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Author Topic: Need help with AL-811 parts  (Read 1129 times)
W3UTD
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« on: December 09, 2008, 04:23:09 PM »

When replacing tubes today, I noticed the resistor inside the coil that connects to the grid cap is split in two.  Looking at the parts list on Ameritron's web page I can't quite figure out which parts to order.

Here a pic I took  http://www.quasc.com/ebay/IMG_0446.jpg

is it 10-15168   SA, CHOKE, COIL Huh
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WB2WIK
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« Reply #1 on: December 09, 2008, 06:46:47 PM »

That's not a "grid cap," it's a plate cap.

But anyway, that's the PC (parasitic choke) assembly, and it's in the parts list.

Not a good sign that the resistor cracked.  Replacing the parasitic suppressor without doing anything else may well have it crack again.

811As are not too swift...evidently they really do have occasional parasitic problems by design.  Of course, this is a tube that is from the 1930s I believe.  It's amazing they're even still made!

You might think about replacing them with 572Bs, which are slightly more modern.  No extra "output power" or anything, but they have longer service life and may help eliminate the problems.

I've heard good reports about people replacing the 811As with 572Bs in this amp...most seem happy they did it.

WB2WIK/6
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W3UTD
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« Reply #2 on: December 09, 2008, 06:54:41 PM »

Thanks for the reply.  First, the resistor cracked when I moved it to replace the old 811's with 572B's. The old tubes had been abused so I figured "stronger" 572B's wouldn't hurt.

Now to the part number, I'm afraid I still don't see it:

is it this?
10-15197   SA, CHOKE, PLATE, ALL AMPLIFIERS
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K0XU
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« Reply #3 on: December 09, 2008, 07:23:45 PM »

I think the part you are looking for is the resistor only. They don't seem to sell the assembly. From the  parts list in the 811H manual that I downloaded I would guess that to be 103-2151-1 a 51 ohm 2 watt resistor. I don't see too many resistors in the online parts list???
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W3UTD
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« Reply #4 on: December 09, 2008, 07:45:59 PM »

Wait, I think I see it.  From the schematic it's called PC, from the parts list PC is:

Parasitic PCB P/N 50-0811-1, which appears to consist of (3) 100ohm, 3 watt P/N 104-2100, (6) 100pF DM15 P/N 250-0100 and (3) CAP 811 P/N 755-6811

On the website list that is given as SA, PCB ASBLY, PARASITIC, 811

All of that for $20.04.  So I think that means 3 of the complete assemblies.  I'll have to call Ameritron tomorrow to confirm.

Interestingly, there's another P/N 50-0572-1 SA, PCB ASBLY, PARASITIC, 572

Perhaps since I replaced the 811's with 572's I should get that one?  I'll update tomorrow after I call.

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VK7JB
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« Reply #5 on: December 10, 2008, 01:31:53 AM »

Hello,
The part you require is #50-0811-1 SA, PCB ASBLY, Parasitic, 811.

I recently installed it in my old 811.  Very easy install and replaces the old style wire inductor-over- resistor parasitic suppressors. It also upgrades  the plate leads with braided wire and the anode caps.  While I was at it, I replaced the Plate DC blocking cap, as you need to unsolder this from the original plate bus bar anyway when you install the parasitic PCB atop the plate choke.  The parasitic PCB assembly for the 811 and 811H are identical, except that there are only 3 units on the 811 board:  the 4th is unpopulated, as it's not needed in the 3 tube version, obviously.

Very easy upgrade and well worth the effort.

I've also replaced the tubes with 572b's. The old 811 runs like a dream now.

Regards,
John, VK7JB
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W8JI
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« Reply #6 on: December 10, 2008, 03:41:25 AM »

The 811 originally used a carbon composition resistor for low inductance. Carbon comps are subject to fatigue and value change as they age. This is greatly accelerated by heat from the tubes. They have a finite life because of tube heat.

They stopped making carbon compositions in the 90's except through special order from only one manufacturing source. There was no alternative source.

This forced a change to metal resistors. Most metal resistors had too much inductance.

This forced a change in the parasitic suppression circuit to a resonant suppressor, which is still currently used.


There are now low inductance metal composition resistors from Ohmite in the OX or OY packages. They are superior to the older carbon comps, that you would almost certainly never find. Most resistors sold aftermarket today as carbon comps are actually carbon films and will have short life and not work well.

This means you have two choices. You can order 100 ohm Ohmite metal COMPOSITIONS in their OX/OY series and dumplicate what you had with greatly superior parts....or you can change to the new suppression board that has less loss at 10 meters and is more immune to effects of heat with its longer life metal film resistors.


73 Tom

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KD8GEH
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« Reply #7 on: December 11, 2008, 06:49:06 AM »

Great points from all. I overhauled the assy on mine, but is was newer. By the way, the reason they smoke is too much exciter drive on ten meters. Alway make sure not to drive it too hard on any band and it will extend the life on the tubes significantly.

If you have any questions, call Mike at Ameritron, he's the expert and had taught me alot about the amp.

73,  Dave  KD8GEH
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