Hi,

what do you think:
The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go
something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms
tap for the tweeter? Must be a perfect biwiring!
Thanks, Heinz

Behold, Heinz signalled from keyed 4-1000A filament:

Hi,

what do you think:
The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go
something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms
tap for the tweeter? Must be a perfect biwiring! Thanks, Heinz

Use a crossover on the *input* of the amps and your set, eh?

:-)

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

Biwiring is the misguided practice of using two sets of wires from one
pair of terminals to another. Biamping is using one amplifier for the
treble and another for the bass, with a crossover before the
amplifiers. What he's doing might be made to work, but not real well,
with a custom transformer and driver impedances or Zobel networks.
Basically not worth fooling with.

"Heinz"
Hi,

what do you think:
The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go
something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms
tap for the tweeter? Must be a perfect biwiring!
Thanks, Heinz


** Connecting between the 8 and 16 ohm output taps effectively gives you a
1.4 ohms tap.




............ Phil

Do not understand why 1.4ohms. If the transformer is winded
symmetrical, the 0-8 and the 8-16 windings must be the same.
Heinz

"Phil Allison" wrote in message ...
"Heinz"
Hi,

what do you think:
The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go
something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms
tap for the tweeter? Must be a perfect biwiring!
Thanks, Heinz


** Connecting between the 8 and 16 ohm output taps effectively gives you a
1.4 ohms tap.




........... Phil

"Heinz"

Do not understand why 1.4ohms.

** Tricks everyone.


If the transformer is winded
symmetrical, the 0-8 and the 8-16 windings must be the same.


** Not at all, the 0-4 and 4-16 windings have the same number of turns
while the 8 ohm will have 1.41% times the 4 ohm one.

Imagine the 16 ohm winding has 100 turns, then the 4 ohm has 50 and the 8
ohm has 71. A 2 ohm winding would have 35 turns. Remember - impedance
goes up with the SQUARE of the turns number.

In the above example there are 29 turns from 8 -16 (ie 100 minus 71) so
the impedance is 0.29 squared times 16 = 1.35 ohms.




............. Phil

Phil Allison wrote:

"Heinz"

Do not understand why 1.4ohms.

** Tricks everyone.

If the transformer is winded
symmetrical, the 0-8 and the 8-16 windings must be the same.

** Not at all, the 0-4 and 4-16 windings have the same number of turns
while the 8 ohm will have 1.41% times the 4 ohm one.

Imagine the 16 ohm winding has 100 turns, then the 4 ohm has 50 and the 8
ohm has 71. A 2 ohm winding would have 35 turns. Remember - impedance
goes up with the SQUARE of the turns number.

In the above example there are 29 turns from 8 -16 (ie 100 minus 71) so
the impedance is 0.29 squared times 16 = 1.35 ohms.

............ Phil

Phil is correct on this tricky subject of transformer turns, although
I wasn't tricked one bit.

People could be almost forgiven for dreaming that the turns between the common
end of a winding
and the 8 ohm tap were equal to the 8 ohm tap and the 16 ohm end point of the
winding and that the CT
of the winding was the 8 ohm tap.

But the 29 turns which Phil nominates is a match for 1.35 ohms,
but who would ever make use of these turns?

The wire dia use for the 29 turns would be the same as for the rest of the
tapped winding,
so if 1.35 ohms were used, the winding resistance would losses would be quite
high,
and the leakage inductance also high, and somewhat limited lossy HF transfer
would occur.

However, with a 0-8-16 ohm winding on an OPT, it would be possible to use each
of the
two outlet points for different Z bi-wired speakers.
There could be two 8 ohms bass-midrange in D'apolito series configured
speakers connected to the 16 ohm outlet,
and the single 8 ohm tweeter may be tried on the 8 ohm tap, rather than have a
resistive series
divider to reduce the tweeter SPL should that need doing, as it so often is.
The amplifier plate circuit would then remain matched to the correct nominal
anode load.

The only bother is that the NFB take off for the amp is usually on the whole
of the
16 ohm winding, and corrections for errors from the tweeter HF content are not
directly fed back,
which may not be an all important issue, but it sure isn't ideal engineering
either.

On the plus side, consider the case where an SET amp without loop FB is
concerned.
A typical 300B might be loaded with an OPT of 5k to 16 ohms,
which is a 312 :1 impedance ratio, so the Ra of 800 ohms is transformed
to 2.56 ohms as measured as Ro at the secondary, plus winding resistances of
say 0.8 ohms,
so Ro = 3.36 ohms, which gives a damping factor of 4.7 for a 16 ohms speaker,
and higher at bass F where typical speakers have a higher Z.
If the tap at 8 ohms is used for a tweeter, then the Ro at this tap is
1/2 that at the 16 ohm point, because the 8 ohm tap is at 0.71 times total sec
turns,
and 0.71 x 0.71 = 0.5.
So the Ro of the amp from the 8 ohm tap for the the tweeter is around 1.7
ohms, giving the same
reasonable damping factor of over 4 for the tweeter if it is 8 ohms.

There is the down side of building speakers needing two values of input
voltages;
they won't be usable with any other single input amp.

And slight variations between amps with various output configs may make
incompatibility
worse, however any amp with 0-4-8 would have the correct relationship of
voltages,
as 0-8-16 has.
8 + 16 ohm speakers could be used on any amp with 0-4-8 tappings
a tube amp with no harm done, and the expense of maximum power delivery,
but with the bonus of less thd and less losses, and a better damping factor.

Many speaker have series resistances added in series with them to attenuate
the SPL of the speakers concerned and its usually the midranges and tweeters,
because a man needs to hear as much bass as possible, and one way some makers
enhance bass
is by cutting midrange and treble.
The down side of this practice is that the mids and treble are driven with far

higher series impedance than that measured at the output of the amp.

Should one want lower voltages to be fed to mids and treble to get a more
balanced eq
and flatter response, they could do worse than use a tapped
toroidal transformer, using a 500 VA rated core, with a cross sectional core
area
of 50mm x 30mm and allowing for a voltage of 30vrms at 0.3 Tesla at
50 Hz, one could have about
300 turns of 1.5mm dia wire for a total whole winding, and with taps all
along it at
10% intervals of the turns.
if the hole dia was 50mm, then approx 98 turns per layer
could be neatly wound onto the core, so 3 layers
with insulation between would be possible, and doable by the keen diyer.

This auto transformer can thus be used for a wide variety of impedance
matching,
and cope with powers of up to 100 watts into 16 ohms, perhaps more
into 8 ohms, at slightly higher winding resistance losses.
THD would be negligible, and bw quite wide, since all the turns are
located in 3 layers on top of each other.
It might seem stange to have yet another transformer in the signal path
but nobody complains about the step up trannies in SL speakers.

If the amp is connected to one end of the auto tranny winding,
then as each tap down from the top of winding is tried, the Ro
becomes lower and lower, which is the opposite effect
of resistances in series with attenuation for speakers.

The other side effect is that the amp sees a higher value load if a speaker is
connected
at a tap lower down the auto tranny.
This may be a good thing if there is plenty of voltage swing headroom,
but not if a high power is required to the increased load effectively
harnessed to the tubes.

All is not completely simple, which is usual for anything to do with
electronics,
and there are tricks galore for the uninitiated novice who has no experience.


Patrick Turner.

"Bob Hedberg"

So I take it that if I were to utilize a split primary step-up
transformer between the two 8-16 windings, I could sum those two
portions of the signal and drive a center-channel speaker?



** No.


........... Phil

So I take it that if I were to utilize a split primary step-up
transformer between the two 8-16 windings, I could sum those two
portions of the signal and drive a center-channel speaker?

bob

Patrick Turner wrote:



Phil Allison wrote:

"Heinz"

Do not understand why 1.4ohms.

** Tricks everyone.

If the transformer is winded
symmetrical, the 0-8 and the 8-16 windings must be the same.

** Not at all, the 0-4 and 4-16 windings have the same number of turns
while the 8 ohm will have 1.41% times the 4 ohm one.

Imagine the 16 ohm winding has 100 turns, then the 4 ohm has 50 and the 8
ohm has 71. A 2 ohm winding would have 35 turns. Remember - impedance
goes up with the SQUARE of the turns number.

In the above example there are 29 turns from 8 -16 (ie 100 minus 71) so
the impedance is 0.29 squared times 16 = 1.35 ohms.

............ Phil

Phil is correct on this tricky subject of transformer turns, although
I wasn't tricked one bit.

People could be almost forgiven for dreaming that the turns between the common
end of a winding
and the 8 ohm tap were equal to the 8 ohm tap and the 16 ohm end point of the
winding and that the CT
of the winding was the 8 ohm tap.

But the 29 turns which Phil nominates is a match for 1.35 ohms,
but who would ever make use of these turns?

The wire dia use for the 29 turns would be the same as for the rest of the
tapped winding,
so if 1.35 ohms were used, the winding resistance would losses would be quite
high,
and the leakage inductance also high, and somewhat limited lossy HF transfer
would occur.

However, with a 0-8-16 ohm winding on an OPT, it would be possible to use each
of the
two outlet points for different Z bi-wired speakers.
There could be two 8 ohms bass-midrange in D'apolito series configured
speakers connected to the 16 ohm outlet,
and the single 8 ohm tweeter may be tried on the 8 ohm tap, rather than have a
resistive series
divider to reduce the tweeter SPL should that need doing, as it so often is.
The amplifier plate circuit would then remain matched to the correct nominal
anode load.

The only bother is that the NFB take off for the amp is usually on the whole
of the
16 ohm winding, and corrections for errors from the tweeter HF content are not
directly fed back,
which may not be an all important issue, but it sure isn't ideal engineering
either.

On the plus side, consider the case where an SET amp without loop FB is
concerned.
A typical 300B might be loaded with an OPT of 5k to 16 ohms,
which is a 312 :1 impedance ratio, so the Ra of 800 ohms is transformed
to 2.56 ohms as measured as Ro at the secondary, plus winding resistances of
say 0.8 ohms,
so Ro = 3.36 ohms, which gives a damping factor of 4.7 for a 16 ohms speaker,
and higher at bass F where typical speakers have a higher Z.
If the tap at 8 ohms is used for a tweeter, then the Ro at this tap is
1/2 that at the 16 ohm point, because the 8 ohm tap is at 0.71 times total sec
turns,
and 0.71 x 0.71 = 0.5.
So the Ro of the amp from the 8 ohm tap for the the tweeter is around 1.7
ohms, giving the same
reasonable damping factor of over 4 for the tweeter if it is 8 ohms.

There is the down side of building speakers needing two values of input
voltages;
they won't be usable with any other single input amp.

And slight variations between amps with various output configs may make
incompatibility
worse, however any amp with 0-4-8 would have the correct relationship of
voltages,
as 0-8-16 has.
8 + 16 ohm speakers could be used on any amp with 0-4-8 tappings
a tube amp with no harm done, and the expense of maximum power delivery,
but with the bonus of less thd and less losses, and a better damping factor.

Many speaker have series resistances added in series with them to attenuate
the SPL of the speakers concerned and its usually the midranges and tweeters,
because a man needs to hear as much bass as possible, and one way some makers
enhance bass
is by cutting midrange and treble.
The down side of this practice is that the mids and treble are driven with far

higher series impedance than that measured at the output of the amp.

Should one want lower voltages to be fed to mids and treble to get a more
balanced eq
and flatter response, they could do worse than use a tapped
toroidal transformer, using a 500 VA rated core, with a cross sectional core
area
of 50mm x 30mm and allowing for a voltage of 30vrms at 0.3 Tesla at
50 Hz, one could have about
300 turns of 1.5mm dia wire for a total whole winding, and with taps all
along it at
10% intervals of the turns.
if the hole dia was 50mm, then approx 98 turns per layer
could be neatly wound onto the core, so 3 layers
with insulation between would be possible, and doable by the keen diyer.

This auto transformer can thus be used for a wide variety of impedance
matching,
and cope with powers of up to 100 watts into 16 ohms, perhaps more
into 8 ohms, at slightly higher winding resistance losses.
THD would be negligible, and bw quite wide, since all the turns are
located in 3 layers on top of each other.
It might seem stange to have yet another transformer in the signal path
but nobody complains about the step up trannies in SL speakers.

If the amp is connected to one end of the auto tranny winding,
then as each tap down from the top of winding is tried, the Ro
becomes lower and lower, which is the opposite effect
of resistances in series with attenuation for speakers.

The other side effect is that the amp sees a higher value load if a speaker is
connected
at a tap lower down the auto tranny.
This may be a good thing if there is plenty of voltage swing headroom,
but not if a high power is required to the increased load effectively
harnessed to the tubes.

All is not completely simple, which is usual for anything to do with
electronics,
and there are tricks galore for the uninitiated novice who has no experience.


Patrick Turner.

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

Bob Hedberg wrote:

So I take it that if I were to utilize a split primary step-up
transformer between the two 8-16 windings, I could sum those two
portions of the signal and drive a center-channel speaker?

bob

I am not sure what you mean.

I'll take a complete wild guess, and agree with Phil, to be on the safe side.
That way I can't be blamed for any smoke in your lounge.

Patrick Turner.

Kind of like using a primary to split secondary transformer as a phase
splitter for a push-pull amp, but in the opposite direction. Seems
like it would work to me, as long as the primary windings were not in
a phase-cancelling orientation.

Bob Hedberg


Patrick Turner wrote:



Bob Hedberg wrote:

So I take it that if I were to utilize a split primary step-up
transformer between the two 8-16 windings, I could sum those two
portions of the signal and drive a center-channel speaker?

bob

I am not sure what you mean.

I'll take a complete wild guess, and agree with Phil, to be on the safe side.
That way I can't be blamed for any smoke in your lounge.

Patrick Turner.

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

"Bob Hedberg

Kind of like using a primary to split secondary transformer as a phase
splitter for a push-pull amp, but in the opposite direction. Seems
like it would work to me, as long as the primary windings were not in
a phase-cancelling orientation.



** The two primary windings of your additional (centre mono) transformer
couple magnetically to *EACHOTHER* !!

When used the way you suggested the two channels of the stereo amp would
have their outputs effectively wired in parallel - very bad news.






............... Phil

An independant winding across the unused taps (8 to 16 ohm) for each
channel?.


"Phil Allison" wrote:


"Bob Hedberg

Kind of like using a primary to split secondary transformer as a phase
splitter for a push-pull amp, but in the opposite direction. Seems
like it would work to me, as long as the primary windings were not in
a phase-cancelling orientation.



** The two primary windings of your additional (centre mono) transformer
couple magnetically to *EACHOTHER* !!

When used the way you suggested the two channels of the stereo amp would
have their outputs effectively wired in parallel - very bad news.






.............. Phil




Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

"Bob Hedberg"

An independent winding across the unused taps (8 to 16 ohm) for each
channel?.


** Please read carefully:

" The two primary windings of your additional (centre mono) transformer
couple magnetically to ****EACHOTHER*** !!

When used the way you suggested the two channels of the stereo amp would
have their outputs effectively wired in parallel - very bad news. "

Two windings on the same core are never "independent" - that will act just
like a primary and secondary.





............ Phil

Yes, I suppose channel seperation would take a bit of a hit.
How about a couple of small transformers, each with their primary
winding across the 8-16 ohm tap, but their secondaries combined?

Bob H.

"Phil Allison" wrote:


"Bob Hedberg"

An independent winding across the unused taps (8 to 16 ohm) for each
channel?.


** Please read carefully:

" The two primary windings of your additional (centre mono) transformer
couple magnetically to ****EACHOTHER*** !!

When used the way you suggested the two channels of the stereo amp would
have their outputs effectively wired in parallel - very bad news. "

Two windings on the same core are never "independent" - that will act just
like a primary and secondary.





........... Phil



Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

"Bob Hedberg"

Yes, I suppose channel seperation would take a bit of a hit.


** Much, much worse that that - one channel sees the other as a near dead
short on its output.

Only when a mono signal is fed to the amp could it work OK.


How about a couple of small transformers, each with their primary
winding across the 8-16 ohm tap, but their secondaries combined?



** With the two secondaries wired in parallel the exact same problem
exists.

If the secondaries are wired in series there is no cross coupling -
UNTIL you connect the speaker.

Then it is just the same again.




............... Phil

Heinz wrote:

Hi,

what do you think:
The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go
something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms
tap for the tweeter? Must be a perfect biwiring!
Thanks, Heinz
I'd say this is a safe experiment. I'm assuming that you'll be using the
appropriate crossover components on the speakers. You're gonna get more
voltage outta the woofer though as the 8-16 Ohm winding has far fewer
turns than the 0-8 winding. What may be more useful, if you have
un-matched drivers, is to use the 0-8 for the woofer and 0-16 for the
tweeter. This is gonna serve up more voltage on the tweeter which may in
turn have to be padded.

FDJ