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Old 20th Nov 2017, 10:40 am   #1
Argus25
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Default 12V HT 'tube' homebrew radio

This is a bit of an odd one, at least the way it ended up.

I was always fascinated with 12v HT tubes such as the EF98 and ECH83 after seeing them in hybrid car radios.

So I thought I'd have a go at making a dual band set, 550 to 1700KHz and 5.5MHz to 18MHz shortwave. I started with the best ferrite rod I could get, 1/2" diameter and wound the coils for it, 60 strand litz for the MW coil.

The radio runs from one 12V battery.

I'll briefly mention the mechanical construction:

Dial, Eddystone with epicyclic reduction drive, small incandescent lamps added.

Chassis pre-painted steel (Hammond), pressed machine stainless nuts for bottom plate screws (self threading screws forbidden in this radio) All screws stainless steel 4-40 or 6-32 UNC.

Front panel 1/8" thick stainless with engine turning finish, chrome brass handles.

Knob/switch plates from Akihabara Tokyo markets.

Ceramic valve sockets and Teflon wiring.

Self designed & hand wound RF & osc coils, commercial IF transformers.

Output choke 22:8 ohm auto-transformer.

Band change switch: Telemechanique industrial power switch, operates 3 hermetically sealed Teledyne relays for band change switching. (That way I never end up with a switch that wears out or needs cleaning! Plus these switches feel great).

The radio chassis and rear of the front panel and all surfaces were protected during hole cutting/filing with many layers of plastic tape, so despite all the chassis work/holes etc, everything is free from surface scratches. The cut edges in the steel chassis were re-painted to prevent rusting.

The audio circuit has an EF98 transformer coupled to the OC16 in class A on a 1/8" thick copper heatsink. The bracket for the EF98 transformer is green anodized aluminium.

Most of the adjustments are mil grade ceramic trimmers and are chassis mounted facing upwards for easy alignment/access.

The circuit didn't work out exactly as I had planned. There is an EF98 RF stage, and ECH83 converter and EF98 IF. However I found that the triode section of the converter wasn't much use over 12 to 13MHz and the capacitance of the grid & socket assembly too high to get the tuning range up to 18MHz, so in place of this triode I used an MPF102. I also added another one in as an extra IF amplifier and added a 2N3643 transistor to alter the audio gain in SW mode.

So it turned into a really oddball hybrid.

I wanted to keep an even number of tubes to 4 so the pairs of heater voltages added up to 12V . The radio is a very good performer, the SW band is good up to about 12MHz on the ferrite rod, after that it benefits from a wire antenna.

It is fun to work on a valve radio where you don't get zapped from the HT.
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Old 20th Nov 2017, 11:11 am   #2
paulsherwin
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Default Re: 12V HT tube homebrew radio

Another beautiful construction job, well done.
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Old 20th Nov 2017, 11:23 am   #3
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Default Re: 12V HT tube homebrew radio

A work of art.
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Old 20th Nov 2017, 11:46 am   #4
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Default Re: 12V HT tube homebrew radio

Even to my novice eye, that is a beautiful radio. Well done.
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Old 20th Nov 2017, 12:02 pm   #5
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Default Re: 12V HT tube homebrew radio

Hey Hugo,

That is an incredible piece of engineering and design and and artwork. How does it look alongside your Blake7 TV??

Thanks for sharing it, amazing.
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Old 20th Nov 2017, 12:06 pm   #6
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Default Re: 12V HT tube homebrew radio

How lovely! You really do go to town on the aesthetics of your radios! And kudos for making an 'interesting' hybrid with solid-state where-appropriate.

[I built a few 12V-HT things: best was a HF converter [EF98 RF amp, ECH83 mixer/xtal oscillator] so I could get the 49metre band {shortwave Radio Luxembourg!} on a MW/LW car radio].
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Old 20th Nov 2017, 12:42 pm   #7
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Default Re: 12V HT 'tube' homebrew radio

Lovely job- I'm always impressed with the carefully thought-out aesthetics of your projects, as well as the thorough technical grounding (!). Capped by their off-the-beaten track, different-approach nature.

There were a handful of receivers intended for direct HT operation from 24/28V systems, e.g. the allegedly (personally not convinced) disposable-on-arrival simple BC1206 LF beacon set to the blank-cheque/engineering-for-the-sake-of-it R392.

Last edited by turretslug; 20th Nov 2017 at 12:49 pm.
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Old 20th Nov 2017, 1:49 pm   #8
Argus25
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Default Re: 12V HT 'tube' homebrew radio

Quote:
Originally Posted by turretslug View Post
Lovely job- I'm always impressed with the carefully thought-out aesthetics of your projects
It is pretty easy to get a valve radio to look good from the upper chassis side, if one spends time deciding where all the parts would best be located. But it is a lot harder to get a component layout that looks good on the underside so it doesn't look as neat as I would like. Though with valve gear sometimes tag boards help with geometric placing of the parts.

I built another MW/SW radio on about the same sized chassis as this one, but I realized due to the larger number of parts and smaller parts (being all transistorized) I would need a pcb. So I cut the top off the chassis and replaced it with a pcb. Again a bit of an oddball concept but it worked. I'll post a thread on it when I can.

It is nice with this radio though , as all valve radios, to see the valve heaters glowing in low ambient light and having to wait for it to warm up after turning it on. It is sort of magical and its harder to get an all transistor radio to top that.
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Old 20th Nov 2017, 1:59 pm   #9
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Default Re: 12V HT 'tube' homebrew radio

Quote:
Originally Posted by turretslug View Post
Lovely job- I'm always impressed with the carefully thought-out aesthetics of your projects
It is pretty easy to get a valve radio to look good from the upper chassis side, if one spends time deciding where all the parts would best be located. But it is a lot harder to get a component layout that looks good on the underside. Though with valve gear sometimes tag boards help with geometric placing of the parts.

I built another MW/SW radio on about the same sized chassis as this one, but I realized due to the larger number of parts and smaller parts (being transistorized) I would need a pcb. So I cut the top off the chassis and replaced it with a pcb. Again a bit of an oddball concept but it worked. I'll post a thread on it when I can.

It is nice with this radio though , as all valve radios, to see the valve heaters glowing in low ambient light and having to wait for it to warm up after turning it on. It is sort of magical and its harder to get an all transistor radio to top that.
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Old 20th Nov 2017, 3:07 pm   #10
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Default Re: 12V HT 'tube' homebrew radio

Beautiful looking!

I like your ideas in combinations of technologies, using the strengths of each.
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Old 20th Nov 2017, 5:31 pm   #11
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Default Re: 12V HT 'tube' homebrew radio

It does look lovely! Super job, Hugo.

The stud-mount OC16 (germanium) is an odd transistor to have chosen, couldn't you find any silicon?
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Old 20th Nov 2017, 5:35 pm   #12
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Default Re: 12V HT 'tube' homebrew radio

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Originally Posted by kalee20 View Post
The stud-mount OC16 (germanium) is an odd transistor to have chosen, couldn't you find any silicon?
That is a landmark transistor! The first ever mass-produced power transistor!
It was boasted as a 24W dissipation capable device; I wonder if that was ever even close! I love the bespoke (Berrylium) copper heatsink, it's a work of art in itself!
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Old 20th Nov 2017, 8:01 pm   #13
Argus25
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Default Re: 12V HT 'tube' homebrew radio

Quote:
Originally Posted by kalee20 View Post
It does look lovely! Super job, Hugo.

The stud-mount OC16 (germanium) is an odd transistor to have chosen, couldn't you find any silicon?
Most car radios which used the EF98 , transformer and single output transistor arrangement used a germanium transistor, maybe an AD149 or an earlier OC equivalent TO-3 type etc. In American sets they often they used the 2N441. I have all of these, but when I acquired the matching transformer (Made by Beacon Radio NZ) I noticed it had EF98 - OC16 stamped on its bracket. Plus as Al noted, the OC16 is one of the the very first power transistors made by Philips, released around 1957-58 and example circuits of it with the transformer-EF98 are shown in vintage Philips manuals.

The audio circuit is quite something, an input impedance at the grid of the EF98 of 10meg ohms and an output impedance from the autotransformer of 8 ohms. The amp only needs a volt or two drive for good volume and just the two active devices, which represent two entirely different fields of amplifier technology. It is a spectacular impedance transformation for two stages. As I recall the transformer between the EF98 and OC16 is something like 23:1 turns ratio.
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Old 20th Nov 2017, 9:57 pm   #14
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Default Re: 12V HT 'tube' homebrew radio

I'm not disagreeing - the OC16 was indeed a breakthrough transistor (I have a Philips car radio using one, driven by an EF98 exactly as said! Works well. Quiescent current is 1.5A which, after deducting the valve's heater currents and the pilot lamp current, leaves about 1A through the transistor. With battery on charge, nearly 14.5V, dissipation would have been 14W or just above).

Having one, or an AD149 or whatever is a good argument for using it, I admit!
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Old 20th Nov 2017, 10:18 pm   #15
Argus25
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Default Re: 12V HT 'tube' homebrew radio

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Originally Posted by kalee20 View Post
leaves about 1A through the transistor. With battery on charge, nearly 14.5V, dissipation would have been 14W or just above).
With this one the OC16's transistor's quiescent current is close to 420mA, so it dissipates about 5W, regardless of the volume level, which is a bit more conservative. It readily provides a few watts output which is plenty for a desk radio especially with the efficient ceramic magnet speaker, but I'd agree, more can be needed in a car.
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Old 21st Nov 2017, 12:06 pm   #16
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Default Re: 12V HT 'tube' homebrew radio

Absolutely stunning! Well done,

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Old 21st Nov 2017, 12:16 pm   #17
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Default Re: 12V HT 'tube' homebrew radio

I agree with everyone else, beautifully built, better than most manufacturers of that era, similar in quality to the ex WD equipment widely available after the war but much prettier.

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Old 21st Nov 2017, 1:52 pm   #18
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Default Re: 12V HT 'tube' homebrew radio

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Originally Posted by astral highway View Post
The stud-mount OC16 (germanium) is an odd transistor to have chosen.......

It was boasted as a 24W dissipation capable device; I wonder if that was ever even close! I love the bespoke (Berrylium) copper heatsink, it's a work of art in itself!
Not unreasonable given the likely constraint on that rating of the mounting base held at 25C.

Actually doing that, of course, would require some interesting heatsink/refrigeration arrangements.

What it could do in practice would be a matter for calculations around max junction temperature (about 90C for Ge) and the thermal resistance from there via the device case and heatsink to ambient temperature.
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