[Alex Pogossov]

"Patrick Turner" wrote in message
news:02ac8343-a10e-48e9-

I raise my hat to all the designers of tube AM radios. They worked
under duress and had no liberty to produce the best radio which could
be built because the companies all had to compete with each other for
sales so they all agreed to make the lowest common denominator
radios.

But DIYers were always free to do whatever they damned well liked, and
some did, but there's very little record of the
0.000000000000000000001% of radios made by ppl MORE intelligent that
99.99% of staff employed by companies.

If you consider the bulk and inefficiency of using pre WW2 octals and
most tubes, then why would making a radio with a couple of extra tubes
be a hanging offense? 1950's 7pin and 9pin tubes are far smaller and
compact than th octals they replaced, so you could use more tubes.
When FM radio and TV came into being the number of tubes became a
minor concern - you needed plenty of tubes, sure, and a shirt&trouser
load of other "stuff". Prices for such goods wasn't based on the cost
of production + a margin for profit because the industry had learnt
far higher margins over cost were needed to fund the phasing in of
Solid State and a huge increase om manufacturing infrastructure.

one wants to go that far, op-amps would be far better. Take TLE2074. With
10MHz bandwidth and 45V/us slew rate it can work as IF amplifier to
increase
sensitivity, tone control, buffer, AGC integrator, etc.

Sure. Then you have a solid state radio. **** that. Gimme a coupla
triodes.


And after that why do you need to retain a lousy tube amp? Why not to use
an
IC? All the radio can be powered from a 6.3V winding with a bipolar
voltage
doubler (four caps, four 1N4004 diodes)

I build tube radios, OK.

...................
I like to use paralleled 12AX7 as V1 and EL34 in triode as V2 for the
audio amp and with 12dB global NFB.
This works far better than anything with 6V6 or EL84 etc.

Alex:
Works better in which respect? EL34 takes as many amps for its heater as
the
whole original radio circuit. What about the poor lousy power transformer?

I replace PT, maybe add another if I have to. Anything goes, but what
goes out my door gives excellent AM and SW if there are coils, plus I
put in a couple of RCA sockets to take CD player L&R or an FM tuner
L&R so you get mono sound and real Hi-Fi. The trioded EL34 makes
better sound than anu single EL84/6V6. 807 and 6L6 in triode are also
excellent, and need less heater power.

I have a pile of PTs from which I can use for old radios where the PT
has become very fragile and poor insulation after running hot for 70
years.

So when someone getds me to fix a big beautiful floor standing radio,
they get the glory of the nice woodwork and they are not compelled to
listen all day to maybe 1 or 2 stations which sometimes broadcast
something worth listening to. They need to be able to play CDs and and
listen to AM and then they discover the glories of SET amps and wide
AF bandwidth. Most people don't need the sound to be loud, they don't
need stereo, and what they want is GOOD sound, and that's what I give
them. And in 3 years time they don't have to find a tube type
originating from pre-WW2 Europe, they will find spares quite easily
for the next 20 years at least.

Some radio ideas never became popular. The 1947 Tucker Synchrodyne was
such a thing. Synchronous detection really had to wait until the solid
state era where you could become more precise with circuits which all
too often were a nightmare for anyone to get running easily with
tubes.

Patrick Turner.
====================
Alex replies:
I see your approach to radio design. You are prepared to improve a boring
typical aussie radio by adding a cathode follower, another audio stage for
tone control, replace PT, drill/punch a couple of holes for the extra 2
tubes, etc.

What I can not understand is the craziness. Why go through all this bother
if a better improvement can be done with transistors and op-amps, and the
stuff can be hidden on a small board under the chassis. Unfortunately there
is no area where tubes outperform solid state, except for (literaly) warm
feeling.

Sometimes I do redesign as well. For example, the best AM detector is the
emitter detector. It virtually combines your cathode follower (emitter
follower in this case) with a diode detector (emitter junction). It has
rather high input impedance: beta times higher than of a bare diode
detector. And beta is quite high for the modern transistors. One problem
though. Reverse breakdown voltage of an emitter junction is about 8V, so the
carrier shall not exceed 1Vm (with a margin). Thus the radio needs to have a
perfect AGC.

An AGC in a boring radio is bad. On a week station AM detector gives 0.5V,
while on a strong local station can reach 5...10V. What I do, I put an
integrator in the AGC circuit so that no matter how strong is a station, the
carrier level is about 1V, while the AGC can be anywhere from 0V to --30V.
The radio becomes very convenient. Set volume and cruse the dial. All the
stations will have the same volume. The radio does not blast when running on
a strong local station.

Now I ask you: how would you make an integrator on a tube? Possible, but
cumbersome. Why lika communist, create difficulties and then overcome them
with a loud chest-pounding and shouting slogans?

Well, normally I do not do deep redesign, but just improve whatever possible
keeping the same topology. That is where the idea of low cut off in the
feedback came from. Once you begin to apply NFB to a lousy boring amp (to
reduce the speaker boom and make crispier sond), you want not to extend the
low frequencies below what the lousy OPT can handle naturally. And you can
not use RC high-pass filters on the input because the input impedance must
be 10M or more (not to load the detector). And you can not use a Hi-Z RC
filter of say 470pF and 10M, as hum and noise caused by flickering grid
current would be enormous.