In article , "Tim Wescott"
writes:

"Avery Fineman" wrote in message
...
In article , "Tim Wescott"
writes:

Your solid state devices will either be really light-weight or will have
significantly higher capacitances than the tubes, so it would be very
difficult to get your oscillator to track.

-- snip --
On checking the HV supply of this little receiver, the OB2 strike
voltage had increased beyond manufacturer's spec so it did not always
strike and start regulating with power-on. The lone OB2 in spares,
also 40 years old, was just the opposite, striking below spec.

Antique Electronics Supply has good replacement tubes -- but solid-state
regulators for old tube equipment is more fun, kinda like putting EFI on
your flathead Ford engine (no kidding, it's done).

There still exist vacuum tubes off someone's shelf even though the
prices for small receiving-type tubes are $5 to $12 each (with any
sort of guarantee of performance)..

It was a bit of a shock to find those old, seemingly trustworthy OB2s
to be quite off specification, especially with the strike voltage off on
the high side. The receiver I was restoring was my own design and it
depended on the shunt regulator to help the stability of two local
oscillators that were L-C tuned, not quartz.

In the early 1960s and the conception of this receiver, regulation of
B+ (as we quaintly referred to HV then, heh heh) was only for expensive
things such as high-quality test instruments. A shunt regulator is fine
for critical things like oscillator stability but it does waste power which
is converted to heat, variable heat depending on AC power line stability.
Changes in local heating (within the cabilnet, chassis) means another
random variable to throw off oscillator stability. Series regulators waste
less heat (general statement, not an absolute) and can hold the HV
more constant over a wider current demand range.

I had accomplished a fairly sensitive and tuning-stable SW receiver
for my father using only 6 tubes for active devices, solid-state rectifiers,
and a single gas shunt regulator. Filament power was a mere 7.6 W
total; a common 5Y3 rectifier needs 10 W for its filament by itself (2 A
at 5 V). Internal heating was minimal and there was no such nonsense
as "waiting for it to warm up," to reach some thermal equilibrium above
room temperature. That "warm up wait" doesn't assure anything about
internal radio stabiity since room temperature may be variable and the
internal power dissipation varies with everything from AC line voltage
to plate current variation from AGC stages changing with signal
strength.

Tube series regulators waste power (and thus dissipate heat) because
the cathode-plate voltage is relatively high to stay within a linear
operating range. NPN transistors are more forgiving for series regulation
work since their emitter-collector voltage can be lower for the same
series current. Two NPNs in my prototype series regulator (plus one
zener) waste only 5 W for at least 2% voltage stability over AC line
(I can tweak that to at least 1$), An OB2 shunt regulator wastes 1.9
W alone and the total dissipation from all in the old supply is about
8 W. I'm already ahead of the game in reducing wasted heat by going
to transistors and zeners.

-- snip more --
With a 6.3 V filament arrangement, that would be on the hairy edge
of regulating at -10% AC line voltage. Filter capacitor needs to be
high enough for only 5% ripple and rectifier diodes can't be much more
than 0.7 V forward drop to fit the minimum 7806 regulator 2 V head-
room.

I would use a new LDO regulator, if I used one at all. On thinking about it
after my post, it occurred to me that for the filaments I would be happy
with a well-filtered DC supply to keep the filament hum from modulating my
RF. If all you have is a 6.3V supply you need really low-drop diodes,
however, and your filament transformer's RMS current will go up no matter
what.

I had NO "hum in the RF" to begin with...the filament regulation is an
afterthought since varying filament voltage in small receiving tubes used
as oscillators will result in varying fixed-tuned frequency. How much
variation depends on a whole bunch of other factors of the oscillator
circuit design. Using a variable autotransformer for AC mains adjustment
may lead to (sometimes) shocking discovery that 6.3 V filaments do
NOT stay at 6.3 when the AC mains varies +/- 10%. Frequency
counter measurement of the LOs in my case showed some drift with
AC mains voltage change that could only be accounted for by the
filament voltage changing.

The 7800s (positive) and 7900s (negative) all-in-one series regulators
are very common. Their minimum voltage drop for regulation specs is
only 2 VDC for most in each series. Minimum extra space, minimum
components added. Using a negative output and regulator yields
a stable bias supply which can be applied to other circuits or additions.
The 7800s and 7900s are legacy devices and still in production.

Note: The original AC filament voltage was made within certain
(seldom specified) limits; if that is on the low side also, the 7806
may poop out during droops in ripple voltage. 7906s were originally
made in the series for negative voltage regulation, also good for
providing a minus bias voltage. Tube filaments for indirectly-heated
cathodes don't care about DC polarity. Slightly-lower than nominal
voltage will improve heater life.

But can decrease cathode life if the cathode current is high -- even oxide
cathodes don't like being temperature starved. Check some of the warnings
and recommended start up procedures for 10kW transmitter tubes for some
_really_ tight cathode care-and-feeding instructions.

MOST homebrew projects involve lower power. Running 6.3 V NOMINAL
filaments at 6.0 V isn't going to degrade anything. In my case it has
12.6 V NOMINAL filament voltage reduced to 12.0 V exact. Filament
voltages are always given as "nominal" since the indirectly-heated tubes
almost always get their filament voltage from transformers. Since
ordinary transformers don't regulate, the percentage change of secondary
winding voltage is the same as primary winding voltage change....for most
of North America that is 115 VAC RMS +/-10 V or about +/-10%. A 6.3
V filament-heater can thus range from 5.7 V to 6.9 V just from the line.

Reduction in the high-temperature thermionic devices' voltage will result
in longer operating life. That was true over a half century ago. A friend
of mine into model railroading has been running lots and lots of #327
28 V rated aircraft indicator lamps in his track layout, all running at
about 18 VAC. NO outages yet due to filament failure in what must be
about 3 dozen or so used for 35+ years. Still quite bright enough in
ordinary room lighting.

I prefer solid-state all around, including LEDs. I'm keeping most of the
vacuum guts of this little receiver still in vacuum tube style just for
sentiment's sake. Everyone's mileage varies.

Len Anderson
retired (from regular hours) electronic engineer person