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

Bill Dzurilla billdz.geo at yahoo.com
Fri May 28 10:33:18 PDT 2010

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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