Running a Tube-Type Radio on "Car Batteries" - PART 1-

This was starting to get a bit looong, so I'm splitting it into two parts... 

I also made a few changes so "Part 2" shows up after "Part 1".

I was having a discussion with Harry over on his blog on the subject of shortwave listening, types of receivers, and other things.

I made my standard "If you're worried about EMP, take your little solid-state (transistorized, but more likely uProcessor controlled DSP radio these days) radio with the batteries out (because if you don't, you will forget they're in there, they will leak, and likely ruin the radio. Trust me, it WILL happen!), and seal it up in a steel ammo can, using adhesive-backed aluminum tape on all the seams. NOT aluminum colored duct tape, but real metal aluminum tape. You can get it at big box home improvement stores. This gives you a homemade Faraday Cage, and should protect the radio".

Have I tried this?

No, mostly due to the lack of a suitable EMP generator, but I consider it sound advice, knowing what I do about EMP.

I'd tell you more about why I know this stuff, but uh...well...you how that goes...

Yes, you could make a special box from "Mu Metal", but Mu metal is expensive, hard to work with, and generally difficult to buy in small quantities.

Then we started talking about types of receivers, and the subject turned to tube-type radios.

Tubes are inherently "hardened" to EMP because of their construction. They have large metal elements, separated by substantial spacing (compared to solid-state devices), and all the metal elements are encased in a vacuum envelope. Granted, the vacuum isn't "perfect", but the breakdown voltage (aka "Dielectric Strength") between two metal elements in a vacuum is is substantially higher than the breakdown voltage in air.

Wikipedia lists the breakdown voltage for air as 3e6 (3,000,000 Volts) per meter of spacing, and vacuum as 10e12 Volts per meter.

That's TEN TERRA Volts per meter, a staggering amount of voltage!

That's a thousand billion Volts.....

Speaking in term of "Volts/mil" (voltage per .001"), air breaks down and conducts at around 800 Volts per .001" of spacing (I always remember "1000Volts/mil") and breakdown in a vacuum is something like a MILLION times higher.

ANYWAY......before I ramble too far off the original topic, tubes should survive an EMP (especially if powered off), while conventional "wisdom" claims solid-state devices will be destroyed.

SO......In all of our favorite TEOTWAWKI/TSHTF novels, there always seems to be some "Old Guy" who just happens to be a Ham operator, always "lives on a hill top", has "big antennas", and he just happens to have some old tube-type radios laying around. Of course, they get pressed in to service powered up by "car batteries" scavenged from all the abandoned/broken/non-functional cars which just happen to be strewn around everywhere.

It makes for a great read, and shows how ingenuity can overcome adversity, but would it work?

Well, that's what I'll try and analyze here (whew! finally got back on topic), and give some recommendations for trying to do this.

The closest radio manual I had at hand is for my Heathkit SB-310, so that's what I'll go with. This radio is not "100% Hollow State", as it has three rectifier diodes in the power supply (B+ and bias rectifiers), two "small signal" diodes for the Automatic Noise Limiter ("ANL"), and two more small-signal diodes in the Automatic Gain Control ("AGC") circuits. Since this is an AC to DC conversion, the rectifier diodes will be bypassed, and you can use the receiver with the ANL and AGC diodes clipped out of their circuits if they get blown from an EMP-type event. It wouldn't be as "pleasant" to listen to with those two circuits disabled, but it will work just fine without them.

The principles involved with doing this to most any tube-type receiver would be the same, but some of the voltages will probably be different.

YMMV, don't try this at home, don't blame me if you get zapped or blow up your receiver, and yes, I am a professional at this stuff!

First, let's look at how the receiver is powered during "normal" operation.

Tube-type receivers use a power supply (transformer, rectifiers, filter capacitors, dropping resistors, and possibly a filter choke) to produce the required voltages from the 120 VAC power line.

There are (usually) three voltages required to make the receiver operate:

1. An "A" voltage, which is the filament voltage, either 6 Volts or 12 Volts. Some receivers may have a mix of both 6 and 12 Volt filament tubes in them. The filaments "don't  know/don't care" if they're being fed AC or DC. Some people claim that running the filaments from DC will give a "quieter" receiver, while others claim that running them from DC will shorten the filament life due to metal transfer off the filament to the other tube elements, similar to what happens during electroplating.
Way back when I was repairing/modifying electronic stuff for all my musician friends, I did a full make-over on a buddy's Fender Twin Reverb amp. I went completely gonzo on it, beefing up the power supply, regulating all the voltages, converting the filaments to a regulated DC supply, redoing all the grounds inside, and shielding the daylights out of it. *I* couldn't tell much difference, but *he* could, and I wound up doing a half-dozen or so for other people. AFAIK, there was NO difference in tube life.

2. A "B" voltage, commonly called "B+", which is the plate/Anode supply voltage, and generally the highest voltage in the radio. Sometimes there are multiple B+ voltages used at different points in the circuit, and these are generally derived from the highest voltage using dropping resistors. This voltage is always well filtered DC, and in some cases is regulated over a narrow range. This voltage is positive with respect to ground.

3. A "C" voltage, which is the bias voltage applied to the tube grids, and used to set the "operating point" of the tube. This voltage is negative with respect to ground.

In "Ye Dayes Of Olde", long before residential homes were wired for that new-fangled "AC" stuff, these voltages were all supplied by batteries, so running tube radios on batteries is nothing new.

An "A" battery powered the filaments, a "B" battery powered the plates, and a "C" battery provided the bias.

First, we have to deal with how to get the voltages we need from "car batteries", and discuss some things about "car batteries" that need discussing. Then we'll move on to modifying the wiring in the radio to use our new external voltage sources.

At this point some of you are probably scratching your head and wondering why all this talk about using ONLY "car batteries" to power the radio, and you'd be correct. The simplest thing to do is get a 12 Volt DC to 120 Volt AC inverter, and just run the radio that way. Most receivers don't draw a whole lot of power, so you don't need a big inverter. The SB-310, for example, only draws 50 Watts AC power, less than 1 Amp, so even a small El Cheep-O inverter like this $20 one from Harbor Freight would work. My Drake R-4B, a superb ham band only receiver, draws 60 Watts, still well under 1 amp of AC line current.

BUT, as all the stories go, we're stuck using "car batteries", so that's why I'm writing this (very long...) post.

You'll notice I keep putting "car batteries" in quotes. There's a reason for that.

"Car Batteries" fall into a couple of broad classifications. First, and most common, are what's called "SLI" rated batteries, for "Starting, Lighting, and Ignition" batteries. This type is designed to put out a huge blast of current (several hundred amps) for a short time to get the vehicle started, and then to be recharged by the alternator immediately. They do NOT like to have a constant, low drain applied to them, like the drain that a radio would cause. If used in this service, they will not produce their rated "Amp Hour" capacity, will go "dead" quickly, and will get to the point that they will no longer hold a charge, or even accept a charge. I've personally ruined several SLI batteries by using them as a back-up power source for my "12 Volt" Ham radios. Even though I kept them charged with a properly deigned "battery tender", and watched them carefully, they never lasted more than 12 months.

Expensive lesson, but well-learned.

The next common type is the "Deep Cycle" type. These batteries are designed uses that require a lower current draw for extended periods. Sometimes they're called "motive" batteries, and are used for wheel chairs, golf carts, trolling motors, and solar power storage. Every time I've replaced my car battery I've always replaced it with a deep cycle battery, as there's times I'll run my radio for extended periods without running the engine, and I want to make sure I have enough power left to start the car.

And within the battery types are some sub-classifications depending on the type of construction used.

"Flooded" types have the liquid electrolyte (aka Battery Acid) and removable caps to check the levels. Most of us grew up with this type, and are somewhat familiar with it. I always used distilled water to top off the level, although many sources say that any potable water is OK. Personally, with the cost of distilled water being so low, I could never see using tap water, especially in areas with hard water.

"Valve Regulated Lead Acid" (VRLA) batteries were first seen in the late 1960's, and were marketed as "Maintenance Free" batteries. They used a slightly different chemistry and construction/

"AGM" or Absorbed Glass Mat batteries are newer still, and have a different construction that keeps all the electrolyte in a fiberglass mesh.

And as of 2016 some cars are using Nickle Metal Hydride and Lithium Ion batteries for their power source.

So, with that said, be aware that using old fashioned, flooded construction SLI batteries would work, but the batteries probably won't last as long as you'd like.

Now as to "How Many Batteries Will We Need", we need to look at voltages, and we'll use my trusty SB-310 as our example.

The SB-310 "requires" 185 Volts for the B+, -85 Volts for the Bias, and 6 Volts for the filaments.

Nominal, fully charged voltage for a "12 Volt" automotive battery varies some with the type. We'll go with the current VRLA batteries as they're most common in newer cars.

Fully Charged = 12.66 Volts

50% Charged = 12.10 Volts

25% Charged = 11.95 Volts

  0% Charged = 11.70 Volts

We'll pick 12.5 Volts just to make the math a bit easier.

185 Volts/12.5volts-per-battery= 14.8 batteries.

Kinda of hard to have 8/10ths of a battery, so we'll say 15 batteries.

Fully charged, 15 batteries gives 189.9 Volts, a few volts higher than the nominal 185 Volts for the B+, but nothing to worry about. These radios were designed with 10% tolerance resistors, and most of the capacitors (except in critical tuned circuits) were about as "loose", and the AC line voltage was never exactly "117VAC", so plus or minus a few Volts on the B+ isn't going to matter.

Fully discharged, we'd have 175.5 Volts, enough to keep the receiver running, but pretty hard on the batteries.

For the -85 Volt Bias Supply, we have it easier. Since the Bias Supply is feeding the grids of the tubes (a very high impedance), the resulting current draw is extremely low, on the order of microamps. Rather than lugging another 7 car batteries to make the bias supply, it's much easier to use ten 9 Volt "Transistor Radio" batteries, or any other combination of standard dry cells that gives about 90 Volts.
Then we'd make a simple resistive voltage divider, and adjust the voltage for the 85 Volts (or other required voltage) we need. The bias voltage is a little more critical than the B+ voltage, because if the tubes aren't biased close to where they should be, the radio won't operate properly.

This radio has  6 Volt filament tubes exclusively, with a total draw of 3.3 Amps.

While there are ways to drill and tap into a 12 Volt battery at the 3rd cell and get 6 Volts from it, I've never done it, and only seen pictures of it. I'm going to assume that whoever is attempting to do this (the old "Ham on the Hill"!) has a pretty extensive "Junque Box", and would just make up a 2:1 resistive voltage divider, and use a single 12 Volt battery for the filaments.

So that gets us our required operating voltages. 15 car batteries in series for the B+, one more for the filaments, and a bunch of dry cells for the bias voltage.


In "Part 2" I'll get into modifying an actual radio