[AMRadio] Too much HV

Wed Oct 19 00:30:27 EDT 2011

Could it be that some one added a capacitor on the input side of the 
choke? The value would determine the HV. Large and it will be close 
to 1.414 *VRMS. Small and it will be close to .9*VRMS. Somewhere in 
between and it might be the 2300V.

Now everybody knows how to up the voltage on their Boatanchors by 50% 
- watch for smoke.

- Paul

At 08:57 PM 10/18/2011, you wrote:
>I think the silly part comes fundamentally from blaming the HV
>increase on the SS diodes. At most, expect to see 15-20 V increase in
>the HV using them. But that's not what we are told is observed. Thus,
>it can't be the SS diodes. What might it be? Let's work the problem:
>
>Most of the older HV PS used choke input filters and a few used
>resonant filters, wherein the inductor and capacitor offered an LC
>circuit resonant at about 120 Hz (for a FW rectifier). Based on all
>the books in which this stuff is discussed, we should expect a
>capacitor input filter to generate approximately sqrt(2)*VRMS, or
>1.414*VRMS, where VRMS is the RMS AC voltage applied to the rectifier.
>
>So, as an example if we neglect voltage drop in the rectifiers, if
>100 VRMS AC is applied to the rectifier and then followed with a
>capacitor-input filter, we may expect the output voltage to be
>somewhere in the neighborhood of 140 VDC. If the filter is replaced
>by a choke input filter, we can expect the output to be about
>(2*sqrt(2)/Pi) * VRMS, or about 0.9*VRMS and is ~ 90 VDC.
>
>Let's look at our example: if we have a 3 kV CT secondary and use a
>FW rectifier with a capacitor input, we should expect a no-load DC
>output voltage to be about 1.4*1500=2100 VDC. That same transformer
>feeding a choke input filter will produce about 1350 VDC open-load.
>In the original posting, we're told that the rectifier is fed 1900
>VRMS. We also know that originally, the PS used a choke input filter.
>So, we expect to see 1710 VDC. Yet, we're told that the output is
>about 2300 V. If the input choke is shorted, then we have nothing but
>the capacitor and so a capacitor-input filter. We expect it to
>produce about 2660 VDC. We're told it's really 2300 VDC, which is a
>bit lower than we might expect but in the ballpark. So, a shorted
>input choke is strongly suspected.
>
>73,
>
>Kim N5OP
>
>At 10:01 PM 10/18/2011, you wrote:
> >I have found the discussion in this thread interesting. What's silly about
> >it?
> >
> >Brad K4RT
> >
> >------ Original Message ------
> >Received: Tue, 18 Oct 2011 09:27:26 PM EDT
> >From: "Bernie Doran" <qedconsultants at embarqmail.com>
> >To: "Discussion of AM Radio in the Amateur Service" 
> <amradio at mailman.qth.net>
> >Subject: Re: [AMRadio] Too much HV
> >
> >look up the voltage drop for Vacuum tube rectifiers, mercury vapor, and
> >diodes. VTs drop around 50 to 60 volts, MV tubes 15 volts, almost identical
> >to a string of diodes. each diode is about 1/2 volt or so.    This is
> >getting silly, just what is on the currant amateur radio tests?    If the
> >voltage changes a measurable amount, something else has happened. not the
> >diodes
> >original Message -----
> >From: "CL in NC" <mjcal77 at yahoo.com>
> >To: <amradio at mailman.qth.net>
> >Sent: Tuesday, October 18, 2011 1:52 PM
> >Subject: [AMRadio] Too much HV
> >
> >
> > > Well, anytime I ever replaced a tube rect. with a diode, I 
> always ended up
> > > with a higher B+.  The HA 10 I am working on does not have the choke in
> > > it.  It arrived with a 1900VCT xfmr, a bridge rect. and  a cap string
> > > totaling 16uf.  Unloaded, the plate volts rests at 2300, to much for
> > > idling 811's.  I removed the bridge rect. used only half the diodes in
> > > full wave configuration and get 1700 no load, which is OK.  I 
> picked up an
> > > original Heath 3kv CT to put in it.  But in conjunction with 
> Bill, who has
> > > the an original HA10 with correct PS plate xfmr, swinging choke, and oil
> > > caps, gets 2500 with a W2AU diode block.  I expect to end up 
> with the same
> > > problem if I switch out the xfmr,  So I'm looking to solve Bill's HV
> > > problem and avoid creating a similar one.  I am putting inrush protection
> > > in it since the original HA10 plate xfmr was a known weak point, along
> > > with an internal  ant/amp bypass relay.  Guess the choke will not make a
> > > return since it is
> > > obscenely expensive when newly manf., so upping the filter to a string of
> > > 6  450uf at 450vdc.  While the amp is not needed for use with the Heath
> > > Apache, it does come in handy with the C Line when running it on AM.  One
> > > fellow questioned the xfmr, both xfmrs have the proper 3kv AC out of the
> > > secondary, with 1500 each side of CT, no imbalance, so does not appear to
> > > be a xfmr issue.
> > >
> > > Charlie in NC
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