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Author Topic: Ameritron Al-572  (Read 3046 times)
W5ER
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Posts: 74




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« on: July 28, 2011, 07:42:50 AM »

It appears that I've lost my  grid current. Wife called for some assistance, so I walked away from the shack with the amp powered on, in operate mode and had current showing, returned about five minutes later and now had negative current showing.

Recalled having read about D117 going bad suddenly, opened her up and replaced D117 which was NOT bad, gave a casual look around the unit saw nothing amiss, buttoned it back up, all for naught.

So other than the 572Bs which I understand cannot be tested without owning an SB201 tester or fabricating one like W8JI , where would be the next area of interest?

BTW 35 years ago I was very comfortable working on tube devices along with their associated high voltages but now find myself quite nervous after years of low voltages. Need to keep the one hand at a time in mind.

I'd truely appreciate any help offered.

73 Ed W5ER
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KA5N
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« Reply #1 on: July 28, 2011, 07:55:27 AM »

Sounds more like a resistor in the metering circuit opened up. 

Allen
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W8JI
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« Reply #2 on: July 28, 2011, 09:43:18 AM »

Negative grid current is caused by a cathode to ground short, always.  Never by a bad diode or an open resistor.

The most common cause of negative grid current is a filament to grid short in a tube.
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W1QJ
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« Reply #3 on: July 28, 2011, 02:03:07 PM »

.  Never by a bad diode or an open resistor.

When the grid shunt resistor opens in an SB-220 you get a negative reading.  I would at least check the grid shunt for an open for giggles.
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W8JI
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« Reply #4 on: July 28, 2011, 03:11:11 PM »

.  Never by a bad diode or an open resistor.

When the grid shunt resistor opens in an SB-220 you get a negative reading.  I would at least check the grid shunt for an open for giggles.

1.) The SB220 has no diode clamp, although it would be a good idea to add one.

2.) The SB220 grid meter can sometimes pin negative with an open grid shunt and not keyed because of the application of positive voltage to the filament CT, but when drive is applied it pins forward.

The only possible cause of a negative grid current reading in the AL572 is a cathode circuit to ground short. Most commonly it is a bad tube.

If the 572 has an open grid shunt, something I've never seen, the grid meter will pin positive with drive. If it has an open plate current shunt the plate current meter will pin positive. The SB220 will do the same, the exception is when there is zero applied drive the grid meter can back up in the 220. That's because of the +120 dc applied to the filament of the SB220 when on standby, a system the AL572 does not have.

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W5ER
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Posts: 74




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« Reply #5 on: July 29, 2011, 08:11:40 PM »

Okay fellas, thanks for the info.  On transmit it does show a little grid current 20~30ma. I was thinking it was a tube or 2,3, or 4.  Have a set coming.

Tom, was looking at your gas discharge mod for the 572, Mouser shows a few thousand choices, among them are Peak Pulse volts and fail safe protection. I haven't got the faintest idea as what to order other than 140 to 170 volt

Additionally, are GDTs a one time usage device like a fuse,.. tube pops then install a new GDT? Or would they offer recurring protection?  Determines how many I order.

73 Thanks again guys
Ed  W5ER
 
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W8JI
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« Reply #6 on: July 30, 2011, 11:05:43 AM »

Okay fellas, thanks for the info.  On transmit it does show a little grid current 20~30ma. I was thinking it was a tube or 2,3, or 4.  Have a set coming.

It's PROBABLY a tube. That is the most common thing that fits your description.

Quote
Tom, was looking at your gas discharge mod for the 572, Mouser shows a few thousand choices, among them are Peak Pulse volts and fail safe protection. I haven't got the faintest idea as what to order other than 140 to 170 volt

Additionally, are GDTs a one time usage device like a fuse,.. tube pops then install a new GDT? Or would they offer recurring protection?  Determines how many I order.

The gas tubes can fire many times and still be good. They recover after a breakdown.


73 Thanks again guys
Ed  W5ER
 
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N6WK
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« Reply #7 on: August 04, 2011, 12:25:04 PM »

I had an AL-572 here yesterday that had the same problem. The grid Meter went in reverse instead of up as it should.
It is NOT the tubes. a filament to ground short could do it, but that was Not the case.
If anyone tells you it HAS to be a bad tube , they are Wrong in this case.
   It turned out to be a shorted Q2 and an open D3 on the power supply board.
The D3 is the 9.1 volt Zener ( NTE5018) and Q2 is a 2n3906 (NTE159)
The amp is working great again and the parts were just a few dollars. The grid meter now works as it should again.

Hope this might help you out.
73,
Gordon
N6WK
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W8JI
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« Reply #8 on: August 04, 2011, 03:01:16 PM »

I had an AL-572 here yesterday that had the same problem. The grid Meter went in reverse instead of up as it should.
It is NOT the tubes. a filament to ground short could do it, but that was Not the case.
If anyone tells you it HAS to be a bad tube , they are Wrong in this case.
   It turned out to be a shorted Q2 and an open D3 on the power supply board.
The D3 is the 9.1 volt Zener ( NTE5018) and Q2 is a 2n3906 (NTE159)
The amp is working great again and the parts were just a few dollars. The grid meter now works as it should again.

Hope this might help you out.
73,
Gordon
N6WK

Hi Gordon,

Be sure you make these mods:

http://www.w8ji.com/arc_protection_al572.htm

The original poster, based on his description, almost certainly has a bad tube.

First, if D3 and Q2 open they open because of a tube short. An open bias circuit can cause a negative deflection *if the amp is driven to force high filament to ground voltage*, but an open bias system is rare. This happens only when something in the filament circuit takes a very hard hit from excessive filament to ground voltage.

Second, let's try to not totally rewrite what other people say. Surely our reading skills are better than that!  :-)

I said an open shunt cannot cause negative grid (the context of this negative deflection was at no drive, and positive deflection with drive). I also said nearly all of the time negative deflection is caused by a tube fault. It can also be cause by any filament to ground fault. It is never caused by an open meter shunt or an shorted or open meter protection diode.

73 Tom







« Last Edit: August 04, 2011, 03:08:01 PM by W8JI » Logged
N6WK
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« Reply #9 on: August 06, 2011, 06:50:48 PM »

Hi Tom,
  Sorry to go against what you have previously posted.
On QRZ you posted : http://forums.qrz.com/showthread.php?295776-Ameritron-AL-811&goto=newpost
I think in the second post you stated 
"Negative grid current is always a short from the cathode (filament) to chassis. "   BTW, That is a QUOTE of your actual post.

Didn't appear to be the case this time.
I am not trying to Re-Write what people say,.. more I am trying to CORRECT erroneous information they might have said.
I have learned to never say Never and Never say Always!

73,
Gordon
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W8JI
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« Reply #10 on: August 07, 2011, 11:04:23 AM »

Hi Tom,
  Sorry to go against what you have previously posted.
On QRZ you posted : http://forums.qrz.com/showthread.php?295776-Ameritron-AL-811&goto=newpost
I think in the second post you stated 
"Negative grid current is always a short from the cathode (filament) to chassis. "   BTW, That is a QUOTE of your actual post.

Didn't appear to be the case this time.
I am not trying to Re-Write what people say,.. more I am trying to CORRECT erroneous information they might have said.
I have learned to never say Never and Never say Always!

73,
Gordon

Hi Gordon,

That statement is totally correct. Negative grid current ALWAYS comes from a filament to chassis path. Always.
Now here is what happens, and why we have to pay attention to ALL symptoms.

The AL811 and 572 series have MOV's at minimum, and gas protection tubes in later models, from filament to ground.  If someone drives the amplifier with RF and if the amplifier has an open center tap or boas system, they can force the tube filament voltage high enough to break down the MOV's or gas protect tubes. When that happens the grid meter will go backwards, but that is because they drove the amp with RF while it had another defect to the point where the filament system broke down and shorted to the chassis.

In this case the user had negative current with no drive when he came back in the room, and later he said some positive current with drive. He was told it was an open shunt, but an open shunt cannot cause that problem.

The ONLY thing that ever causes a negative grid meter reading in the Ameritron series is a filament or cathode to ground fault. As I said, that USUALLY is a bad tube although it can be other parts. For example, an MOV or gas protection tube can bad. If RF is applied with an open cathode path, the MOV's or gas tube can break down. This is because the cathode is forced over the breakdown voltage of the protection devices.

I think it helps to understand what causes the grid meter to go negative. I'll try to be a little more thorough in the future about an open center tap path, because that can cause the MOV or gas protect to break down **if enough RF is applied to break down the MOV's or clamp tubes**.

By the way, the root cause of failures in bias are tube arcs. It is a good idea, if the bias system fails, to keep an eye on the tubes. Also be sure to update the protection system and remove the grid resistors and directly ground the grids.

73 Tom
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