An old timey radio

Tube Depot wants only $4.95 for it.

Also $4.95 for the 12EC8.

But it should be EMP-resistant, thank goodness.

Depends on how big the pulse is and how close.

Regardless of the wive's tale, there are components in tube circuits that will suffer damage in such instances.

A regenerative receiver probably doesn't need an RF stage given the kind of gains it's possible to get out of a single tube with positive feedback. From what I've read it does help prevent re-radiation from the regenerative tube into the antenna and prevents movement of the antenna from disturbing the detector. A really good radio would probably use a pentode with a tuned grid circuit to adjust the antenna coupling, but that requires more coils and 2 or 3 extra variable capacitors that I don't have. My hope is that the untuned RF input with pot will at least give me some crude control of the coupling between the antenna and detector.

I'm actually not sure if using the diode to bias the second triode will work with negative feedback applied - if the feedback signal is too large on the negative swing it will cause the diode to drop out of conduction. That's unfortunate - 0.6 volts was the perfect cathode voltage to set put that triode at a good operating point.

I'm actually not sure that using a diode to bias the second triode will work with feedback; I think if the negative swings are large enough it will cause the biasing diode to drop out of conduction and mess things up. That's too bad, as 0.6 volts on the cathode puts that tube at a good operating point. I think I'll just have to use a resistor and a bypass capacitor.

Here's a fixed schematic:

I changed the position of the audio voltage gain tube's biasing diode; that should allow me to use diode biasing and negative feedback without the biasing diode being cut off when the feedback swings negative.

Here's a fixed schematic:

I changed the position of the audio voltage gain tube's biasing diode; I think that should allow me to use diode biasing and negative feedback without the biasing diode being cut off when the feedback swings negative.

Now that's a deal.

That's why I built a shortwave preamp a long time ago, using a tube. Because the transistor ones died during every other thunderstorm. However, I didn't have LV tubes so I mounted an "inverted" little transformer up there.

You'd be surprised. A tube amp can take a serious beating. Even after something has arced over they often keep on humming. Done it many times.

Not too weird. I built myself a similar receiver when I was a kid. One tube but it needed around 200V on the plates, containing a triode and a pentode. I could listen to shortwave stations across the pond with that thing, amazing. But I was in a hurry and the enclosure consisted of a thick bent and non-painted plate of steel, plus wood. Ugly as can be, very low WAF, some rust spots here and there from when beer bottles hissed off upon opening. Then I married, and ... well, the radio ain't no longer there no more.

In 12th grade physics, they had converted the cloakroom into sort of an electronics lab - they had an old TV and one of those old TEK tube- type scopes, and they let me play there after hours. One day I was curious to see the plate waveform of the horizontal output tube; when the scope probe was about an inch and a half from the plate cap, it drew a nice blue arc.

The scope hardly even noticed. I can imagine what it would have done to a scope with transistor inputs. ;-)

Cheers! Rich

My first scope (still have it) sports two JFETs in there. They are socketed, probably because they assumed that guys like you might buy this scope :-))

There's even hints in the manual how to disgnose whether one of those JFETs has suffered a partial blow, how to pair them up, and all that.

The Soviet pilot that defected to the West with his MIG-25 caused some amusement in 1976 with a lot of tube electronics.

However, this caused some concerns, when the EMP question as examined later.

There's a reason why so many nuvistors are still available cheaply, by the carton.

During a recent design review one guy asked me where my surge cut-off FET was. "There is none, the circuit just keeps on working through the surge, the user won't notice anything" ... "WHAT?"

Actually, you apparently would.

I never said one couldn't.

An arc is an entirely different animal. Hardly a valid comparison.

Goody goody.

Like I said, it can cause damage and you obviously have no knowledge as to the pulse and apparently even less about what it does to other components.

I was contracted to make an EMP pulser for the green zone, back in

They killed the project before we even got started much past concept.

That was a SMALL pulse goal.

My Dad turned me loose with at least one tube based aircraft radio that was pulled from the instrument panel of a Cessna 172 as too far out of date.

Those had vibrators also.

Lots of those junked radios got a second life monitoring Unicom in hangars and admin buildings, usually just hung off a battery eliminator.

Low voltage?? Look at the 1R5, 1U4, etc..where the plate voltage is 12V!!

..especially the synchronous rectifier ones.