Edit: What determines your OT requirements?

[tribi9]

Sorry I edited my original post...

This is the info that I'm after...

How do you decide on the rating of an OT? Do you just add the plate resistance of each tube?

[tribi9]

Bump.

So for example if I have 4 EL34's how would I determine what the secondary tap should be on my OT for my application?

Thx

[Phil_S]

Either you draw a load line or you make an educated guess from the tube data sheet. Tube data sheets: http://tubedata.itchurch.org/index.html or you ask at a forum like this. Knowing expected plate voltage makes a difference.

[Andy Le Blanc]

1: define supply voltage

2: define idle current

3: fix operating point by assuming max plate current

here you combine plate curves with the max dissipation limit

the load line , rL, is 1/4 of the transformer impedance.

you can do it with two paper print outs and red string

where load line intersect the 0v point of the grid is where you find max I

where the string crosses between the graphs is 0ma plate current and operating point

It starts to act like a big slide rule, you slide the two graphs applying load
lines that don't cross the max plate dissipation limit.

BUT..there is a bit more to it, the sweet spots are related to the solutions
for harmonic distortion, the relative percent's of even and odd order
products to the power out. once you have the current data and rL, you can
go as far as you wish.

[Andy Le Blanc]

Loading...... say you have an OPT where the sheet says it has a 4K primary
and a 8 ohm secondary , its a ratio.

So... 4000:8... push pull... that's 500:1 impedance ratio 22.4:1 turns ratio.

so when you change the load its say 16 x 500 or 4x 500, with 16 ohm load
the load reflected back to the plates is 8K with 4ohm speaker its 2K.

when you see an opt with say 4000: 16/8/4 it has taps on the secondary to
maintain the ratio to keep a 4k load on the plates with different loading.

An opt is also a matching device so if you have 4k:8 you can also get a 2k:4
and a 8k:16. You don't want to go to extremes doing this but its how the opt can be utilized.

So if your dead set on using a specific speaker or group of speakers you
choose to use an opt that will match the speaker load and the reflected load you find recommended for the tubes.

so say 4x el34 want to see around 2k but your stuck with a 4k:16

you can put a 8ohm load on it and the tubes will see 2K

[tribi9]

so say 4x el34 want to see around 2k but your stuck with a 4k:16

you can put a 8ohm load on it and the tubes will see 2K

So, it is ok to mismatch the load to the output transformer then?

[Andy Le Blanc]

A mismatch happens when the amp is set up for a condition and it doesn't "see" it. ...
if the bias and operating conditions are set up for a load but the wrong load is applied,
the opt is a matching device but the amp isn't automatic, you set the conditions...know your load.

[Andy Le Blanc]

Some months back I built a power side using KT90, All I had were opt from
A twin and a generic marshal 100w. The twin was 2K/4 and the marshal was
1.7K/4,8,16. I wanted to use a fixed bias and provide a load to use KT90, KT88.
Most of the data sheets I found suggested 3K loading. And I couldn't get it
out of the twin opt, so I put an 8 ohm load on the 4 ohm tap to get around
3.4K, about as close as I could get. Great sounding amp, I had to be conservative
with the bias because the PT was a generic twin and had a limited ma. rating (310ma.).
Its been gig'ed out four nights a week for nearly a year, and the guy won't give it back.

When I loaned it to the guy, I told him it was limited to an 8 ohm load
and asked him what he intended to use, he said he had the appropriate load
and would respect it. That a year of open mic's, bar gig's, and outside
festival stages. (and the guy pounds on his gear)

Know your load.

[tribi9]

Thx for the explanation and the example Andy.