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Author Topic: Ameritron AL-82 With HIGH AC Line voltage  (Read 1560 times)
K1FPV
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Posts: 33




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« on: March 30, 2009, 12:26:59 PM »

I've run into a bit of a sticky situation. My AC line voltage coming into my home is too high. It typically reads between 244 and 247 volts. When checking with the local power company, they allow a ±5% variation which means they allow from 228  to 252 volts.

With this high an AC voltage, my new AL-82 amplifier typically has between 3650 and 3700 volts on the B+ supply to the tubes. It is supposed to run at no more than 3,600 volts tops without a load according to Ameritron. I called them and asked if there was a different tap I could use to lower the B+ by 100 or 200 volts. They said no as it is fixed. I know on the AL-80B, there is a tap that lowers the B+ by a couple hundred volts.

Anyone with a solution or suggestion?
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WB2WIK
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« Reply #1 on: March 30, 2009, 12:55:23 PM »

I don't have the AL-82 schematic...

In the power supply, what do the filter capacitors add up to, in terms of overall voltage rating?

That's really the only limitation; the 3-500Zs are rated 4000V and some amplifiers do run them that high.

If the capacitor string adds up to 4000V or more, I wouldn't really worry about this.

(I'd measure the B+ to be sure the AL-82 meter is indicating it accurately, also.  But use a high voltage probe and a correctly rated meter to do so!)

WB2WIK/6
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WB2WIK
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« Reply #2 on: March 30, 2009, 01:00:07 PM »

I just looked at the manual on-line (Ameritron's website) and see a discrepancy.

It states the power supply is a full wave bridge with 3800Vdc output.  In that case, yours (if your metering is correct) should be fine.

But then the schematic shows a power supply filter PCB assembly with eight 450V capacitors in series.  That only adds up to 3600V.

So, which is it?  3800 or 3600?  Hmmm.

You might see if Tom W8JI chimes in on this one, I think he was involved with the design.

WB2WIK/6
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W8EZT
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« Reply #3 on: March 30, 2009, 01:27:32 PM »

I don't have the schematic handy, but assuming all power is drawn across the 240 line (no current flow in the neutral) then you add an outboard filament transformer in series with the amplifier in "buck" mode to reduce line voltage.  

With an appropriate filament transformer, you can drop a fixed amount of voltage very efficiently by phasing the secondary such that it's voltage is subtracted from the line voltage.  To be safe, I'd drop the maximum transformer voltage, so line voltage never exceeds 240v, however with the addition of a small variac on the buck transformer's primary, you can adjust it to accommodate whatever the line voltage is (up to the limit of the transformer).  The use of a variac also gives you a wider choice of filament transformers that can be used.  

The key requirement of the transformer is that the AC amp rating of the secondary (filament) side exceeds the maximum  current your amplifier draws.  The volt-amp rating of the transformer and variac will be relatively small (12 volts at 15 amps is 180 volt-amps).  

I've done this in boost mode, but I can't see any reason it shouldn't work in buck mode, as long as current and volt-amp ratings are not exceeded.

Incidentally,  high line voltage can not only damage or destroy your filter capacitors, it can also greatly shorten the useful life of your 3-500 filaments.  

If you decide to try this, be safe!!  You are working with a circuit that can produce a potential of 250 volts at 15 or more amps - that's 4000+ watts - a lot of power!  Enclose the circuit in a safe housing with appropriate fusing and grounding (and metering if you desire).  If you are inexperienced with these levels AC power, seek professional assistance before attempting to do this.

Good luck & 73!

       - Frank (still waiting for those sun spots)
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KA2UUP
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Posts: 388




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« Reply #4 on: March 30, 2009, 01:40:10 PM »

Take alook at the manual at http://www.ameritron.com/man/pdf/AL-82.pdf.  There are multiple taps that can be used.  However, seems that the 245V is the highest you can go.

I hope this helps!

Bert @ KA2UUP.
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KA2UUP
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« Reply #5 on: March 30, 2009, 01:41:49 PM »

Also, W8JI has some good information.  He designed some of these amps and can help you out.  Suggest sending him an e-mail.

Bert @ KA2UUP
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K1FPV
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Posts: 33




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« Reply #6 on: March 30, 2009, 01:45:42 PM »

No, the online manual must be wrong. The no-load voltage is supposed to be 3600 volts which is the maximum rating of the capacitor string. I sure hope the bleeder resistors across each cap is right on the button or there will be a big bang when one cap goes!

I checked the voltage with my VOM which goes to 5,000 volts and the onboard voltmeter is right on the nose. I'm uneasy with this voltage. I guess the years of broadcasting work, I never saw caps in a circuit used so close to their maximum rating!

Unfortunately, the transformer and amplifier will only work on 240 volts. There is no double winding to allow for 120 VAC operation. Looks like I'm up the creek without the proverbial paddle!  

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K1FPV
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Posts: 33




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« Reply #7 on: March 30, 2009, 01:48:43 PM »

The unfortunate thing is, those taps are for fan speed and filament voltage. Nothing for the high voltage. The schematic that Ameritron has online and for that matter, with the amp leaves much to be desired.
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W8JI
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« Reply #8 on: March 31, 2009, 02:08:56 AM »

The plate transformer has two taps, 220 and 240. 3800 volts will not hurt anything with that supply since the caps are specified to be aged and tested for leakage by the manufacturer at 525 volts each.

The capacitors are also directly in the air inlet and have very good airflow, so they never even come remotely close to the temperature limits even with added dissipation from higher voltage.

I run and have ran an AL1200 here since 1985 with 4000 volts, although that is certainly not recommended.

Moving the amp to 120 volts would not help a thing. The 120 comes from half your 240, so the net result is exactly the same except for much poorer full-load regulation and having much higher line currents to deal with.

If it was my amp, I'd run it as you have it. Historically there are very few power supply problems, less than 2% of all failures, so the power supply obviously is not a soft spot. The most common failures are from bad tubes. The most likely failures caused by excessive voltage are tuning capacitor arcing or bandswitch failures if you under-load the amp. It is unlikely you will have any problem.
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K1FPV
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Posts: 33




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« Reply #9 on: March 31, 2009, 08:18:03 AM »

Thanks Tom for the information! I wasn't aware of the aging and testing of the caps to the 525 volt spec you mentioned. That makes me feel a little better so as not to worry about the caps failing. That was my main worry. I guess from what I've seen, in a nutshell, the AL-82 is the same basic chassis as the AL-1200 and AL-1500 with the main differences being the different finals and filament supply for those other tubes.

It is a nice amp! My amp will only work on 240 volts......as it doesn't have a 120 volt winding. The AC line coming in is always rock solid as the transformer on the pole is in front of my house. I can usually get 1500 watts out of the amp with about 85 to 90 watts of drive into it. Usually the B+ on the tubes doesn't go below +3,450 volts DC which gives the power supply terrific regulation.

Again, thanks for the input and thank you to all the other guys who replied. Smiley  

73,
Bill
K1FPV
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HFRF
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« Reply #10 on: April 01, 2009, 06:11:25 AM »

A few years ago I picked up a variac off ebay for $100.  It was a strange one with dual 120 vac cores which can be paralleled or series at 30/15 amps. I over load it by at least 50% and it doesn't even get warm. Best $100 I ever spent.  240 vac high current variacs sell for less money on ebay since they are less desireable than 120 vac models.  I have seen the 240vac variacs for about 1/2 of the 120's with the same current.
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