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Author Topic: Ameritron AL-82 Loud Bang  (Read 12260 times)
W1QJ
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Posts: 1515




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« Reply #15 on: March 03, 2013, 07:21:04 AM »

Then use the green wire on the transformer to bump the HV up to 3900 to 4KV.  

Tell me more about this mod Lou as this is first I have heard of it.  

The big box amps really need 10 electrolytic caps instead of 8.  You can add 2 more caps to the string of 8 pretty easily by using a piece of PC type board and dead bug wire 2 caps in series with bleeders.  Attach wires to cap PC board to pick up last one in the string loop it thru the 2 new caps and then back to the PC board.  Then cut the trace going to the metering resistors and use a wire soldered to the resistor side of the trace over to the + of the last cap you added so you get the right HV reading on the meter.  The green wire on the transformer isn't necessary, but I've used it to bump the plate voltage up to 3900 or 4KV.  The green wire on the transformer is there in case your line voltage is closer to 220 than 240.  So if you have 240 and you use the green wire it bumps the plate voltage up and since you added 2 more caps it will now take the higher voltage.  I'm sure the difference in signal is not noticeable but non the less it is an option.  The filament voltage is not affected because the amp has a separate filament transformer. The main thing actually is to add the 2 more caps to relief stress on the string of 8.
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W8JX
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Posts: 6645




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« Reply #16 on: March 03, 2013, 07:33:41 AM »

Then use the green wire on the transformer to bump the HV up to 3900 to 4KV.  

Tell me more about this mod Lou as this is first I have heard of it.  

The big box amps really need 10 electrolytic caps instead of 8.  You can add 2 more caps to the string of 8 pretty easily by using a piece of PC type board and dead bug wire 2 caps in series with bleeders.  Attach wires to cap PC board to pick up last one in the string loop it thru the 2 new caps and then back to the PC board.  Then cut the trace going to the metering resistors and use a wire soldered to the resistor side of the trace over to the + of the last cap you added so you get the right HV reading on the meter.  The green wire on the transformer isn't necessary, but I've used it to bump the plate voltage up to 3900 or 4KV.  The green wire on the transformer is there in case your line voltage is closer to 220 than 240.  So if you have 240 and you use the green wire it bumps the plate voltage up and since you added 2 more caps it will now take the higher voltage.  I'm sure the difference in signal is not noticeable but non the less it is an option.  The filament voltage is not affected because the amp has a separate filament transformer. The main thing actually is to add the 2 more caps to relief stress on the string of 8.

I fully understand that capacitor string has to be changed BEFORE you even try to bump voltage. As you raise voltage on valves the amount of drive required for same output decreases. I always liked that amp and may buy one one day.   It is a shame that Ameritron did not use two more caps to begin with.
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You can embrace new computer/tablet technology and change with it or cling to old fall far behind....
AA4HA
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Posts: 1640




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« Reply #17 on: March 03, 2013, 12:06:36 PM »

This may seem odd but by increasing the string to ten capacitors in series you end up decreasing the capacity of the entire bank. It is the way that mathematics works out for capacitor calculations in series.

You will increase the voltage capabilities of the capacitor bank but the capacity to hold a charge goes down. Here is an example;

8 caps     210uF each, 450V            26.25uF string,3600V      at 3600V (no load)   170.1 Joules
10 caps   210uF each, 450V            21 uF     string,4500V      at 3600V (no load)   136.1 Joules

By changing out the caps to the 220uF at 500V each here is what happens;

8 caps    220uF each,  500V            27.5uF string, 4000V       at 3600V (no load)   178.2 Joules
10 caps  220uF each,  500V            22 uF   string, 5000V       at 3600V (no load)   142.6 Joules

The maximum rated anode voltage for the 3-500Z is 4000V so I would not want to exceed that. (do what thou want);

Increasing to 4000V here is what happens in the four cases above;

8   210uF caps, 450V each             bad idea, cap string is rated to 3600V "caps die early and noisy death"
10 210uF caps, 450V each             168 Joules, very close to what the supply does at 3600 w 8 caps (500V margin on caps)
8   220uF caps, 500V each             220 Joules, higher inrush current, suggest step-start (you have no voltage margin on the caps)
10 220uF caps, 500V each             176 Joules, probably ok, step-start probably not a bad idea (1000V margin on caps)

What would I do? Stick with 8 caps but change them out to 220uF at 500V if the price is right. You get a dual benefit of more Amp/Seconds of capacity (that is what Joules means), you will see less fluctuation of anode voltage during operation, you have 400 volts of protection for the capacitors. You probably really should add step-start to spare the transformer and diodes during power-up. If you plan on increasing anode voltage then go from 8 to 10 of the 220uF values so you could go up to 4000V (or maybe even 4500V if you toss all caution to the winds and own stock in a supplier of 3-500Z tubes).

Drastic increases in power will just move your problems elsewhere in the amplifier.
« Last Edit: March 03, 2013, 12:13:17 PM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
K4RVN
Member

Posts: 812




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« Reply #18 on: March 03, 2013, 04:49:31 PM »

Actually Tisha, I got 142.56 joules for the 10 at 220uf ,3600v no load. Just kidding good job and I learned something thanks,

Frank
« Last Edit: March 03, 2013, 05:07:24 PM by K4RVN » Logged
W1QJ
Member

Posts: 1515




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« Reply #19 on: March 03, 2013, 05:55:50 PM »

The ultimate value of cpacitance for a FWB is 32UF.  The minimum value is 22UF.  Using more than 32UF gains you nothing until the next break point of 88UF.  This is due to the law of diminishing returns.  Utimately the Al-82 and the other 2 big box amps would have 10 320UF caps.  Sure, you can use 500v caps and stay with 8, but try and find them #1 and #2 sit down when you see the price.  Much cheaper to use 10 caps unless you don't mind spending the extra money.  You can always retrofit snap in caps that are cheaper yet.  I've seen too many big box Ameritrons blowing filter caps because of the "right on the edge" voltage rating.  They are also known for opening bleeder resistors.  That will also take out the associated cap.  Our Lunar Link amps use 10 caps in the power supply.  Gives plenty of head room. 
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N1DYX
Member

Posts: 40




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« Reply #20 on: March 03, 2013, 06:14:08 PM »

Well, I removed all the caps and checked them very carefully, no bulged ends of swollen cases, no signs of burn marks.  I also removed the boards and looked them over carefully and found no suspicious abnormalities.  Checked all the bleeder resistors and they all checked out fine.  Very puzzling, I expected to find some evidence.  I'll keep snooping around I guess.

Here's a shot of the cap area, from what I'm interpreting about the insulator, it looks like that wouldn't be necessary since these caps are very short and don't reach the chassis.......correct?  (I guess you can't link a picture directly to display here)

Edit: They all seem to charge up OK also using a multimeter.  Kind of a crude test I guess but hopefully it would show one that was open.

http://i29.photobucket.com/albums/c251/Bill75/DSC02321.jpg
« Last Edit: March 03, 2013, 06:24:25 PM by N1DYX » Logged
W1QJ
Member

Posts: 1515




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« Reply #21 on: March 03, 2013, 09:14:15 PM »

OK, That is a newer version of the AL-82.  They now use the shorter caps.  The older units used much longer ones and they were about 1/8" from the chassis on the bottom hence the insulation.  SO now that I see this unit I see it is pretty new edition.  Could you possibly give us a similar photo of the other side of the amp?  Same angle just the other side.  The only other place the HV can arc would be near the bottom of the plate choke, maybe the bypass cap is blown out. check it closely.  What about the plate choke itself?  Any burned windings that can be shorting to each other? That could make a snapping sound also.  Keep looking closely.
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ZL4IV
Member

Posts: 44




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« Reply #22 on: March 04, 2013, 12:50:44 AM »

Is the amp still going? Does bang sometimes? Take the cover off the amp, turn off the lighting so you can see the outline of the amp. Run it under the conditions the problem manifests it's self and if possible keep both hands in your pockets. It will be obvious where the bang comes from. If it is a cap then B+ will suffer in voltage drop. If it is a flash over then this will continue to happen until you clean off the arc points completely from the carbon deposit and even then this weak spot may continue to be a problem. Can't give any more advice with so little information. Hope this helps.
ZL4IV
 
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N1DYX
Member

Posts: 40




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« Reply #23 on: March 04, 2013, 06:32:01 AM »

OK, That is a newer version of the AL-82.  They now use the shorter caps.  The older units used much longer ones and they were about 1/8" from the chassis on the bottom hence the insulation.  SO now that I see this unit I see it is pretty new edition.  Could you possibly give us a similar photo of the other side of the amp?  Same angle just the other side.  The only other place the HV can arc would be near the bottom of the plate choke, maybe the bypass cap is blown out. check it closely.  What about the plate choke itself?  Any burned windings that can be shorting to each other? That could make a snapping sound also.  Keep looking closely.

OK sure, when I get home from work.  I left out the 9,247 screws holding the cover Smiley
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N1DYX
Member

Posts: 40




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« Reply #24 on: March 04, 2013, 06:36:41 AM »

Is the amp still going? Does bang sometimes? Take the cover off the amp, turn off the lighting so you can see the outline of the amp. Run it under the conditions the problem manifests it's self and if possible keep both hands in your pockets. It will be obvious where the bang comes from. If it is a cap then B+ will suffer in voltage drop. If it is a flash over then this will continue to happen until you clean off the arc points completely from the carbon deposit and even then this weak spot may continue to be a problem. Can't give any more advice with so little information. Hope this helps.
ZL4IV
 

Amp is still going, it only gives bang perioidically after it'e been in use for a half hour or so.  About the time you let your guard down.  Actually I haven't seen a flash but as you stated the cover has always been on.  The inlet holes for the cooling air is near the caps and is facing me and have not noticed any lightning but I'll give yor idea a test.

Thanks

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N1DYX
Member

Posts: 40




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« Reply #25 on: March 04, 2013, 06:39:26 AM »

This may seem odd but by increasing the string to ten capacitors in series you end up decreasing the capacity of the entire bank. It is the way that mathematics works out for capacitor calculations in series.

You will increase the voltage capabilities of the capacitor bank but the capacity to hold a charge goes down. Here is an example;

8 caps     210uF each, 450V            26.25uF string,3600V      at 3600V (no load)   170.1 Joules
10 caps   210uF each, 450V            21 uF     string,4500V      at 3600V (no load)   136.1 Joules

By changing out the caps to the 220uF at 500V each here is what happens;

8 caps    220uF each,  500V            27.5uF string, 4000V       at 3600V (no load)   178.2 Joules
10 caps  220uF each,  500V            22 uF   string, 5000V       at 3600V (no load)   142.6 Joules

The maximum rated anode voltage for the 3-500Z is 4000V so I would not want to exceed that. (do what thou want);

Increasing to 4000V here is what happens in the four cases above;

8   210uF caps, 450V each             bad idea, cap string is rated to 3600V "caps die early and noisy death"
10 210uF caps, 450V each             168 Joules, very close to what the supply does at 3600 w 8 caps (500V margin on caps)
8   220uF caps, 500V each             220 Joules, higher inrush current, suggest step-start (you have no voltage margin on the caps)
10 220uF caps, 500V each             176 Joules, probably ok, step-start probably not a bad idea (1000V margin on caps)

What would I do? Stick with 8 caps but change them out to 220uF at 500V if the price is right. You get a dual benefit of more Amp/Seconds of capacity (that is what Joules means), you will see less fluctuation of anode voltage during operation, you have 400 volts of protection for the capacitors. You probably really should add step-start to spare the transformer and diodes during power-up. If you plan on increasing anode voltage then go from 8 to 10 of the 220uF values so you could go up to 4000V (or maybe even 4500V if you toss all caution to the winds and own stock in a supplier of 3-500Z tubes).

Drastic increases in power will just move your problems elsewhere in the amplifier.

Thanks for all this, great information.  I see they do make a soft start for this that has some mild reviews, maybe I'll make one. 

Thanks again!
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N4TTS
Member

Posts: 175




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« Reply #26 on: March 04, 2013, 03:23:27 PM »

It should already have a soft/step start circuit.
See the relay under the interlock switch with the big resistor connected to it?

Don N4TTS

Thanks for all this, great information.  I see they do make a soft start for this that has some mild reviews, maybe I'll make one. 

Thanks again!
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W1QJ
Member

Posts: 1515




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« Reply #27 on: March 04, 2013, 04:34:55 PM »

It should already have a soft/step start circuit.
See the relay under the interlock switch with the big resistor connected to it?

Don N4TTS

Thanks for all this, great information.  I see they do make a soft start for this that has some mild reviews, maybe I'll make one. 

Thanks again!

Yeah, the Al-82 already has a soft start but the delay is mighty quick.
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N1DYX
Member

Posts: 40




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« Reply #28 on: March 04, 2013, 05:03:51 PM »

Pics of the other side.  TTS .....Also noted the info on the soft start switch, thanks.

http://i29.photobucket.com/albums/c251/Bill75/DSC02325.jpg
http://i29.photobucket.com/albums/c251/Bill75/DSC02322.jpg
http://i29.photobucket.com/albums/c251/Bill75/DSC02323.jpg
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9A4WY
Member

Posts: 24




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« Reply #29 on: March 04, 2013, 11:47:10 PM »

Hi Bill.
Try to measure resistance on the output conector(must be few ohms)...when amp. is OFFLINE and DISCONECTED, but with extreme care(can be "safety choke" open, and then blocking caps discharging through output coax or conector or somewhere...

Load cap is spark gap maybe?? Wink
73 Kiko 9A4WY
« Last Edit: March 04, 2013, 11:51:19 PM by 9A4WY » Logged
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