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Old 21st Jul 2020, 7:54 pm   #1
ChristianFletcher
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Default Bodger's transformer Installation Lafayette HE-40

When it comes a ham radio equipment I cannot help wondering if it is actually a license to commit atrocities again radios ?

I have a Lafayette HE-40 these came out the Trio factory of Japan back in the day under various names Hallicrafters S-120, Star-Line SR-40 and in England the HAM-1

The radio I have is in a bit of a state all the valve missing and a really appallingly installed transformer I think these were a AC-DC set originally designed for 120 AC. I haven't found a picture or circuit for the HAM-1 version sold in England so dont know if it was AC DC or transformer circuit. But this has a really badly installed transformer of the 230/120/6.3V type.

The original radio used series strung 12 volt valves I have tried to find 6 volt alternatives that I plan on using with the 6.3V transformer . I couldn't find a direct replacement for the output valve 50C5 and I am using a 6CU5 would this be OK ?

Currently the phantom has wired all the filaments in parallel, but using only one wire to the transformer and is using the radio chassis ground as the return for the heater supply to the transformer. I have never seen this done on any other radio and I am assuming its going to cause more mains hum than keeping the circuits separated. So I plan to disconnected one side of the 6 volt winding from the chassis and run two wires to the filaments in the convectional manner.

Just wondered if anyone had any thoughts or advise before I take the plunge. I have included a picture showing the original circuit and also the valve base. The brown pen marks show the existing heater arrangement as I find it. If I do rewire the heaters does the sequence I wire the heaters in matter at all. Should I start at the frequency changer or the output end on the valves line up.

As always thanks in advance for the helping hand from those more knowledgeable than myself

Thanks Regards Chris
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Old 22nd Jul 2020, 12:37 am   #2
JohnBG8JMB
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Default Re: Bodger's transformer Installation Lafayette HE-40

The Lafayette HE-40 Hallicrafters S-120, Star-Line SR-40 were AC-DC sets originally designed for 120 AC

The Star SR-150 was a 230 VAC transformer set- cct attached. not allowed NB the transformer was unusual, it PRVIDED 110v FOR HT TAPPED AT 75v FOR THE SERIES HEATERS CHAIN, AND 6.3 v FOR DIAL LAMPS ONLY. Also sold as the Veritone CR-150. Valves 12BE6 12BA6 12AV6 19C5, 150mA heaters.


you could look at a 6aq5 for the op valve, but you will have to adjust the cathode resistor

Coud you use the mains tr as a 230-120 v and the original series acdc setup?

regards
John G8JMB
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Old 22nd Jul 2020, 7:32 am   #3
SeanStevens
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Default Re: Bodger's transformer Installation Lafayette HE-40

There are lots of 240-110v transformers about. I have one I found in France that looks to be a domestic portable unit. That kind of thing would allow you to run it as intended.

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Old 22nd Jul 2020, 8:47 am   #4
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Default Re: Bodger's transformer Installation Lafayette HE-40

Unfortunately, a lot of people back in the day used autotransformers and war-surplus headphones, leaving only the insulation of the output transformer between one end (either end!) of the mains and their lug 'oles.

That set may have had an atrocity performed on it, but it did start out as an abomination.

I think it was all part of an Oriental/American plot to kill off British SWLs.

The drawings of them looked good in the Henry's Radio and G W Smith adverts, but I'm glad I saved up a bit extra and bought an AR88.

I looked up 'Lafayette' in the school's Brittanica. He was a French Aristocrat who commanded American troops in the American revolutionary war, so it's one of those names Americans are supposed to revere or stand up and salute when they hear. I suppose the maker didn't go quite as far as putting a stars and stripes background to the tuning scale, but they were trying unsubtly to hitch a ride on American history.

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Old 22nd Jul 2020, 2:50 pm   #5
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Default Re: Bodger's transformer Installation Lafayette HE-40

As always thanks for the information and circuit diagram. I think this was an AC/DC set and the transformer was installed by the phantom. The transformer should be ok as it has a 6.3 volt heater supply to suit the valves I already bought. The high voltage output from the transformer is 120 AC that may need a bit of taming down but I have thought I may try fitting a shunt regulation tube as used on the TRIO 9R

Any preference on rewiring the heaters and loosing the heat ground return via chassis.

Thanks regards Chris
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Old 22nd Jul 2020, 3:54 pm   #6
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Default Re: Bodger's transformer Installation Lafayette HE-40

Hi Chris, it should be possible to wind you an isolation transformer for your set that could be configured for series or parallel as you require as well as a suitable HT tapping


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Old 22nd Jul 2020, 11:34 pm   #7
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Default Re: Bodger's transformer Installation Lafayette HE-40

Quote:
Originally Posted by ChristianFletcher View Post
Any preference on rewiring the heaters and loosing the heat ground return via chassis.
There are differing schools of thought re. optimum heater wiring arrangement depending on whether minimising hum is the highest priority (generally higher-fi AF amp usage) or preventing stray coupling between stages (much radio usage). By returning one side of the heater to chassis at each valve, the chance of propagating stray pick-up along the heater wiring is reduced- after all, the other side of the heater is then only a low impedance away from local chassis and if heater decoupling is still needed at a particular stage, only one capacitor is needed. With balanced heater wiring connected to chassis only at the transformer centre tap, the heater wires are more likley to propagate strays between stages without those local and definite chassis connections, and any necessary decoupling capacitors then need to be applied in pairs. The Hartley local oscillator beloved of US (and followers) designers with their pentagrid mixers (6BE6, 12BE6 etc.) results in a cathode "live" to LO and therefore likely to induce LO into the heater wiring via cathode-heater capacitance, risking whistles and other sprogs by coupling into other stages.

So single-wire, chassis return heaters would be the usual choice- but by all means try other configurations!
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Old 23rd Jul 2020, 4:44 am   #8
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Default Re: Bodger's transformer Installation Lafayette HE-40

The other way of getting RF isolation of coupling via the heater system is to have decoupling in the heater system. Most usually capacitors to chassis, less often including series chokes.

This increases cost. and the shunt impedances aren't zero, but then the single wire versus chassis approach isn't perfect either. It might get a shunt capacitor or two if trouble is hit.

It gets problematical with series string heaters in AC/DC sets. If the chassis metal is RF ground as well as neutral, shunt capacitors can be used, but they have mains AC across them and will take current, distorting the distribution of heating amongst the valves if the capacitors are large. If it's one of those sets with isolated chassis and hot circuitry, you can wind up with more safety-critical capacitors bridging taps on the heater string to what should be mains-isolated metalwork.

I avoid AC/DC sets.
I prefer single wire for radios and balanced heater supplies for hifi.

In sme cases like calcode RF amplifiers in VHF/UHF front ends, you'll see the top valve with twin RF chokes in the heater feed, possibly done as a common mode choke. In this case the top cathode must not have capacitance to ground, and the heater/cathode capacitance is too much.

Another place where heater circuitry had to get creative is for thermionic diodes in AC voltmeters, particularly RF ones. Sometimes the operational circuitry had to be designed around the limitations of the heater scheme.

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Old 23rd Jul 2020, 9:30 am   #9
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Default Re: Bodger's transformer Installation Lafayette HE-40

Thanks so it looks like the single wiring for the heater may not be such a bodge after all. I am just waiting the arrival Of the output valve on Saturday hopefully before I can power the set up but I may give it a try using my signal tracer as the PA. funny that I have done quite a lot of domestic radios Restorations and I have never seen the one wire heater Circuit using a chassis return Line. Great thing about the hobby every day is a school day. I can experiment with the heater wiring. Just to clarify it isn’t using an auto transformer it’s a conventional transformer with the primary isolated from the secondary.

Thanks for all the great comments always food for thought. Best regards Chris
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Old 23rd Jul 2020, 6:21 pm   #10
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Default Re: Bodger's transformer Installation Lafayette HE-40

Quote:
Originally Posted by Radio Wrangler View Post

That set may have had an atrocity performed on it, but it did start out as an abomination.

I think it was all part of an Oriental/American plot to kill off British SWLs.
The circuit is basic to say the least....no RF amp, no BFO (unless it uses the hideous idea of a regenerative IF.....) which I think are minimum essentials for a half-decent SW set.

Edit.....second look, it DOES use regenerative IF for a BFO.......Yuk! It might be OK as a boys first foray into SW but I think the radio I built from a design by F G Rayer back in 1968 probably worked far better....
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Old 23rd Jul 2020, 6:43 pm   #11
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Default Re: Bodger's transformer Installation Lafayette HE-40

One-side-of-the-heaters-grounded and using the chassis as return is just fine: RCA were quite happy to use it in the AR88!

And an auto-transformer to power the heaters is OK too - quite a few domestic receivers/record-players used the idea [some record-players used a tap on the motor's field-coils for this, reducing production-costs and parts-count, to the delight of 'product engineering' and shareholders alike]
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Old 23rd Jul 2020, 7:51 pm   #12
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Default Re: Bodger's transformer Installation Lafayette HE-40

It is what it is, seen worse, at least the chassis is in effect floating according to the schematic.

I think that where possible the original circuit should be preserved but that's just my view.

Lawrence.
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