Unknow O.T.

[chocopower]

Hi,

Please check my maths for a push-pull ULTRALINEAR transformer. Cathode biased power secction. 320vdc into the plates.

I put 36.6vAC into the plate leads and measured 1.65 in the secondary.

So...
36,6/1,65 = 22,2, call 22.

That means a Turn ratio of 22:1

And an Impedance Ratio of 484.

For EL34 i expected 8ohm, (3K8)
For 6V6, should be 16ohm. (7K7)
Is this right, even with a U.L. trafo?

thanks in advance,

david

[Phil_S]

I would round after all the calculations.
22.2^2=493
8*493=3944
16*493=7888

These numbers are close to the result you show.

I think the real problem now is to guess at whether this OT can take the input from a pair of EL34. They are going to 2x the power of a pair of 6V6.

Good luck.

[chocopower]

Hi,

this is the amp.

http://www.vintagehofner.co.uk/britamps ... haft1.html

It cames with 2 EL34 into 16ohm, so i used that way for several moths, but O.T. always get too hot, so i decide to test it just for learn. After the calculations, i connected it to a 8ohm load and after 2 hours playing really hard, trafo was just warm.
With both load, power transformer never get really hot.
Original amp had 4 preamp tubes, 1 P.I. tube and the 2 EL34. (plus the rectifier, but that use a diferent secondary, as you supose)
I moded it into a head using the power section.
Now, the Power trafo just have to serve 6.3v to 1 preamp tube, instead 4, so the heaters current is not a problem.

Any way, Are my supositions right?
8 ohm for EL34 and 16ohm for 6V6?

thanks, and sorry for my english.

[David Root]

Yes, close enough. A standard Marshall 2xEL34 OT primary is 3K4. A "standard" 2x6V6 OT primary is 8K these days although Fender did use 6K6 years ago, if I recall correctly.

If you put 2 6L6GC in it, they want to see a 4K primary.

[rsalinger]

3 ef86 tubes in the pre... hmmm... that's different.

Can anyone point me to a schematic?

[tubeswell]

Sounds like a 4k PriZ. (FWIW Its more 'accurate' to hook the secondary up to a small ac supply and measure the primary.)

[chocopower]

3 ef86 tubes in the pre... hmmm... that's different.

Can anyone point me to a schematic?

i didn,t find a schemo, so i draw it, not finish... is a rat nest!

check this topic:

http://music-electronics-forum.com/t10117/

In the Power amp schemo there are a mistake (or more...).
the cathodine plate and "sub-cathode" resistors are not 74k, but 56K.

Salinger! we live quite close. Nest time i travel to Aveiro looking for "arroz con marisco" I get with me the little beast!
Moito brigado!!!

[chocopower]

Yes, close enough. A standard Marshall 2xEL34 OT primary is 3K4. A "standard" 2x6V6 OT primary is 8K these days although Fender did use 6K6 years ago, if I recall correctly.
If you put 2 6L6GC in it, they want to see a 4K primary.

thanks! aleady i have a swicht to change bias betwen EL34 and 6V6.
I need to check the 6L6 option.

[chocopower]

Sounds like a 4k PriZ. (FWIW Its more 'accurate' to hook the secondary up to a small ac supply and measure the primary.)

Sorry... who means 4k PriZ?

I´ll repeat the test as you said, Thanks!

[chocopower]

upss.. just for share it.
i´m buiding a head using the modular power section and a home made preamp.
still tweaking values.

https://picasaweb.google.com/lh/photo/0 ... directlink

[Structo]

Sounds like a 4k PriZ. (FWIW Its more 'accurate' to hook the secondary up to a small ac supply and measure the primary.)

Sorry... who means 4k PriZ?

I´ll repeat the test as you said, Thanks!

4K ohm primary impedance.

[chocopower]

Sounds like a 4k PriZ. (FWIW Its more 'accurate' to hook the secondary up to a small ac supply and measure the primary.)

Sorry... who means 4k PriZ?

I´ll repeat the test as you said, Thanks!

4K ohm primary impedance.

Silly me....

Question. The primary impedande is in rellation with the tube plate to plate load and the secondary. So, in a multitap (4-8-16), what Plate load and secondary is used to calculate PriZ for a unknow transformer?

Sorry if i couldnt explain me better, but my fathers spend some money in my education... but i was playing guitar meanwhile....

[Structo]

Sorry if i couldnt explain me better, but my fathers spend some money in my education... but i was playing guitar meanwhile....

At least you're honest!

Seriously, if you Google "unknown output transformer"

You will get a lot of hits about it.

Wella!

http://www.google.com/search?sourceid=c ... ransformer

[chocopower]

I google it a lot of times. I think i understand most of the theory, but the reason why a OT is marked as "4k" or "6k6", when those values are totally diferent for diferent valves, reamain a mistery for me.....

Language problem, i suposse....

I´ll keed trying!!

[tubeswell]

Try the secondary-to-primary VAC measurements with each set of secondaries. The ones that give you the highest Pri:Sec voltage ratios will be the highest impedance ratios (and vice versa). Remember that an OT is all about reflected load.

And be careful when taking those measurements off the primary. And you only want to put a small AC voltage onto the secondary for this test (but you will still get high VAC readings on the primary).