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ot- why are p-p tube output transformers unbalanced?

tnmgcarbide

tnmgcarbide

Diamond
Joined
Jul 6, 2004
Location
N. GA- 33.992N , -83.72W usa
why are p-p tube output transformers unbalanced?

i have 8-10 of them , and they all typically measure

250 ohm/100/150 , 300 ohm/180/120 resistance on the primary windings . i know
that ac impedance and inductance , and center
tapped 350-400volts , and an output load come into
the equation...... but is this intentional , or
does it just exist by default? does it just
"balance out" in practical application?

it would seem that one 6l6 or el34 would see more
current on the lower resistance?
 
tnmgcarbide said:
why are p-p tube output transformers unbalanced?

i have 8-10 of them , and they all typically measure

250 ohm/100/150 , 300 ohm/180/120 resistance. i know
that ac impedance and inductance , and center
tapped 350-400volts , and an output load come into
the equation...... but is this intentional , or
does it just exist by default? does it just
"balance out" in practical application?
Click to expand...

SWAG unbalanced so you get 3 choices of impedance, center tapped would only give you 2.
Tubes outputs are much higher impedance than transistors and impedance matching is critical for max power transfer.

CarlBoyd
 
so , @ a 60/40 differential, with 400v input to the
center .. and a balanced phase splitter tube or
intermediate center tapped transformer- the load on
the output tubes would be equal from the input and
not the plate voltage?
 
hmm. i could,
but it was a question in general.
ALL PP output transformers that i've encountered are
not even close to balnced . i just don't understand
why. windings on a fixed ohm 4/8/or 16 secondary
wouldn't possibly be standard?

a single ended, class-a only has one path .

who cares.


Can you post a photo of the transformer label, or at least a manufacturer and model number?.

all different from 50L6 to eL34 .

no point really .... if they aren't balanced?
some from hifi,guitar, pa, lab stuff . like i said
all similar. i dont know that it matters unless there's more than one tube ?

but two tubes push/pull have to be close.
 
The answer is simple as can be. The winding are done in one shot, pausing only to attach the center or other taps. Naturally, the outer half of the windings will have more feet of wire on it than the inner half. It doesn't make much difference since the number of turns on each is the same and the voltage on each will also be close to the same, although leakage reactance will be somewhat diffeent. The DC resistance is not high enough to have much effect.

I have made transformers where this factor was important, solved by winding both strands of wire together. Naturally, this will only work if the voltages are low. Another solution is to wind one layer of one winding, then a layer of the other. You can also have each occupy half of the available length on the core.

The hardest I ever did like that was 6 strands of #6 square to maximize coupling between windings. You wouldn't pay for one like that.

Bill
 
thank you bill, that was close to the answer i was
asking here . what about ULTRALINEAR ot's ? they have
a ct and 4 output taps .i don't have one to measure
today , but could they be any different( and does it matter in sound) ?

it would seem that in the real world, the AC output
transformer issue doesn't exist outside of my thoughts....
 
9100 is right. Ultralinear hooks a tap at like 80% to the screen grid to impart some nfb or something like that, and it improved thd. Usually, they end up making more power like that too. Been a long time since I worked with the stuff.
 
i think i get the picture...the sum of losses.

center charged
hi voltage b+ DC charges the plates from the ends, so shouldn't be critical. the output's AC signal , on the same wires as the DC b+ passes back through the xformer, and is not as critical as the dc resistance
would imply .
 
Bill is of course correct.

There is a slight effect, unbalancing the half cycles of output a small amount, which is somewhat equivalent to even harmonic addition, so can be a small contributor to "tube sound".

That can be minimized by winding the transformer "bifilar", both windings together as described, and also by breaking the winding into sections so that the innermost winding also has sections out farther in the layers. That can maintain insulation, and still get a good balance.

Since the voltage between the ends of the plate winding can have from 500V to over 1200V between them, winding bifilar, or "two in hand", is impractical, because the wire insulation is not good for anything like that voltage.

It's a great question, though.
 
thanks for the insight from everyone.

sometimes i just
refuse to shut up and accept the fact that some things
just work well without explanation. i've a need to question everything .

it's a machinist thing.
 
TDegenhart said:
As I recall this from MANY years ago, some of the circuits had pots in the finals cathode circuit to balance the tube current.

Tom
Click to expand...

Not uncommon to have an overall grid bias, and a separate balance control. Or even two separate bias controls.

Cathode bias circuits often had nothing for balance other than the tube characteristics.

As for balance at max output, the difference in resistance was often considerably less than the variation of tube voltage drop on peaks, which could easily be 30 to 50 volts.

Most outputs were pentodes, which tend to act as current sources on their plates (that is what the screen grids did), so the difference would only show up as a limit on max peak voltage (clipping point).

Triode outputs were often so "soft" that the wire resistance imbalance was quite swamped-out under other effects. If the designers were going for extra special performance, they could have the transformers wound with "sectional balance" to even out the resistance as mentioned above.

Lots of tricks to use when you cannot rely on more than a few dB of feedback to stabilize gain and linearize the amplifier.
 
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