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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 17th Feb 2012, 9:55 am   #1
Chris Parry
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Default Barretters and neons: contact cooling?

I have an R-390 and an R-390A receiver, each using an Amperite type 3TF7 barretter to provide stabilised AC heater current to the VFO and BFO valves. These barretters have ordinary B9A glass envelopes. The authentic valvecan is a fluted type with ventilation slots. I have IERC heatsink cans that could be fitted instead.

Similarly, there are several radios here with OA2 or OB2 neons to stabilise the HT fed to the LO, and in some cases the BFO as well. These devices have tall B7G glass envelopes. Once again the authentic valvecans are usually fluted types fitted with ventilation slots, and I have suitable IERC heatsink cans that could be fitted instead. But as before, I am unsure whether this would be a good idea.

Any views?

Chris Parry
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Old 19th Apr 2012, 10:24 pm   #2
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Default Re: Barretters & neons: contact cooling?

Cooling a discharge voltage stabiliser doesn't seem like a bad thing to do, but finned cans are there to cool the glass rather than the anode, though a reflective can would send radiated heat straight back.

If the cooling can does affect a barretter, then cooling is not a good thing. The barretter relies on establishing an equilibrium between its current and its temperature, leading to a small but useful constant current region. I'd want to leave this just as it was designed.

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Old 20th Apr 2012, 12:13 am   #3
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Default Re: Barretters & neons: contact cooling?

Hello Chris.
I have a Hallicrafters R-274 receiver and a barretter is fitted in the set to regulate the filament current to the VFO and first mixer valves. The octal base for the barretter is a standard base used in so much military equipment of that era. It has the ring around the base to fit a valve shield. However the Army manual goes to great lengths to say that a valve shield is not to be fitted to this valve.

Last edited by QQVO6/40; 20th Apr 2012 at 12:15 am. Reason: Bad gramatical error. "oops"
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Old 20th Apr 2012, 2:53 pm   #4
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Default Re: Barretters & neons: contact cooling?

Voltage regulator tubes contain convection currents of hot neon gas. So by cooling the envelope, the cathode bucket will most certainly run cooler. But does that matter and is it beneficial, I wonder?

Barretters also contain convection currents of hot gas, hydrogen this time. By cooling the envelope, I reckon the filament may run hotter than it otherwise would. If a barretter is extremely cold and then switched on, I can imagine that the glass envelope might crack because it gets maximally hot immediately after switch-on. Then it will start to run a little cooler, as the valves warm up.

I have never seen a contact-cooled can fitted to either of these devices on a production radio, but it's common to find a special fluted can with partly open sides used on both neons and barretters.

Over decades, I have systematically fitted IERC contact-cooled valvecans to barretters and neons in my american radios. Whether it's doing me any good, I have no idea! As far as I know, Hallicrafters was the only company to actively advise against this practice. At the moment, I'm inclined to revert to standard cans next time these radios are on the bench.

Chris
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Old 20th Apr 2012, 3:20 pm   #5
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Default Re: Barretters & neons: contact cooling?

I don't think that the cathode being cooler will affect anything much - rvoltage regulators don't run hot in any case.

What might be significant however is not the temperature, but the gas pressure. A rise in temperature will result in a rise in pressure, and this will affect the characteristics (in what direction, I'm not sure).

The barretter is a different matter - there are various things going on, first the temperature coefficient of the iron filament (clearly temperature affects resistance directly) but there is also a reversible chemical reaction between the filament and the hydrogen, which effectively alters the filament resistivity even more than the pure temperature effect. So, altering the cooling could shift the characteristics. Having said that, if the thing is unusually critical with respect to ambient temperature, I'd have thought that it would have limited use in practice - and I've not seen any temperature limitation on any barretter data sheets.
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Old 29th Apr 2012, 10:56 am   #6
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Default Re: Barretters & neons: contact cooling?

It occurred to me, whilst clutching for explanatory straws, that the spec-sheets for conventional (i.e non-radioisotope) stabilisers invariably quote a minimum illumination requirement for reliable striking. Perhaps open fluted cans help to admit a bit more light- after all, many receivers etc. haave scale lights with generous back-scatter into the works. Admittedly, the opaque cathode cylinder shields the bit that counts, i.e. anode-cathode gap, from direct light but allowing as much envelope illumination as possible consistent with physical retaining would allow a few more photons to scatter around within.

I'll concede that the R390/R390A is one of the few cases of no internal lighting scatter but perhaps, by then, references with radioisotope-assisted striking were available. One can imagine that a received-wisdom culture might develop that "stabilisers always have open retaining cans". At least the R390 has an internal "night-light" from those un-canned 6080's with their open anode structure and fiercely glowing cathodes!

With barretters, the availability of these cans would at least allow both physical retaining and good ventilation.

Just an idea...

Colin.
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Old 29th Apr 2012, 11:22 am   #7
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Default Re: Barretters & neons: contact cooling?

A barretter with a fancy cooling can, a Philips C1 from an 838HU

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Old 29th Apr 2012, 12:50 pm   #8
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Default Re: Barretters & neons: contact cooling?

I don't remember seeing a minimum ambient illumination requirement in the datasheet for the pre-war S130, for example.

My understanding is that popular miniature type OA2 does not contain a radioactive isotope, whereas military type OA2W/WA does. This is to depress the striking voltage.

The R-390 uses a pair of beefy 6082 series regulator triodes, but the later R-390A did not use active HT stabilisation.

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Old 29th Apr 2012, 12:54 pm   #9
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Default Re: Barretters and neons: contact cooling?

Further to the above, I reckon that Phirrips C1 can may possibly be an RF shielding device rather than a cooling aid. I wonder if it's grounded? Chris
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Old 29th Apr 2012, 1:00 pm   #10
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Default Re: Barretters and neons: contact cooling?

It's not grounded, I meant C1C as well not C1
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Old 29th Apr 2012, 1:03 pm   #11
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Default Re: Barretters and neons: contact cooling?

Okay. The only Philips C1 I ever saw didn't have an integral can. It's all very very puzzling, I reckon.
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Old 29th Apr 2012, 9:16 pm   #12
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Default Re: Barretters and neons: contact cooling?

Oops, yes, I should have remembered that the R390 uses the 6082, with its nominal 25V heater. Could be an expensive mistake, otherwise!

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