This is from the Amp Slab website.

http://www.ampslab.com/SCHEMATICS/Altec260A.gif

What is the purpose of the transformer/choke(s) just before the output
tubes? The only thing I can think is that they are there for stability.
I've seen schematics with a small value cap in this place, we're
talkin' a few nanoferads, to roll off unwanted highs. Is this for
rolling off lows? And since they are wound on the same core there will
be reactance between the push and pull sides. I've never seen this
before.

In article .com, "Karl"
wrote:

This is from the Amp Slab website.

http://www.ampslab.com/SCHEMATICS/Altec260A.gif

What is the purpose of the transformer/choke(s) just before the output
tubes? The only thing I can think is that they are there for stability.
I've seen schematics with a small value cap in this place, we're
talkin' a few nanoferads, to roll off unwanted highs. Is this for
rolling off lows? And since they are wound on the same core there will
be reactance between the push and pull sides. I've never seen this
before.

I would guess that there are two purposes, the main purpose of the chokes
is to provide a low impedance bias source for the output tubes to prevent
thermal runaway under conditions of grid emission. A second purpose is to
couple the two driver tubes together in a push-pull configuration for
increased drive to the output tubes along with lower distortion.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

"Karl" wrote in message
oups.com...
This is from the Amp Slab website.

http://www.ampslab.com/SCHEMATICS/Altec260A.gif

What is the purpose of the transformer/choke(s) just before the output
tubes? The only thing I can think is that they are there for stability.
I've seen schematics with a small value cap in this place, we're
talkin' a few nanoferads, to roll off unwanted highs. Is this for
rolling off lows? And since they are wound on the same core there will
be reactance between the push and pull sides. I've never seen this
before.

I got this far on the web. I'll look again later. This appears to be a
theater amp and L1 is some sort of elaborate feedback. The core of the
inductor passes through both windings so this probably balances the
push-pull with emphasis on a certain bandwidth. Possibly an inductor is
used because of the phase lead. This type of circuitry is very interesting.
Probably most of those who really understand it are passing away.

Theater amps worked very hard and they surely made them ultra-stable so they
wouldn't give out due to overheating. Mark

http://alteclansingunofficial.nlenet.net/Library.html

"Karl" wrote in message
oups.com...
This is from the Amp Slab website.

http://www.ampslab.com/SCHEMATICS/Altec260A.gif

What is the purpose of the transformer/choke(s) just before the output
tubes? The only thing I can think is that they are there for stability.
I've seen schematics with a small value cap in this place, we're
talkin' a few nanoferads, to roll off unwanted highs. Is this for
rolling off lows? And since they are wound on the same core there will
be reactance between the push and pull sides. I've never seen this
before.

I found something else, but not a whole lot.

http://www.voiceofthetheatre.com/260A.th.htm

so the 260 is a theater amp. I don't know if the spec sheet will enlarge or
not. Mark

Karl wrote:

This is from the Amp Slab website.

http://www.ampslab.com/SCHEMATICS/Altec260A.gif

What is the purpose of the transformer/choke(s) just before the output
tubes? The only thing I can think is that they are there for stability.
I've seen schematics with a small value cap in this place, we're
talkin' a few nanoferads, to roll off unwanted highs. Is this for
rolling off lows? And since they are wound on the same core there will
be reactance between the push and pull sides. I've never seen this
before.

I tried that link yesterday but no luck. This AM I Googled & got a good
schema at

http://www.triodeel.com/al260a.gif

but it is a very large file.

As John B has already pointed out the split choke is in there to provide a
low DCR to the control grids of the 813's. The spec is 30K max in the 813
G1 circuit, given in RCA Transmitting Tubes TT-4.

It also assures good drive balance to the 813's should the output from the
6AU6's differ for whatever reason. However, it will not result in more
drive. The 6AU6's, being pentodes are quite capable of that on their own.
To get more drive the split choke would need to be in the plate circuit of
the driver tubes. In that case the drivers would more than likely be medium
mu triodes such as a pair of 6J5's, 6C5's or a 6SN7. Even so, the split
choke will need to have a very large inductance in order not to degrade the
LF response & gain of the rather high impedance of the driver (6AU6) plate
circuit.

See also http://www.voiceofthetheatre.com/images/260A.1.jpg
and http://www.voiceofthetheatre.com/images/260A.2.jpg

Cheers, John Stewart

"John Stewart" wrote

As John B has already pointed out the split choke is in there to
provide a
low DCR to the control grids of the 813's. The spec is 30K max in
the 813
G1 circuit, given in RCA Transmitting Tubes TT-4.

It also assures good drive balance to the 813's should the output
from the
6AU6's differ for whatever reason. However, it will not result in
more
drive. The 6AU6's, being pentodes are quite capable of that on
their own.
To get more drive the split choke would need to be in the plate
circuit of
the driver tubes. In that case the drivers would more than likely
be medium
mu triodes such as a pair of 6J5's, 6C5's or a 6SN7. Even so, the
split
choke will need to have a very large inductance in order not to
degrade the
LF response & gain of the rather high impedance of the driver
(6AU6) plate
circuit.

Good explanation, thanks. Not difficult to get the high inductance,
considering there is very little current and the windings are in
series.

Are those trimmers that the choke windings pick up the bias from?
That seems to me the strange part. That would put a variable
resistance between the two choke windings. Diagram isn't very clear.

Otherwise the inductor can be seen as a differential mode choke, and
hence a common mode shunt. A perfectly balanced signal is pure
differential mode, so the choke should improve balance and offer a
high impedance differential mode load to the drivers.

Incidentally, it provides a low impedance path for both DC and AC
components of the bias supply, which therefore needs to be
reasonably smooth I suppose, even though consequent hum would be
mostly cancelled in the output transformer.

cheers, Ian

Ian Iveson wrote:

"John Stewart" wrote

As John B has already pointed out the split choke is in there to
provide a
low DCR to the control grids of the 813's. The spec is 30K max in
the 813
G1 circuit, given in RCA Transmitting Tubes TT-4.

It also assures good drive balance to the 813's should the output
from the
6AU6's differ for whatever reason. However, it will not result in
more
drive. The 6AU6's, being pentodes are quite capable of that on
their own.
To get more drive the split choke would need to be in the plate
circuit of
the driver tubes. In that case the drivers would more than likely
be medium
mu triodes such as a pair of 6J5's, 6C5's or a 6SN7. Even so, the
split
choke will need to have a very large inductance in order not to
degrade the
LF response & gain of the rather high impedance of the driver
(6AU6) plate
circuit.

Good explanation, thanks. Not difficult to get the high inductance,
considering there is very little current and the windings are in
series.

Are those trimmers that the choke windings pick up the bias from?
That seems to me the strange part. That would put a variable
resistance between the two choke windings. Diagram isn't very clear.

That is a good point Ian. I had to magnify the schema to see what the
circuit is. One end of the Pots P1 & P2 goes to -125 volts while the
other goes to common thru 10K, so that sure seems to be the bias for
813's setup.

Otherwise the inductor can be seen as a differential mode choke, and
hence a common mode shunt. A perfectly balanced signal is pure
differential mode, so the choke should improve balance and offer a
high impedance differential mode load to the drivers.

Yes, since both windings are on a common core the split choke performs
very well as a common mode sink. AC Balance to the 813 G1's should be
very good. But it would be nice to be able to control that should the
813's differ, for whatever reason.

Incidentally, it provides a low impedance path for both DC and AC
components of the bias supply, which therefore needs to be
reasonably smooth I suppose, even though consequent hum would be
mostly cancelled in the output transformer.

cheers, Ian

Another interesting circuit feature IMO is what appears to be shields on
the coupling caps C1 & C2. Each shield? is returned to common thru what
looks to me like a 150K resistor.

The G1 to plate cap for the 813 is large at 0.25 picofarad, so that may
be one of the HF corners. But 0.25 pf in a tube that size ain't bad, at
all!!

Cheers, John Stewart

"Karl" wrote in message
oups.com...
This is from the Amp Slab website.

http://www.ampslab.com/SCHEMATICS/Altec260A.gif

What is the purpose of the transformer/choke(s) just before the output
tubes? The only thing I can think is that they are there for stability.
I've seen schematics with a small value cap in this place, we're
talkin' a few nanoferads, to roll off unwanted highs. Is this for
rolling off lows? And since they are wound on the same core there will
be reactance between the push and pull sides. I've never seen this
before.

The 1937 Thordarson sound amp manual shows this type of inductor. At least
three of the smaller amps use this. Part number T-7431 is called "tapped
input impedance." in T-7530-2A3 amp.




http://www.audiophool.cjb.net/Misc/SoundAmp.pdf