[Amps] Ameritron Al-1500 excessived grid current & shut down
jim.thom at telus.net
Mon Aug 27 12:36:53 EDT 2018
Date: Mon, 27 Aug 2018 09:40:13 -0400
From: w8hw <w8hw at comcast.net>
To: Herbert Schoenbohm <herbert.schoenbohm at gmail.com>, Amps group
<amps at contesting.com>
Subject: Re: [Amps] Ameritron Al-1500 excessive grid current & shut
<90% of the time it is a diode on the hv board that is near the edge. If
I were home, I could give you the exact one, but won't be home in
Florida for a week (Sept 3). I repair amps and have worked on that
problem on that exact amp many times, and only once was it the tube.
Hope that helps. 73, Bruce, W8HW
On 8/26/2018 4:30 PM, Herbert Schoenbohm wrote:
> Today I turned on the Al-1500and immediately notice that the grid current
> went all the way over to the right and the amp grid protection circuit kick
> in. Even with only 5 watts of drive, the grid current is about 175 ma. I
> also notice that I don't see any HV indication. I hope this is not a bad
> 8877 final tube. Any suggestion to confirm this would be appreciated.
> Herb, KV4FZ
## IF its a case of a power supply diode arcing to metal, with not enough clearance,
a thin sheet of teflon etc, can be inserted in the problem area. Local plastic store
has loads of scrap teflon sheeting. IF the B+ arced to the chassis, it will try and return back to the
B- of the filter caps.... via the single safety diode.... which is located between the chassis..and B-.
That safety diode will typ short out. Its purpose is to keep the B- above chassis potential.... but
keep it within .7 volt. Then you can do normal metering. The safety diode should not short out.
Ameritron used a single 1N5408..with its 200A surge rating..which is for 8.3 msecs. They should have used
a 6A10, with its 400A surge rating. A 6A10 is the same body length as a 1N5408, but bigger diameter. They both
have the same 16 gauge / .051 diameter wire leads..and same length leads. A 6A10 will drop into where a 1N5408
once stood, ditto with the rectifier diodes in the FWB.
## with no B+.... on any GG triode amp, and with drive applied, you will peg the grid meter every time.
With no B+, the tube now thinks the grid is the anode. All the electrons are now attracted to the grid.
## Check all the diodes on the diode board on the B+ supply. If they fail, they will fail shorted.
## To stop any potential blow ups, I install a fast..single HV fuse between one leg of plate xfmr sec..and input to
FWB. That HV fuse can go into either leg of the HV xfmr sec. I typ use a 50 ohm glitch resistor in the B+ output.
A 2nd HV fuse is located in the B+ output, just prior, and in series with the 50 ohm glitch resistor. In the event of a
B+ to chassis arc, or internal anode to grid arc, etc, the 50 ohm glitch R limits the fault current to a safe value. The
HV fuse in the B+ interrupts the fault current... in less than 1-2 msecs.
## IF the fault is on the main FWB board, or between FWB board..and input to filter caps, or between B+ of filter caps..
and input to 2nd HV fuse.... then HV fuse #1 will blow open asap. I use several paralleled 6A10s for greater surge capacity,
installed between chassis..and B-. At that point, anything can be shorted to anything else, B+ to chassis, B+ to B-.... raw DC
to chassis, anode to grid, etc, etc and nothing bad happens. One or both hv fuses opens up, thats it. No blown tube, no destroyed
safety diodes, no damaged diodes on FWB..or FWD. Then its fully protected.
More information about the Amps