I just noticed in this ad: http://hamstation.com/ybstkftmk5.htm that
Yaesu's FT-1000 finals are MOSETS in push-pull. I don't ever recall
seeing a pair of final tubes in an xcvr or linear in push-pull except
perhaps way back in a very old QST. Can someone explain why push-pull is
used in the Yaesu and why we don't see it in "modern" tube amps?
tnx

--

73
Hank WD5JFR

On Fri, 15 Apr 2005 19:07:19 GMT, "Henry Kolesnik"
wrote:

I just noticed in this ad: http://hamstation.com/ybstkftmk5.htm that
Yaesu's FT-1000 finals are MOSETS in push-pull. I don't ever recall
seeing a pair of final tubes in an xcvr or linear in push-pull except
perhaps way back in a very old QST. Can someone explain why push-pull is
used in the Yaesu and why we don't see it in "modern" tube amps?
tnx

Cuz a single 3CX3000A is all you need?

Seriously, tubes are (relatively) high impedance devices and except
for distributed amplifiers, are usually tuned. The high impedance
step down to 50 Ohm requires large transformation ratios that are more
easily implemented with tuned circuits. Push-pull tuned circuits are
a pain in the ass to implement, especially when you want band
switching.

Bipolar and MOSFET amplifiers are by comparison low impedance devices
and are usually untuned. Instead of tuned circuits, broadband
transformers are used for coupling, impedance transformation and in
this case, power combining. The push-pull circuit requires lower peak
supply current and has inherent suppression of even-order harmonics.

Bipolar and MOSFET amplifiers are by comparison low impedance
devices

==============================
Actually they are not. But everything else you say is quite correct.

On Sat, 16 Apr 2005 00:27:09 +0000 (UTC), "Reg Edwards"
wrote:


Bipolar and MOSFET amplifiers are by comparison low impedance
devices

==============================
Actually they are not.

Please expand upon your remarks.

But everything else you say is quite correct.

Wes Stewart wrote:
On Sat, 16 Apr 2005 00:27:09 +0000 (UTC), "Reg Edwards"
wrote:


Bipolar and MOSFET amplifiers are by comparison low impedance

devices

==============================
Actually they are not.


Please expand upon your remarks.


But everything else you say is quite correct.



Mosfets , consider them cold cathode tubes and ignore the fact they are
not :_)

Bipolar and MOSFET amplifiers are by comparison (with tubes) low
impedance
devices

==============================
Actually they are not.

Please expand upon your remarks.

==============================

For example, the internal resistance, Ra, of a triode tube is -

Ra = Mu / Gm

where Mu is the amplification factor and Gm is the mutual conductance.

Typical values for a triode are Mu = 10 and Gm = 5 milliamps per volt
which gives Ra = 2000 ohms.

Corresponding values for a silicon NPN bipolar transistor are Mu =
1000 and Gm = 40 milliamps per volt which gives an internal resistance
of 25,000 ohms.

Typical values for a beam tetrode are Mu = 200 and Gm = 5 mA/volt
which gives Ra = 40,000 ohms.

For a field effect transistor Mu = 1000, Gm = 10 mA/volt and an
internal resistance of 100,000 ohms.

Typically, transistors have higher internal resistances than tubes.
The volts and amps at which they work, the power ratings, are
coincidental and have nothing to do with it except that when comparing
one device with another they should have similar power ratings.

Take a look at their DC characteristic curves. Internal resistance is
the SLOPE of the anode (collector) current versus anode (collector)
volts characteristic for constant grid (gate) volts (or constant base
current).

OK?
----
Reg, G4FGQ

"Reg Edwards" wrote -

Take a look at their DC characteristic curves. Internal resistance
is
the SLOPE of the anode (collector) current versus anode (collector)
volts characteristic for constant grid (gate) volts (or constant
base
current).

=================================

And, incidentally, this is the internal resistance which determines
whether or not a Conjugate Match exists between the PA device and its
load resistance.

Note that it is not under the control of the circuit designer or
tuner-upper who is not permtted to change tubes during a tune-up
operation in order to obtain a 1-to-1 SWR.
----
Reg, G4FGQ

The internal impedance of the device is irrelevant.

My correct previous statement was that transistor power amplifiers operate
"at lower impedance levels" because of the lower DC supply voltages and
higher DC current, therefore the much lower values of output load and driver
impedance levels.

These lower levels make broadband designs easier to accomplish. The
fixed-tuned lowpass filters at the output are easier to bandswitch. The old
"dip and load" procedures are no longer needed.

Bill W0IYH

"Reg Edwards" wrote in message
...
Bipolar and MOSFET amplifiers are by comparison (with tubes) low
impedance
devices

==============================
Actually they are not.

Please expand upon your remarks.

==============================

For example, the internal resistance, Ra, of a triode tube is -

Ra = Mu / Gm

where Mu is the amplification factor and Gm is the mutual conductance.

Typical values for a triode are Mu = 10 and Gm = 5 milliamps per volt
which gives Ra = 2000 ohms.

Corresponding values for a silicon NPN bipolar transistor are Mu =
1000 and Gm = 40 milliamps per volt which gives an internal resistance
of 25,000 ohms.

Typical values for a beam tetrode are Mu = 200 and Gm = 5 mA/volt
which gives Ra = 40,000 ohms.

For a field effect transistor Mu = 1000, Gm = 10 mA/volt and an
internal resistance of 100,000 ohms.

Typically, transistors have higher internal resistances than tubes.
The volts and amps at which they work, the power ratings, are
coincidental and have nothing to do with it except that when comparing
one device with another they should have similar power ratings.

Take a look at their DC characteristic curves. Internal resistance is
the SLOPE of the anode (collector) current versus anode (collector)
volts characteristic for constant grid (gate) volts (or constant base
current).

OK?
----
Reg, G4FGQ

On Sat, 16 Apr 2005 09:47:25 +0000 (UTC), "Reg Edwards"
wrote:

Bipolar and MOSFET amplifiers are by comparison (with tubes) low
impedance
devices

==============================
Actually they are not.

Please expand upon your remarks.

==============================

For example, the internal resistance, Ra, of a triode tube is -

Ra = Mu / Gm

where Mu is the amplification factor and Gm is the mutual conductance.

Typical values for a triode are Mu = 10 and Gm = 5 milliamps per volt
which gives Ra = 2000 ohms.

Corresponding values for a silicon NPN bipolar transistor are Mu =
1000 and Gm = 40 milliamps per volt which gives an internal resistance
of 25,000 ohms.

Typical values for a beam tetrode are Mu = 200 and Gm = 5 mA/volt
which gives Ra = 40,000 ohms.

For a field effect transistor Mu = 1000, Gm = 10 mA/volt and an
internal resistance of 100,000 ohms.

Typically, transistors have higher internal resistances than tubes.
The volts and amps at which they work, the power ratings, are
coincidental and have nothing to do with it except that when comparing
one device with another they should have similar power ratings.

Take a look at their DC characteristic curves. Internal resistance is
the SLOPE of the anode (collector) current versus anode (collector)
volts characteristic for constant grid (gate) volts (or constant base
current).

OK?

No.

All of this is very interesting, but you are referring to the
*devices* that are used in the amplifier, while I said, "Bipolar and
MOSFET *amplifiers* are by comparison low impedance.

Reg Edwards wrote:
Bipolar and MOSFET amplifiers are by comparison low impedance

devices

==============================
Actually they are not. But everything else you say is quite correct.


Well their INPUT IMPEDANCE can be high, but their
OUTPUT IMPEDANCE when used as a power amp is just as
low as bipolar transistors, so the broadband transformers
are just as valid. And as far as their input impedance,
well these are class AB or B power amps, so they take
some driving power. In this case because of the required
driving power, the input impedance isn't very high.