[Patrick Turner]

On Jun 30, 8:03*pm, "Alex Pogossov" wrote:
"Patrick Turner" wrote in message

...
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In most old radios with DC flow across the volume control pot track
the adjustment of the volume is often very noisy after the pot has
aged a few years.

So instead of the conventional arrangements put forward by RDH and
most others to avoid parts costs I will have the last IFT coil biased
at say +50V at one end, and the live end goes to a triode grid of 1/2
12AU7 CF buffer to remove any loading effect of diode detection on the
last IFT LC. Then I use Ge diode feeding RC circuit, and this can
directly feed second 1/2 12AU7 CF buffer and then usual CR coupling to
any a volume control and while employing time passive poles to give
say -3dB at 30Hz before any power amp which has NFB. I often add in
another 12AU7 gain stage for tone control to boost/cut treble; bass in
AM is usually OK.

Alex:
Is adding two extra tubes a reasonable "mod"? It is the radio redesign.

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.