[Patrick Turner]

"Bob H." wrote:

On Feb 19, 12:22 am, Patrick Turner wrote:
"Bob H." wrote:

A lot of vintage 4k to 5k pp outputs have 16 and even 32 ohm taps.
Putting 8 ohms on the 16 or 32 ohm taps should get you into a better
loading range for 6080's. A lot of PP 7591 output transformers have
pretty low refected impedance, I believe.

Bob H.

This is a solution which could be tried.
However, there is a downside.
A typical old OPT with ratio of 5k:15 will have 10% winding losses, ie
looking into the primary, the total winding resistance will be 500 ohms
of the primary dcr wire resistance and the reflected secondary winding
resistance.

So when you have 8 ohms connected to a 16 ohm tap,
or to a 16 ohm winding which was most common in 1955,
then the primary load is 2k5, but the winding resistance remains at 500
ohms
so losses rise to 20%.
When 4 ohms is connected to the 16 ohm winding, losses = 40%.

The main problem with the 6080/6AS7 tube used for an amp is that once it
is built,
there isn't any other tube that will suit the PS and OPT easily.

With an amp designed for EL34, one can usually fiddle with the
circuit to make it suit 6L6, KT66, KT88, KT90, 6V6, 6CM5, 6DQ6 etc, etc.

Patrick Turner

I'm used to more like 200 to 300 ohm resistance in 7591 primary
windings, and a Heathkit mono 7591 amp on my bench right now shows 220
ohms plate to plate on the output tranny.

This is about the usual fare, and if the P losses are 10%, primary RL =
2k2.
If 5%, RL a-a = 4k4.

I'm sure the reflected load
is around 4 to 5k, so the 16 ohm tap would yield around 2 to 2.5 k
impendance.

Total Rw with secondary usually total around 10% in average quality OPT,
5% or less for real good ones,
and up to 25% Rw losses ( copper losses ) for a crummy mantle radio, and
usually
the secondary has more Rw than the P, perhaps because most makers think
there is more danger of a P winding fusing with too much DC, and they
are right.
Small average amounts of music will never fuse a secondary winding on an
OPT.
So they use thn wire to help pay for their BMW.

With a Z ratio of 4k:8, the ZR = 500:1.
If the winding R of the sec = 1 ohm, then at the P this appears as 500
ohms.
So if the P = 220ohms, total Rw lokking into the P = 220 + 500 = 720
ohms.
S dcr can only be checked with something that reads low dcr accurately,
and you need to know the unloaded voltage ratio at 400Hz for the TR,
which squared gives the ZR,
and you then have the voltage ratio for a perfect tranny, andthe dcr of
P and S can be added to it to draw up
the model of what you have in an amp.

Clever dicks measure the OPT with 400Hz, or 1kHz, something around these
F,
and with 1kohm in series with the P, and a low Z out voltage gene
capable of say
50Vrms to simulate a tube in the output.
Such folk don't need to make dc resistance measurements, they make these
parameters clear
by careful routine ac signal measurements and by later calculation
because the clever dick has the mental picture of the tranny model
firmly in his head.
The leakage inductance and shunt C can also be checked out during the
process...

Patrick Turner.



Bob H.