PP Output Trafo Measurement

[jkeny]

I've measured the primary resistances in a Rogers Cadet OPT. One measures 257-0-233 ohm (145) the other measures 256-0-240 ohms (147). The numbers in brackets = the ohms on a fourth tap on the each OPT. The secondary side has taps for grnd, 3/5R & 10/16R.

I have 2 questions:

Is the imbalance in readings between the PP sides of each trafo a big problem? Can this be fixed?

Does anybody know what the fourth tap on the OPT might be used for? There are 5 taps but one is unconnected (I had hoped that they might have been ultra-linear connections - but no).

John

[SY]

Is the imbalance in readings between the PP sides of each trafo a big problem? Can this be fixed?

In order:

No, as long as you understand that compensation may be different side to side. It's pretty common and derives from the coil diameter increasing as more turns are added; the "outer" half will have a higher DCR than the inner half.

No, not unless you rewind the transformers. Lots of segmentation helps this, but carries its own issues.

[jkeny]

Thanks SY,

Could I just unwind some wire until both sides are nearer to balance?

Any ideas on fourth tap designation?

John

[Tom Bavis]

No, they are wound for the same number of turns - a different number of turns is even worse than differing resistance. The only place I ever saw a push=pull transformer with ONE primary tap was in a Grundig console - the tap drove the electrostatic tweeter.

[jkeny]

Thanks Tom,
I won't do that so. What do you mean by

ONE primary tap

?

This OPT has five taps on the primary -
one is unconnected to trafo wiring,
three are used for mid & both end points of PP output stage,
and one is not connected to output stage but a wire runs to it from trafo so it gives the ohm readings given in brackets above.

Just wondering what this might be?

John

[cerrem]

Not to woryy about the slight difference in DC resistance....
The important thing is the turns are equal and that means the AC impedance is balanced...
When you model the output stage..the DC resistance gets added into the plate resistance of the tube...
With pentodes this DC resistance amounts to nothing...
With high power TRIODES the DC resistance plays a bigger role so there are techniques to wind the primary to get the DC resistance almost perfect...but in most cases no really needed..

Chris

[jkeny]

Help Moderators,

My username no longer works - I have re-registered under the same name JKeny but am now a new user on probation.

I'm not sure if my posts still exist - all my posts are refused display and appear as guest and are "on my ignore list"?

Can you help SY, Planet10, anybody?

John

[SY]

You should be OK now.

[jkeny]

Hi SY,
Not OK yet all my post still appear with no author name ( look further up in this thread) and

This person is on your Ignore List. To view this post click [here]

When I search for post under my name I don't find any?

Thanks for your help - has the database been corrupted?

John