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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 16th Oct 2021, 1:16 pm   #1
Bazz4CQJ
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Default Possible HRO Mod; 12VDC Heater config?

I’d appreciate any comments on the following. To simplify the discussion, let’s imagine that an HRO has an even number of valves, and all use the same heater current.

Is there any reason why the heaters could not be configured to run on 12VDC; valves in pairs (in series) across the line?

I’m not sure if there any pros and cons to having the 12V floating, or whether it would be better to have the negative line go via the chassis?

This idea would allow the HT to be produced via one of the 12V inverter units that were discussed on the forum a while ago (with good screening and filtering).

Thanks
B
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Old 16th Oct 2021, 1:26 pm   #2
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Default Re: Possible HRO Mod; 12VDC Heater config?

I see no problem.
 
Old 16th Oct 2021, 1:55 pm   #3
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Default Re: Possible HRO Mod; 12VDC Heater config?

I have seen a HRO so modified, used with a 12-to-240V inverter for the HT and used on 'field day' in the early 1970s.
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Old 16th Oct 2021, 2:30 pm   #4
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Default Re: Possible HRO Mod; 12VDC Heater config?

OK, thank you both. I could not see any problem, but appreciate that confirmation. As you will know, the HRO originally came with separate PSU, speaker and coil packs, and the old "dog kennel" psu was a weighty beast.

Some time ago, I started a complete rebuild of the HRO, which I bought back in 1968; it was completely dead. As I removed paper caps and resistors, many fell apart as soon as they were taken out. The electrolytics were dry as desert sand!

As I mentioned in a previous thread, I'm building a "Set Top Box" for the HRO, the same width as the HRO, and about 3 or 4 inches high, which will be the home of a new (lightweight) power supply, a digital frequency counter, and maybe some other things (a Product detector and AF processor).

The small (and light) inexpensive inverter units, first mentioned by Joebog on the forum, seems entirely adequate for the purpose, but operates at 75kHz, so will need good isolation.

This task has been in progress for a while now and I think I should try and finish it this winter!

B
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Old 16th Oct 2021, 3:03 pm   #5
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Default Re: Possible HRO Mod; 12VDC Heater config?

What about a 12 to 6V buck converter, such as https://www.ebay.co.uk/itm/202801514...QAAOSwmh1Z3ZZx
 
Old 16th Oct 2021, 4:13 pm   #6
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Default Re: Possible HRO Mod; 12VDC Heater config?

Quote:
Originally Posted by merlinmaxwell View Post
What about a 12 to 6V buck converter, such as https://www.ebay.co.uk/itm/202801514...QAAOSwmh1Z3ZZx
Yes, that's worth thinking about. I need to take a look at the set again and assess how easily the heaters could be rewired. If that looks tricky, the buck convertor might be an easy solution. Do they produce much RF noise?

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Old 16th Oct 2021, 4:22 pm   #7
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Default Re: Possible HRO Mod; 12VDC Heater config?

If you have an HRO fitted with octal valves (6K7; 6J7; 6V6; 6SQ7) then you could fit 12-volt versions e.g. 12K7GT; 12J7GT; 12A6 and 12SQ7. All are pin-equivalent to the originals.
If you have a version with the earlier UX-based valves, then you will need an add-on as already suggested.
I rebuilt an HRO from the chassis up a few years ago and fitted single-ended 'toobs' with 12-volt heaters (12SK7; 12SJ7 etc.) to cut down the current (and hence the voltage drop) in a long lead to the PSU. NOS 'JAN' versions cost less than their 6-volt equivalents.
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Old 16th Oct 2021, 5:07 pm   #8
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Default Re: Possible HRO Mod; 12VDC Heater config?

After all, it's what innumerable vehicle/ship/aircraft mounted receivers did when running 6.3V heater valves from 6- or 12-cell lead-acid systems. Certainly, refer one side to chassis to keep the stray RF tendencies down and, bearing in mind that 10-100nF capacitors of 50-63V rating will cost peanuts, a few of them in appropriate places will do no harm just in case- I can't recall the HRO circuit offhand but US designers rather liked Hartley oscillators with the possibility of their H-K capacitance modulating the heater line. It's a valid point that 12V operation will keep the umbilical loss down, I always thought that the Pye CAT ought to have done something like this!
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Old 16th Oct 2021, 6:45 pm   #9
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Default Re: Possible HRO Mod; 12VDC Heater config?

My HRO is full of 6D6 and 6C6's etc, which I don't think come in a 12V version, and anyway, I have a stock of a few new ones of those, so changing over to 12V valves would be a distraction. Having parted with the dog kennel psu, the idea was to go with something lighter and smaller, and this recent idea of using the 12V inverter for the HT and and 12V on the heaters is the smallest lightest option yet (taking 12V from a small bench-top PSU, used for many other tasks). The inverter could probably be squeezed inside the HRO, and may be a small speaker too (for general listening), which would give me a self-contained HRO... except for a few coils packs . I do quite like this idea.

The inverter will run the HRO happily when fitted with a 50mm fan, but I need to take some care to put it in a good screened box and filter the connections. Running at 75kHz, I guess it will put out a lot of RF noise. Any thoughts on how best to keep that quiet would be appreciated.

The set hasn't been in regular use since the early 1970's!

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