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Author Topic: AL-811H plate and load capacitors  (Read 8482 times)
N0SQ
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Posts: 53




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« Reply #15 on: April 30, 2012, 05:24:57 PM »

  First off you didn't  "change" your amp to grounded grid by just grounding the grids instead of using a resistor.  The amp was still grounded grid with the resistors.

What? That statement flies in the face of logic. But, I'm not going to argue the point. I'm done with this thread, anyway, since I didn't get any positive responses.
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N3JBH
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Posts: 2358




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« Reply #16 on: April 30, 2012, 06:36:42 PM »


Quote
What? That statement flies in the face of logic. But, I'm not going to argue the point. I'm done with this thread, anyway, since I didn't get any positive responses.


If i may before you run off If it was not grounded grid before then may i please just ask what did you call it ?
« Last Edit: April 30, 2012, 06:39:08 PM by N3JBH » Logged
AB0WR
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Posts: 77




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« Reply #17 on: April 30, 2012, 06:45:12 PM »

I assume the grid resistors are bypassed, right? So the only reason to get rid of the resistors is to eliminate a possible failure point?
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N3JBH
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Posts: 2358




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« Reply #18 on: April 30, 2012, 07:24:51 PM »

AB0WR In short what 8JI was saying was removing the resistors and adding Gas discharge tubes would reduce the problem with a arc during tube failure. Or at least that is how i make of what it said.
 
http://www.w8ji.com/al811_important_modifications_changes.htm
« Last Edit: April 30, 2012, 07:27:41 PM by N3JBH » Logged
W1QJ
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Posts: 1516




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« Reply #19 on: May 01, 2012, 04:43:59 AM »

  First off you didn't  "change" your amp to grounded grid by just grounding the grids instead of using a resistor.  The amp was still grounded grid with the resistors.

What? That statement flies in the face of logic. But, I'm not going to argue the point. I'm done with this thread, anyway, since I didn't get any positive responses.

With all due respect my friend, I suppose you do not understand what a ground grid amp design is.  if you did, you would not have made this statement.  Also, we hate to see you run off is a huff.  I don't know where else you can go to get any better advice on ham amplifiers.  Some of the best ham amplfier experts are assembled right here on this group.  The cumulative knowledge here is about the best you'll find anywhere.  If you don't like the advice you get here. and can find better advice someplace else would you kindly let us know where that is.  I for one would like to be there as well.  Thanks and good luck with your project.
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AB0WR
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Posts: 77




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« Reply #20 on: May 01, 2012, 03:34:19 PM »

An arc where?

An arc would indicate high momentary current, perhaps enough to pop one of the resistors/capacitor grounding paths.

A blown resistor in the grid circuit surely wouldn't be good. Wouldn't the grid wind up at about the same potential as the plate?

If the resistor/capacitor pair were replaced with a heavy wire, I'm not sure what the gas discharge tube would accomplish.

....... Oh, I see. The gas discharge tubes are in the filament leads. They would prevent the filaments from going high during a filament to plate arc.

The two mods are for separate things.

AB0WR In short what 8JI was saying was removing the resistors and adding Gas discharge tubes would reduce the problem with a arc during tube failure. Or at least that is how i make of what it said.
 
http://www.w8ji.com/al811_important_modifications_changes.htm
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NO2A
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Posts: 843




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« Reply #21 on: May 02, 2012, 06:22:11 PM »

If you amp is a commercial amp then theoretically the cap values and plate spacing are correct.  If it's arcing, find the cause as QJ suggests. 

If you increase the plate spacing then you will probably shift the problem to the next vulnerable component like the bandswitch.  Smoke that one and "you in a heapa trouble boy!"
I agree,and the bandswitch is usually the weakest link in any amp. Replacing it is a major PITA!
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N0SQ
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Posts: 53




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« Reply #22 on: May 07, 2012, 04:55:15 PM »

If you increase the plate spacing then you will probably shift the problem to the next vulnerable component like the bandswitch.  Smoke that one and "you in a heapa trouble boy!"

It can be replaced.

Anyway, I ordered the original plate capacitor. I don't know why it arced because I followed Ameritons instructions. I've tried tuning the amp by using a keyer running at 60 WPM (to keep duty cycle low) also.

After reading what I felt like reading about amplifiers, I wonder why anyone would want an amplifier. They're too easily damaged. It seems that it's tricky to get the drive and loading set right. And if you're a contester, re-tuning after each band change is a hassle and undesireable.

I have thought about buying an auto-tune amp like an Alpha but I've read negative comments about them also. I once had an ALS-600 but kept blowing up the finals. The ALS-600 would have been nice since you didn't have to tune it after changing bands but it couldn't handle the repeated, initial transceiver key up.

So, what is a ham to do?
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K8KAS
Member

Posts: 570




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« Reply #23 on: May 09, 2012, 05:24:41 AM »

Why would anyone screw around with a basic 811 amplifier like this, a pair of vac caps are not cheap for sure and once you made this mod no one would touch it on the used market. Why don't you look for a Quality used amp with a real transmitting tube in it
and parts rated for the application. Nice used Henry for example..73 Denny K8KAS
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DL2MDU
Member

Posts: 15


WWW

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« Reply #24 on: May 09, 2012, 08:26:06 AM »

Hi guys,

I would save my money, going to sell the AL-811 and buy a rugged AMP instead of putting plenty of time into that project.  Wink

73 Chris, DL2MDU
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K8AXW
Member

Posts: 4001




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« Reply #25 on: May 09, 2012, 09:31:27 AM »

 
Quote
After reading what I felt like reading about amplifiers, I wonder why anyone would want an amplifier. They're too easily damaged. It seems that it's tricky to get the drive and loading set right. And if you're a contester, re-tuning after each band change is a hassle and undesireable.

SQ:  I agree and have felt the same way for a long time.  I built a homebrew amp 30 years ago and have never had a bit of problem with it..... so far.  But I bought a used SB-200 that has driven me crazy and has made me feel the way you do.

However, with that being said, there still isn't any feeling like hearing a DX station on in the middle of a pileup ..... pushing the PTT button, saying my callsign and have him come right back.  Or better yet, have a DX contact and after have my own pileup from DX stations.

I'll keep my amps and put up with this PITA along with my hemorrhoids.

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K1ZJH
Member

Posts: 1185




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« Reply #26 on: May 09, 2012, 06:23:27 PM »

If you amp is a commercial amp then theoretically the cap values and plate spacing are correct.  If it's arcing, find the cause as QJ suggests. 

If you increase the plate spacing then you will probably shift the problem to the next vulnerable component like the bandswitch.  Smoke that one and "you in a heapa trouble boy!"

Plus 1. Exactly what I was going to post.
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KM3F
Member

Posts: 525




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« Reply #27 on: May 10, 2012, 12:16:57 PM »

Normally many amplifiers and Tuners for that matter are designed to have reasonable plate spacing for the intended power level.
However, if they are misstuned at high power levels such that the RF voltage goes too high accross the plates, the air as a dielectric will break down causing the arc.
Once a pit or raised peak develops on the plates from the original arc, that sharp point will be the place the next arc will usually occur at 'and' do it at an even 'lower' voltage the next time and there after.
There are some tuner designs that should not be used to tune certain antenna lengths and configurations especially at higher powers due to the high RF voltages that develop in the matching network. Same is true trying to load an amplifier into an improper antenna.
Then there is the opposite, (but don't do this).
It's just a point of info.
You can take many 150 watt rated tuners and work them at 300 watts, BUT, only if it is match down 'before' that power is applied.
Why, is that the RF voltage present by being in a matched condition is within the spacing limits for that hardware.
But once you adjust out of tune very far, all bets are off so again don't do it.
This is often why the loss of bandswitchs occurr from RF voltges going to high from miss tuning and poor configurations attempted.
Good luck.
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K1ZJH
Member

Posts: 1185




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« Reply #28 on: May 10, 2012, 01:28:58 PM »

The gas discharge tubes are there to protect the transceiver! Having 1700 volts on the filaments during TX could take out the transmitter PA devices.

I'd also try using jewelers files to clean up those plates before replacing the variable cap. Then
i'd try to determine the cause for the arcing in the first place. Arcing can be caused by
poor tuning procedures, or high SWR.

Pete
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K8AXW
Member

Posts: 4001




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« Reply #29 on: May 10, 2012, 06:06:11 PM »

Quote
I'd also try using jewelers files to clean up those plates before replacing the variable cap.

I went through this with a TUNE cap on my SB-200.  The arc in many cases causes the plates to bulge at the point of the arc as well as eroding the edge of the plate(s).  In order to dress up these points, it's almost a given you have to remove the cap from the amp.

Not only do you have to dress the plates until there are no bulges or sharp points with a FLAT jewelers file.  It's then necessary to polish these spots with a very fine garnet paper or crocus cloth to remove the file scratches.

After the polish then it's necessary to hose the whole thing down with contact a non-residue contact cleaner to remove all of the aluminum filings.

It took me two tries before I got it right.  I lost a bit of capacitance so had to reposition the knob so that my settings matched my notes.
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