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

Subject: Re: [Amps] OK to run HV power supply with no load?
From: Bill Dzurilla <>
Date: Fri, 28 May 2010 10:33:18 -0700 (PDT)
List-post: <">>
Well, it seems it is OK to run the p/s without a load, but there may be other 
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 

73, Bill NZ5N 

--- On Thu, 5/27/10, Bill Dzurilla <> wrote:

> From: Bill Dzurilla <>
> Subject: [Amps] OK to run HV power supply with no load?
> To:
> 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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