[Amps] OK to run HV power supply with no load?

[Bill Dzurilla]

Tried it again.  At the suggestion of folks who contacted me off-line, this time ran the Drake B- to ground through a different one ohm resistor, a resistor that is totally separate and apart from the Harbach board, added bypass capacitors and used heavier wire.

This time, no sign of any arcing or other problems with the power supply.  However, RF output is zero.  I am reading some plate current on the amp's plate current meter, although quite a bit lower than with the internal p/s.  I do not know for sure if the amp's meter is reading accurately, but I can't think of any reason why it would be inaccurate, as on the L-7 the B- also goes to ground through a one ohm resistor.

With the internal supply, idling plate current is 100ma.  With the Drake, it's reading only about 10ma.  With 5 watts of drive, the internal supply shows 200ma of plate current, while with the Drake supply the meter reads 100ma.  I suppose I need a high voltage probe to verify HV and a separate ammeter to confirm the plate current reading.  If there's a bad connection somewhere I'd expect to see zero plate current.  

Any further ideas or thoughts would be appreciated.

73, Bill NZ5N  

--- On Fri, 5/28/10, Bill Dzurilla <billdz.geo at yahoo.com> wrote:

> From: Bill Dzurilla <billdz.geo at yahoo.com>
> Subject: Re: [Amps] OK to run HV power supply with no load?
> To: amps at contesting.com
> Date: Friday, May 28, 2010, 1:33 PM
> Well, it seems it is OK to run the
> p/s without a load, but there may be other issues.
>  
> What I have here is a homebrew 6m amp with a 3-500Z
> tube.  The amp has a built-in power supply using an
> SB-200 transformer and a Harbach SB-200 diode board. 
> It works, but output is only about 350 watts, because the
> power supply is not beefy enough to deliver more. 
> Under load, plate voltage is less than 2000v, and the 3-500Z
> likes to see higher than that for full output.
> 
> I also have a Drake L-7, which uses two 3-500Zs and has a
> separate power supply.  What I wanted to do, on a test
> basis, was to disconnect the HV from the amp's internal
> supply and replace it with HV from the Drake p/s.  Just
> wanted to see how much more output, if any, I could get from
> the Drake p/s.  If output is substantially higher, I
> could think about some sort of more permanent arrangement,
> perhaps involving removal of the internal supply.
> 
> So I removed the B+ connection from the internal P/S to the
> plate tank and replaced it with a connection from the plate
> tank to the B+ on the Drake p/s.  I was unsure about
> where to make the B- connection from the Drake p/s to the
> amp -- directly to ground or to the high side of the 1 ohm
> metering resistor?  I ended up connecting to the high
> side of the resistor, on the theory that this would enable
> me to monitor plate current on the amps meter.  Also
> ran a ground wire from the Drake p/s to the amp.
> 
> Fearing a big bang that would destroy both the amp and the
> Drake p/s, I nervously hooked it all up and applied
> power.  No bang.  With the transceiver set on SSB
> and low power, I pressed the PTT.  With the internal
> supply, this results in about 100ma of plate current. 
> With the Drake supply, the reading was zero.  I then
> switched the transceiver to RTTY and applied a carrier on
> low power.  The meter then showed about 80ma of plate
> current.  After a few seconds, just as I was reaching
> for the amp's knobs to tune up, I heard a low noise from the
> Drake p/s, perhaps a small arc.  I then stopped the
> test and shut down.  Fortunately, the Drake p/s appears
> to be undamaged, as it is still working OK on the L-7.
> 
> Not sure what I did wrong, if anything, but it looks as
> though it will not be as easy to complete this test as I had
> hoped.  Any comments or advice would be appreciated.
> 
> 73, Bill NZ5N 
> 
> --- On Thu, 5/27/10, Bill Dzurilla <billdz.geo at yahoo.com>
> wrote:
> 
> > From: Bill Dzurilla <billdz.geo at yahoo.com>
> > Subject: [Amps] OK to run HV power supply with no
> load?
> > To: amps at contesting.com
> > Date: Thursday, May 27, 2010, 12:28 PM
> > In connection with my homebrew 6m
> > 3-500Z amp that we discussed in an earlier thread, I
> would
> > like to try a test with a power supply from a
> different
> > amp.  The homebrew amp's internal power supply has
> B+
> > and B- wires running from the Harbach board.  For
> the
> > test, I'd like to remove just these 2 wires and
> connect the
> > B+ and B- to the external p/s.  However, the
> internal
> > p/s would remain powered up to run the filaments, fan,
> and
> > relay.  
> > 
> > Is there any problem with this idea?  In
> particularly,
> > is it OK for the Harbach board to be connected with
> no
> > load?
> > 
> > The test would be far more difficult if I have to
> > disconnect the internal p/s completely.
> > 
> > 73, Bill NZ5N 
> > 
> > 
> >       
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> 
> 
>