I bought a pair of Harman Kardon citation II output transformers off e-bay,
and I'd like to design an amp using them, but I'm unsure of their impedance.
They have cloth covered wires. The SAMS booklet lists them as 3200k, other web
sites list them as 3,800k and 4k. Another question is how much current can be
run thru them. I am considering using kt88/6550 pair as output tubes, or maybe
two el34 in parallel. Last question- I've searched the net for tutorials in
using loadline graphs, but haven't found any with ultralinear examples. Are
there any differences that I should be aware of when drawing the load lines
in ultralinear vs. pentode or triode modes?
Thanks,
Don

1) plug primary in the wall
2) measure voltage across the secondary
3) Zprimary=8*((Vpri/Vsec)^2)

:-)

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

Gregg wrote:

1) plug primary in the wall
2) measure voltage across the secondary
3) Zprimary=8*((Vpri/Vsec)^2)

This might give some smoke.

Before using the mains to test an OPT, think of what your'e doing.
If the neutral of the mains is at 0V, and that goes to one end of the
pri,
and the active at 110v @ 60 Hz goes to the other end of the pri,
then you have a problem because the CT is grounded to 0V via the power
supply
caps, so you have 110v working into a short circuit.

FAR BETTER, is to use a low voltage, say the 6.3 volts from a heater
circuit from an external transformer, that has one end of this 6.3v
winding grounded,
and apply the active end to ONE anode connection,
with the amp turned off, of course, because you don't wanna
short the B+ rail.

Then you should measure 12.6v across the primary, because one side
of the primary rises 6.3v while the other falls 6.3v, like a see saw.

Then the voltage ratio to the secondary winding can be measured,
and without any load connected.

This gives the unloaded voltage ratio of the transformer,
and suppose you get 12.6v at Pri to 0.5 v at the Sec, then the VR is 25
: 1
which is the ratio of P turns to S turns,
and if you square the VR, or the TR, this gives you the impedance ratio,
ZR,
which in this case is 625 : 1.

This means that if you have 8 ohms connected to the Sec,
then the tranny converts this to 625 x 8 = 5,000 ohms at the Pri,
which could be a nice loading for the tubes.
The tranny itself has no impedance.
It has only got an impedance ratio.
If a 4 ohm speaker is connected, then 2.5k is the load
that the tubes have to work into.
This Pri load is called the "reflected load resistance"
and the ratio of the tranny should be suitable for the tubes to drive,
ie, at least what is in the data books for the tubes concerned.

The OPT is like an electronic gearbox, which matches fast revs, and low
torque
of the tube "motor" to the slow revs and high torque required at the
secondary
load "wheels".

Alternatively, turn the amp on, apply an input signal at
an F between say 60 Hz and 1 kHz, and turn this up
until 1 vrms appears at the output, with no load.
Hopefully, the amp won't oscillate without a load connected, so if you
get queer results,
or if you can't monitor what your'e doing with a CRO, then use the
method
I first described.
Then measure the voltage between one anode and the other,
and this should be say 25 vrms in the case of a 25 :1 transformer.

Square the VR, and there is your ZR.

Patrick Turner.


:-)

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

Patrick Turner wrote:

Gregg wrote:


1) plug primary in the wall
2) measure voltage across the secondary
3) Zprimary=8*((Vpri/Vsec)^2)


This might give some smoke.

Before using the mains to test an OPT, think of what your'e doing.
If the neutral of the mains is at 0V, and that goes to one end of the
pri,
and the active at 110v @ 60 Hz goes to the other end of the pri,
then you have a problem because the CT is grounded to 0V via the power
supply
caps, so you have 110v working into a short circuit.
[...]

Good advice, if you're planning on doing this measurement "in-circuit."
But then you wouldn't need the external voltage source, and can just use
the amp normally, and measure the signal voltage on primary and secondary.

But the original poster apparently has just the transformers, so the
plug-it-in-the-wall approach would be fine. (I do that quite regularly
myself).

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Fred Nachbaur wrote:

Patrick Turner wrote:

Gregg wrote:


1) plug primary in the wall
2) measure voltage across the secondary
3) Zprimary=8*((Vpri/Vsec)^2)


This might give some smoke.

Before using the mains to test an OPT, think of what your'e doing.
If the neutral of the mains is at 0V, and that goes to one end of the
pri,
and the active at 110v @ 60 Hz goes to the other end of the pri,
then you have a problem because the CT is grounded to 0V via the power
supply
caps, so you have 110v working into a short circuit.
[...]

Good advice, if you're planning on doing this measurement "in-circuit."
But then you wouldn't need the external voltage source, and can just use
the amp normally, and measure the signal voltage on primary and secondary.

But the original poster apparently has just the transformers, so the
plug-it-in-the-wall approach would be fine. (I do that quite regularly
myself).

Yep, you are quite right, but nevertheless,
beginners should use only low test voltages applied to OPTs, because if
110v is applied accidently to a secondary for a speaker.
perhaps a loud hum and 2,500 volts appears at the primary!

The ratios measured with low voltages will be just as accurate as with
higher voltages.

Safety first!

Patrick Turner.



Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

Behold, Patrick Turner flashed in code from a keyed 4-1000A filament:

snip

Safety first!

Patrick Turner.

You are of course correct there Pat ;-)

Experienced tech's like us and Fred know the way to the fire extinguisher
by instinct. LOL!

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

Don wrote:

I bought a pair of Harman Kardon citation II output transformers off e-bay,
and I'd like to design an amp using them, but I'm unsure of their impedance.
They have cloth covered wires. The SAMS booklet lists them as 3200k, other web
sites list them as 3,800k and 4k.

3.2 k should be OK for beam pentode operation at anode supply
up to 400v for a pair of KT88/6550.
Above 400v, say 500v, the load should be about 4k to 5k,
but these loads are for class AB1 operation.
For all pure class A, you need 30% higher load values,
and the load depends on the turns ratio, as I pointed out in a previous post.



Another question is how much current can be
run thru them. I am considering using kt88/6550 pair as output tubes, or maybe
two el34 in parallel. Last question- I've searched the net for tutorials in
using loadline graphs, but haven't found any with ultralinear examples.

Probably, the primary would take at least 50mA in each half of the OPT
primary, and if the DCR is say about 125 ohms each side of CT,
then 0.312 watts of heat is liberated from each half P, so the
OPT should handle that OK, or even twice that.

Are
there any differences that I should be aware of when drawing the load lines
in ultralinear vs. pentode or triode modes?
Thanks,
Don

The 6550 plate resistance line for 0V grid bias is all you need to
have, to estimate the limitations of load voltage swings with say 50% UL taps, if
there are any.
Usually, for UL, voltage swing is 10% to 20% less than pentode,
but a lot more than triode.
The UL curve slopes from the 0v / 0 mA up and across to the right, at a
slope about 20% less vertical than the pentode, and then there
will be a broad knee, and then the line will slope up to the top right
indicating a plate resistance of say 5 kohms.
The triode sloped curves indicate plate resistances
mainly between 3k and 1k, whilst besam pentodes
have nearly horizontal plate resistance lines,
which indicates Ra between 50k and 14k,
and the actual slope of the curve varies with each mode for
different levels of plate current, and voltage.
Gm and Ra vary considerably across the working range of the cycle.

For triodes, with B+ = 450v, 7k to 14k is a best load value for 6550, whilst UL
is OK with 6.5k to 14k.
This gives power of around 25 watts in triode, AB1, and 40 watts in UL, AB1,
and good fidelity.

Understanding your transformers is vital, and how to set up the speaker
impedances to give the right load, so perhaps you will need to run
the midranges in series to increase the load seen by the amp in UL.

Patrick Turner.

Thanks for all your replies.


In article T8%Wa.36465$Oz4.10768@rwcrnsc54,
says...

I bought a pair of Harman Kardon citation II output transformers off e-bay,
and I'd like to design an amp using them, but I'm unsure of their impedance.
They have cloth covered wires. The SAMS booklet lists them as 3200k, other
web
sites list them as 3,800k and 4k. Another question is how much current can be
run thru them. I am considering using kt88/6550 pair as output tubes, or
maybe
two el34 in parallel. Last question- I've searched the net for tutorials in
using loadline graphs, but haven't found any with ultralinear examples. Are
there any differences that I should be aware of when drawing the load lines
in ultralinear vs. pentode or triode modes?
Thanks,
Don