On Thu, 06 May 2004 17:19:30 GMT, "Frank Dresser"
wrote:


"matt weber" wrote in message
.. .
On Wed, 05 May 2004 13:40:55 GMT, "Frank Dresser"
wrote:


"matt weber" wrote in message
.. .

The send puts the receiver in standby, filaments remain on but that is
about all. I suspect it disconnects the B+, or at least mutes the
audio.

The switch is between the center tap of the high voltage secondary and
ground. Opening the switch disables the B+ circuit. Closing the switch
with the tubes warmed up forces a large surge current through the
rectifier
as it charges the filter capacitors.
Not really. The vacuum tube rectifiers have very high internal
resistance, it is why you can safely use cap input filtering.

Yes there really is a large surge current when hot switching vacuum tube
rectifiers! In fact, they went to the trouble to develop a spec for the
maximum hot-switching transient plate current. The RCA tube manual says
it's 2.5 amps for the 80/5Y3. That's with the tube manual's recommended
input filter cap value of 20uFd. This radio uses an input cap something
like 40 uFd.

I'd rather play it safe, and not hot switch the rectifier.

The tube
itself is a very effective surge limiter that gets better with age as
the thorium on the cathodes is evaporated off.

The hot switching problem gets worse as the tube ages. As the emissions go
down, the voltage drop goes up. Hot switching old, high voltage drop tubes
can cause internal arcing.

Hot switching vacuum tube rectifiers needlessly reduces their useful life.
Not appreciably. Rectifier failures are almost invariable the result
of filaments burning out. Remember that both the anode and cathode
have considerable mass, so they tolerate short term gross overloads
very very well. In fact that is the basis of the so called vacuum tube
sound. Most tubes that are rated for a few watts, can easily put out
tens of watts for a few seconds without damage, and a Tube like a
4PR400 which is rated 400 watts can actually take a several hundred
thousand Kilowatts for a few milliseconds at a time. If you tried
that with a solid state device, it would be toast . Short of getting
them hot enough for the seals to fail, or elements or connections to
the elements to vaporize, there really isn't any such thing as short
term damage from overload unless the tube is already at the end of
service life anyway. (you eventually boil off all of the thorium on
the cathode).

By the way, there's no thorium in the 80/5Y3. These tubes use oxide coated
filiments. I'm not aware of any thoriated tungsten filament tubes used in
normal consumer electronics since the days of the '01A.


If you did it with a solid state rectifier, the rectifiers burn up
unless you protect them. Some of the high voltage/high vacuum
rectifiers like the GZ34 had trouble delivering 250ma with 800 volts
on the plate.

That's what happens when tubes are operated out of their ratings. The RCA
tube manual says the maximum plate voltate allowed for the GZ34/5AR4 is 475V
rms, with a capacitor input filter. That's a peak voltage of 672V. The
recommended rms voltage with a choke input filter is 600V, but the output
voltage from the choke input filter will be lower than the rms voltage
input.
I suggest you review the design of the EICO 720 transmitter, which
used a GZ34, and plate voltage on the 6146 was a nice round 600 volts.
Never heard of the GZ34 failing, The 5AR5 is no an absolute
replacement for the GZ34. The GZ34 was a lot tougher.

I would add that it was permissible to operate tubes well above the
commercial ratings, in fact many had two rating CCS (Continuous
Commercial Service), and a much higher, manufacturer sanctions, ICAS
(Intermittent Commercial and Amateur Service). The ICAS ratings were
often 25-30% higher.

The recommended maximum hot-switching transient plate current for the
GZ34/5AR4 is 3.7 amps.

I'm sure some people exceed these ratings, but at the cost of life of the
tube.


That's one of the reasons really big, vintage power
supplies user Mecury Vapor rectifiers. They have much lower
resistance, and you haven't seen a rectifier until you have seen a big
3 phase 800 amp mercury pool rectifier.

I'm sure such rectifiers also had a much higher hot-switching spec than
either the 80 or the GZ34. Although it doesn't matter much. Mercury vapor
rectifiers were almost always used with a choke input filter to avoid
letting the thing turn into a relaxation oscillator. Mercury vapor
rectifiers aren't often found in consumer equipment, either.


It's a poor circuit design which was
commonly used back then.
I disagree. Unlike a solid state rectifier, the vaccum tube rectifier
provided surge protection. That is just the way they work.

Exceeding the maximum hot-switching transient plate current will reduce the
life of the rectifier tube.
Why do you believe that? What is the physics involved?
Unlike solid state devices, they have such large thermal mass that you
really needed GROSS long term overload to damage them. Eimac built a
whole line of tubes that were designed for massive (orders of
magnitude) short term overload.



The triode section of a 6K8 will normally draw about 3 or 4 ma. All of that
current flows through the oscillator coil. The mixer section draws 2 or 3
mils through the plate and maybe 6 mils through the screen. So, in a normal
Hartley hookup, at least 3ma flows through the top section of the oscillator
coil, and and at least 11 ma flows through the bottom section of the coil.
I know that's still not much DC current, but is much higher than your
estimate. But wait! There's more!! The oscillator has a parallel resonant
circuit, and as all good EEs know, a parallel resonant circuit has a very
large circulating current, which is limited by the Q of the circuit. Higher
Q means more current.
Perhaps, but the more resistance there is in the coil, the lower the Q
will be. Q is the ratio of reactance to resistance. So if you have
enough resistance to make the I^2 losses reach milliwatts, the Q will
be lousy (and usually is).

even if you work from 10ma, 10 ohms will give you a whole milliwatt to
dissipate.

The tubes in fact cool by both convenction and radiation, both are in
large part take up by the chassis, which is metal. Metal is a pretty
good conductor of heat, so to suggest that heat generated on the top
of the chassis doesn't also end up underneath is nonsense, but I will
concede that resistor to set up screen and bias voltages as well as
the bleeder across the B+ cap are likely to generate substantial
amounts of heat as well, far more than the I^2R losses in any coil
except perhaps in a filter choke.


The oscillator's feedback ratio will also have a
large effect on the current of the coil. More feedback means more coil
current. All that extra current warms the oscillator coil a bit. Not
finger burnin' hot, just a bit. I don't know how many orders of magnitude
the AC current is higher than the DC current, nor do I know how many degrees
the temperature of the oscillator coil changes as it warms up. I don't
care. The effect is that the frequency drifts. The radio has better
frequency stability if the B+ isn't interrupted.

I have
never seen the coils in a receiver get even slightly warm from I^2 R
heating.

It doesn't take a huge temperature rise of the oscillator coil to cause a
few hundred parts per million drift at 10MHz. I'll get about 100 Hz just
from the air conditioner cycling on and off in the same room.
How much of that is temperature, and how much is a result of voltage
reduction as a result of the AC unit cycling on?
Very
annoyiing on SSB. But these aren't very good radios for that sort of work.

No, it's not the voltage shift, either. My SX-62 drifts with temperature,
and that one has a voltage regulated oscillator.


They are heated far more by radiated and convection energy
from the filaments, rectifier, and Audio output tube heat dissipation.
In most receivers, the filament power dwarfs everything else.


In most receivers, the tubes are above the chassis and the coils are below
the chassis.

If you're saying that there's more to the observed frequency drift as the B+
is switched on and off, you have a point. But most radios, including the
S-20R, have the coils under the chassis and the tubes above the chassis.
However, there's some under chassis power resistors which will also
contribute to oscillator coil heating and frequency drift.


If you
have a reciver that is rated 40 watts, and has an audio output of 1-2
watts, the power isn't in the B+. In an All America 5 design, 90+% of
the power dissipated is in the filaments.


Well, let's run the numbers. The typical AA5 uses 150ma tubes in a series
string rated at 120V. That's 18Watts. If the input power is 40 watts, the
total percentage consumed by the filament string is 45%, not 90+%. But AA5s
use a higher percentage of their power in the heaters because the audio
output tube has a high power heater to optimize it for lower plate voltge
use.
If what you say is true, then were pray tell does the other 22 watts
go. It certainly cannot be output. A 50C5 cannot delivery anything
like that, and frankly, if you tried to dissipate 22 watts in the
other 4 tubes, they'd burn out in a matter of hours. YOu are also
assuming an AA5 uses 40 watts, most are more like 25.


For example, the 50L6 uses a 7.5 watt heater, while the 6F6 uses a
4.4
watt heater. Hallicrafters substituted a 6K6 for the 6F6 in their later mid
level radios. The 6K6 had an even more economical heater, at 2.5 watts.

Anyway, these tubes are above the chassis, the coils are below.


Radiated heat goes up at
T^4, so a reduction in power input of 10% results in a change in
temperature that is tiny (on the order of 1.7%)....

Radiated from the above chassis tubes to the below chassis oscillator coil?
Ignoring the actual heating effects in the oscillator coil itself, I have to
figure the under chassis B+ dropping resistors have a far larger effect on
the temperature of the oscillator coil than the above chassis tubes do.
Anyway, the frequency drift after switching the B+ starts right away, and
that implies the source of the drift is right in the oscillator circuit.


That is often measured in tens of wattts. What is dissipated in the
coils is microwatts to milliwatts. Ambient temperature inside the
cabinet had far more to do with coil temperatures then the current in
the coil.

But the under chassis temperature will rise relatively slowly after the B+
is switched. The frequency drift after switching starts immediately.


The tube and coils cool a bit in the
send position, and rewarm up in the receive position. The frequency
shifts
as the temperature shifts.
Not it if was well designed. Designer did two things. They used
regulated voltage on the oscillator,

Oh. How many S-20R radios have been designed with regulated voltage for the
oscillator?

and NPO caps, negative
temperature coefficient, so the temperature of the coils would drive
the inductance one way, the caps went the other way, cancelling the
changes out.

Temperature compensation might work over a small range of frequencies. As
the tuning capacitor is closed, the reletive effect of the temperature
compensation will be reduced. These radios use a bimetal sort of temp
compensating capacitor a couple of inches away from the oscillator coil.
It's nowhere near the above chassis tuning cap, which is subject to the heat
from the tubes. The temperature compensation doesn't work very well. The
compensation capacitor is also microphonic.

As long as we're on radio design, a good frequency stability technique would
be use of low expansion coefficent coil forms such as some of the ceramics.
The forms on this radio are bakelite. Bakelite isn't as good as low
expansion ceramic, but it better than cardboard.

This radio wasn't designed for a high level of frequency stability. It was
designed to be a good value for the money. From that point it was a