondary taps. In order to measure turns ratio I put a 16 volt AC source on the primary between the CT and one of the outside leads (0.73 volt). The voltage ratio was 22 and I take that to be equivalent to turns ratio. That squared should be the impedance ratio of 484. With an 8 ohm speaker load that should be 3872 ohms.

Now, assuming that the above methodology is sound (?), I'm wondering how this relates to EL84 application. Most reference data shows a required "Plate to Plate Load Resistance" of 8K ohms. Plate to plate suggests to me that we are measuring load impedance from one plate to the other, whereas I have measured from the CT to one end of the winding (CT to plate rather than plate to plate). That suggests to me that I need to double the measured value in order to compare it with the plate to plate load resistance numbers. IS THAT CORRECT?

If so, I would think these transformers applied with EL84s would be a good match with an 8 ohm load. This is a little puzzling because I'm pretty sure they were used with a pair of 8 ohm speakers in parallel (4 ohm load). It's kind of a head scratcher so I suspect that I might not thinking about it correctly.

What can you tell me?

pointy · 1st December 2009, 10:46 PM

this will confuse the issue "if the speakers were in series to start with".

hemgjord · 1st December 2009, 10:49 PM

Hello

Your reasoning is sound up to the point where you duoble the impedance instead of the turns ratio.
CT to anode lead have a turns ratio of 22 then anode to anode has 44. If you square that and assume 4 ohms load you end up at approx the usual 8k a-a for an EL84 output stage.

Regards
/olof

pointy · 1st December 2009, 10:51 PM

whoops..............!

Captn Dave · 2nd December 2009, 07:12 PM

D'oh. I should have figured that out! Thanks for the help.

I suppose one should always measure across the full winding on a push pull transformer as the data is stated for that convention; ie. plate to plate - both tubes / both windings.

It had not occured to me before, and I suppose it's obvious, but I would imagine that is why the SET numbers are always about half the PP numbers. SET numbers are just for a single tube tube.

Isn't it great when the lights come on!!!

Thanks for the help, Olof. I do appreciate it.

1st December 2009, 09:57 PM	#1
Captn Dave diyAudio Member Join Date: Jun 2008	Load Resistance (Ra) question applied to OTs I have a couple of push pull EL84 transfos with single unknown secondary taps. In order to measure turns ratio I put a 16 volt AC source on the primary between the CT and one of the outside leads (0.73 volt). The voltage ratio was 22 and I take that to be equivalent to turns ratio. That squared should be the impedance ratio of 484. With an 8 ohm speaker load that should be 3872 ohms. Now, assuming that the above methodology is sound (?), I'm wondering how this relates to EL84 application. Most reference data shows a required "Plate to Plate Load Resistance" of 8K ohms. Plate to plate suggests to me that we are measuring load impedance from one plate to the other, whereas I have measured from the CT to one end of the winding (CT to plate rather than plate to plate). That suggests to me that I need to double the measured value in order to compare it with the plate to plate load resistance numbers. IS THAT CORRECT? If so, I would think these transformers applied with EL84s would be a good match with an 8 ohm load. This is a little puzzling because I'm pretty sure they were used with a pair of 8 ohm speakers in parallel (4 ohm load). It's kind of a head scratcher so I suspect that I might not thinking about it correctly. What can you tell me? Last edited by Captn Dave; 1st December 2009 at 10:00 PM.

1st December 2009, 10:49 PM	#3
hemgjord diyAudio Member Join Date: Apr 2007	Hello Your reasoning is sound up to the point where you duoble the impedance instead of the turns ratio. CT to anode lead have a turns ratio of 22 then anode to anode has 44. If you square that and assume 4 ohms load you end up at approx the usual 8k a-a for an EL84 output stage. Regards /olof __________________ I'm not allowed to do magic, union rules...

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Op amps: output impedance and load resistance	mudihan	Chip Amps	17	20th April 2009 08:09 AM
how to estimate the load resistance of a circuit	pchw	Power Supplies	0	5th January 2007 11:47 PM
Load-resistance for MC's	rho	Analogue Source	10	28th July 2005 08:08 PM
Load Resistance as seen by PSU??	Zero Cool	Solid State	8	22nd September 2004 05:04 AM
current DAC load resistance?	dimitri	Digital Source	0	20th June 2003 05:51 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

1st December 2009, 10:46 PM	#2
pointy diyAudio Member Join Date: Mar 2009	this will confuse the issue "if the speakers were in series to start with".

1st December 2009, 10:51 PM	#4
pointy diyAudio Member Join Date: Mar 2009	whoops..............!

2nd December 2009, 07:12 PM	#5
Captn Dave diyAudio Member Join Date: Jun 2008	D'oh. I should have figured that out! Thanks for the help. I suppose one should always measure across the full winding on a push pull transformer as the data is stated for that convention; ie. plate to plate - both tubes / both windings. It had not occured to me before, and I suppose it's obvious, but I would imagine that is why the SET numbers are always about half the PP numbers. SET numbers are just for a single tube tube. Isn't it great when the lights come on!!! Thanks for the help, Olof. I do appreciate it.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)