[AMRadio] Re: WTB: HV Tranny

[John Coleman]

The description of the "economy supply" circuit is accurate but I have found
that some folks (me) have tried to lift with a switch or relay the
connection of the bridge that would normally go to ground as method of
standby.  This will tend to make the center tap go negative.  Placing a
single diode in series with the center tap (low voltage output take off)
before filter capacitors (anode to center tap) will eliminate this trouble.

John, WA5BXO


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Greg
Sent: Wednesday, February 06, 2008 12:32 PM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Re: WTB: HV Tranny

OK JT

So I was right? Wow!
Finding the replacement transformer will be difficult for the Multi Elmac 
due to the tapped secondary and current requirement.
An "Economy" supply puts out two voltages: high voltage and half the high 
voltage using only one winding of a center tapped transformer.  You build it

like the classic full wave bridge with two diodes and a filter.  Then you 
unground the center tap and use the center tap as the half voltage and then 
filter and bleed it.  The schematic is on the internet and the radio 
handbooks and is well accepted by everyone.
If you are shooting for 500 vdc on the 6146 plate what I suggest is find a 
400 vac 200 ma or so transformer as the main power source.  The pi net 
filter will boost the dc output to the 500 volt range then do the 1/2 
voltage trick with another pi net filter and bleeder.  The Multi Elmac is 
tolerant to a degree of over/ under voltage but I would not recommend taking

the 6146 to 600 vdc or above.
What I did here is took a Hallicrafters transverter power supply and 
modified it.  Used a large dropping resistor with sliding tap to drop the 
750 vdc to 500 vdc and added a soild state relay and 12 vdc power supply to 
key the HV transformer thru the hand mic. This avoids keying the radio with 
no load and the full 750 vdc appearing in the Multi Elmac.  The filaments 
are lit thru a separate transformer that is not keyed.
It works ok and with 500 vdc on the rig I get 25-30 w of AM out on 75m.  The

audio is ok but not spectacular. VFO is stable.
I think the bottom line on your transformer quest is that you won't be able 
to find the exact right transformer any time soon and will have to do 
engineering/ adaption to get you to where you need to go. I am stocked out 
of any transformers that can be adapted to your rig. Some where there's a 
Elmac net that meets on 75m am. You wouldn't have a reference for that would

you?
73's
Greg
WA7LYO
Kinston NC

----- Original Message ----- 
From: "JT Croteau" <jt.n1ese at gmail.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Wednesday, February 06, 2008 12:42 PM
Subject: Re: [AMRadio] Re: WTB: HV Tranny


> On Feb 6, 2008 12:31 PM, Greg <bluebirdtele at embarqmail.com> wrote:
>> Is this for an AF 67? I just built up a supply for my AF 67 and that's
>> pretty much what the circuit calls for.
>
> It sure is.
>
>> Do you need a tapped secondary for the dual HV or can you do it via the
>> "Economy Supply" method of lifting the secondary CT ground and taking 1/2
>> total voltage for the low B plus supply?  To do that you will need a 400 
>> vac ct
>> transformer at 225 ma. and a full wave bridge rectifier.
>
> I was going to just build it per the PS-2V schematic, I'm not familiar
> with this "economy" method.
>
> Thanks
>
> -- 
> JT Croteau, N1ESE - Manchester, NH (FN42gx)
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