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Author Topic: AL-811H HV  (Read 4865 times)
N0SQ
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Posts: 53




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« on: May 15, 2012, 11:25:18 AM »

Since restrapping my amp for 240VAC my HV has been as high as 1850V. Is there a way to strap the amp to get the voltage down to the nominal 1.7 KV? It was around 1.7 KV when strapped for 120 VAC. I could go back to 120 VAC but since I went to the time and expense to install 240 VAC to the shack I'd like to use it. Or is 1850V HV a non-issue? I suppose another option would be to replace the 811A's with 572B's.
« Last Edit: May 15, 2012, 11:27:35 AM by N0SQ » Logged
K0CWO
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Posts: 419




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« Reply #1 on: May 15, 2012, 11:47:33 AM »

Measure your line voltage with a volt meter and then read page 4 of your owners manual to determine the correct buck boost connections.

Here is what page 4 says about max HV:

NEVER REWIRE THE POWER SUPPLY TO BOOST THE
HIGH VOLTAGE ABOVE 1800 VOLTS. ALSO, DO NOT
ATTEMPT TO REWIRE THIS AMPLIFIER WHILE IT IS
CONNECTED TO POWER.
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WX7G
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Posts: 6220




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« Reply #2 on: May 15, 2012, 01:54:04 PM »

The weak link is the use of four 450 volt electrolytic capacitors. At 1800 volts each cap has 450 volts assuming the bleeder resistors are of equal value and the capacitor leakage current is not very high.

So, 1850 volts is too high. If the AC line voltage is 240 VAC why would the HV measure too high?

1) AC line peak-to-RMS voltage deviates from a sinewave
2) The HV transformer ratio is incorrect
3) The AL-811 HV metering has an error causing it to read high

Number 3 is the most probable followed by 1. You can measure the AC line voltage with an oscilloscope but since most oscilloscopes are specified to 3%, and your 1850 VDC reading is 3% above 1800 volts, you cannot count on the oscilloscope measurement. It would need to have a transfer cal from a DC source (about 350 VDC) measured with a DVM accurate to better than 1%.

What would I do? I'd measure the line voltage with a decent DMM and if it measures 240 VAC or less I would assume the AL-811 metering is off a bit and not worry about it.
« Last Edit: May 15, 2012, 02:58:40 PM by WX7G » Logged
KE3WD
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Posts: 5689




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« Reply #3 on: May 16, 2012, 08:16:35 AM »

The weak link is the use of four 450 volt electrolytic capacitors. At 1800 volts each cap has 450 volts assuming the bleeder resistors are of equal value and the capacitor leakage current is not very high.

So, 1850 volts is too high. If the AC line voltage is 240 VAC why would the HV measure too high?

1) AC line peak-to-RMS voltage deviates from a sinewave
2) The HV transformer ratio is incorrect
3) The AL-811 HV metering has an error causing it to read high

Number 3 is the most probable followed by 1. You can measure the AC line voltage with an oscilloscope but since most oscilloscopes are specified to 3%, and your 1850 VDC reading is 3% above 1800 volts, you cannot count on the oscilloscope measurement. It would need to have a transfer cal from a DC source (about 350 VDC) measured with a DVM accurate to better than 1%.

What would I do? I'd measure the line voltage with a decent DMM and if it measures 240 VAC or less I would assume the AL-811 metering is off a bit and not worry about it.

This is good advice IMHO. 

73
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N0SQ
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Posts: 53




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« Reply #4 on: May 16, 2012, 05:01:53 PM »

Measure your line voltage with a volt meter and then read page 4 of your owners manual to determine the correct buck boost connections.

I measured 246 VAC. The amplifier is strapped for 240 VAC.
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WX7G
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Posts: 6220




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« Reply #5 on: May 16, 2012, 05:13:29 PM »

It looks like you need a bucking transformer to reduce the voltage.

The Triad VPS20-8800 ($40.79 at Digikey and in stock) can be configured to transform 230 VAC to 10.0 VAC at 17.6 A. This is more than large enough for your AL-811H.

With 246 VAC in it will provide a bucking voltage of 10.7 volts (actually more due to this being a bit of a "soft" transformer). The 246 VAC will be reduced to 235 volts and the AL-811H HV will drop from 1850 to 1770 V. This is calculated for an ideal transformer. 

I would place a 1 amp SLO-BLO fuse in series with the transformer input.
« Last Edit: May 16, 2012, 05:22:17 PM by WX7G » Logged
KJ4WS
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Posts: 10




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« Reply #6 on: May 17, 2012, 02:15:54 AM »

There is another thought! The divider resisitors in the metering circuit may have changed value which would give you an incorrect reading of Plate Voltage. How old is your Amp? You could try changing them with 1% tolarance resistors before spending Buckets O Money. Then look at the Plate Voltage. Changing to 572b tubes would insure 2 things. 1 Your voltage problem would be cured. 2. You could pretty much guarantee that the tubes would last a lifetime or better. I have worked with tubes since 1971 when most TV Sets had those glass things that glow and get hot and IMHO 50 volts over the 1800 should not cause a problem where the tubes are concerned. The rating on the power Supply Capacitors would worry me a bit. There are 500 volt Caps out there that would raise the Max Voltage to 2000 volts. If,your amp has some age on it the replacement of the Power Supply Caps would be a good thing to do.
IMPORTANT >>>>>>>>>>> The voltages in your amp can KILL YOU. Don't just unplug the amp before service. Tape the end of the power cord up so there is NO WAY it can be plugged in. In the past we used a device called a Chicken Stick to assure that the Power Supply Capacitors were discharged. A long screw driver makes a good Chicken Stick. Short between the Tube Plate Caps to the Chassis and then again from the Power Supply to Chassis. Always make sure that the power supply has been discharged. I own a Chicken Stick ( screw driver ) that has many scars from failed Bleeder Resistors and the power supply was still HOT with voltage. IMPORTANT>>>>>>>>> Be Very Careful >>>>>>>>>>
I can assure you that there is NO Shame in using a Chicken Stick, you can be positive that without them I would not be writing this message ( most likely )
                                                                                        73 de Wade/KJ4WS ex KE4UTQ
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K8AXW
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Posts: 3965




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« Reply #7 on: May 17, 2012, 09:08:45 AM »

4WS:  Wade, no offense meant, but a screwdrivers isn't a chicken stick!  According to everything I've read for decades shorting HV to ground with a screwdriver or similar device can potentially overstress components in the power supply.

Although you may have been doing this for years I think it's worth noting that this is not a good practice.

A "chicken stick" is a thin piece of wood with a piece of at least #16 stranded wire fastened to one end. (Read flexible here) The stick end of the wire has a series resistor of around 100K ohm, 2W connected to it.  The other end of the resistor lead is bent into a hook. The other end of the wire has an alligator clip that attaches to the chassis ground.

To use it you first ground the alligator clip and then holding onto the stick furthest from the wire, hook the resistor over a HV lead.  It take several seconds to bleed off the HV.  Then it's a good idea to use the screwdriver procedure.

Wade, this is directed to those who read these forums and are not familiar with the term "chicken stick."

If I would have shorted the transmitter HV to ground to bleed filter capacitors at the radio station where I worked, I would have been fired.
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K4RVN
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Posts: 794




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« Reply #8 on: May 17, 2012, 01:11:20 PM »

Good post there axw. There are no agreements on the value of the resistor to use, but it is important from what I have read. Here is a link to a photo of a chicken stick which appeared in QST 2003 in an article about a Heathkit amp. It shows no resistor but a 10 ohm 20 watt was suggested to be added to the shorting wire. The resistor should protect the transformer and components that a screwdriver will not. I like a 2 watt resistor better because I have more of those in the junk box. I have never blown one as I wait some time after unplugging the amp and my bleed resitors have not failed.
Scroll down if anyone is  interested to see a chicken stick, but there are many versions. http://www.w6kan.com/sb200.htm

Frank
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N0SQ
Member

Posts: 53




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« Reply #9 on: May 18, 2012, 10:08:15 AM »

It looks like you need a bucking transformer to reduce the voltage.

The Triad VPS20-8800 ($40.79 at Digikey and in stock) can be configured to transform 230 VAC to 10.0 VAC at 17.6 A. This is more than large enough for your AL-811H.

With 246 VAC in it will provide a bucking voltage of 10.7 volts (actually more due to this being a bit of a "soft" transformer). The 246 VAC will be reduced to 235 volts and the AL-811H HV will drop from 1850 to 1770 V. This is calculated for an ideal transformer. 

I would place a 1 amp SLO-BLO fuse in series with the transformer input.

I'm assuming that this transformer would replace the existing transformer so I'm wondering why I would need an additional fuse for the new transformer.
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WX7G
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Posts: 6220




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« Reply #10 on: May 18, 2012, 02:51:25 PM »

The bucking transformer would be placed external to the amp in a metal box. It reduces 246 VAC to somewhere around 240 VAC. I would post a schematic here if eham had such a feature. Here is a schematic that might work. The transformer 240 volt winding is connected across the 240 VAC line. The two fuses use the ~ symbol. The transformer 10 volt winding is connected in series with the AC LINE in a direction such that it bucks, or subtracts from the line voltage. Hooked up the other way it would add to the line voltage.

The fuse in series with the 10 volt winding is the same type and amperage that the AL-811 uses or 10 amps. The fuse in series with the 240 volt winding is a 1 amp SLO-BLO. The transformer is now protected from over current. And the wiring to the bucking box is protected if the transformer fails shorted. It can be used without an ON/OFF switch and it will probably burn just a few watts when the AL-811 is not drawing power. The metal box should be connnected to the AC power GREEN wire (GND).



                   10V winding
LINE-------------~----------mmm--------AMP
            ~          
            |
            |
            )
            ) 240 V winding
LINE---)---------------------------------------AMP
« Last Edit: May 18, 2012, 04:58:20 PM by WX7G » Logged
N0SQ
Member

Posts: 53




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« Reply #11 on: May 18, 2012, 03:59:43 PM »

The bucking transformer would be placed external to the amp in a metal box. It reduces 264 VAC to somewhere around 240 VAC. I would post a schematic here if eham had such a feature. Here is a schematic that might work. The transformer 240 volt winding is connected across the 240 VAC line. The two fuses use the ~ symbol. The transformer 10 volt winding is connected in series with the AC LINE in a direction such that it bucks, or subtracts from the line voltage. Hooked up the other way it would add to the line voltage.

The fuse in series with the 10 volt winding is the same type and amperage that the AL-811 uses or 10 amps. The fuse in series with the 240 volt winding is a 1 amp SLO-BLO. The transformer is now protected from over current. And the wiring to the bucking box is protected if the transformer fails shorted. It can be used without an ON/OFF switch and it will probably burn just a few watts when the AL-811 is not drawing power. The metal box should be connnected to the AC power GREEN wire (GND).



                   10V winding
LINE-------------~----------mmm--------AMP
            ~          
            |
            |
            )
            ) 240 V winding
            )
NEUT---------------------------------------AMP

I think that you're missing something on the schematic that you drew. 240 VAC is picked up across Line1 (L1) and Line2 (L2) - not across Line and Neutral. That said, shouldn't there be a buck winding on both L1 and L2 where identical voltage would be dropped on each leg?

And I did a little research on buck and boost transformers. It seems that a buck/boost transformer is ideal since there might be substantial swings in voltage if my wife is running the dryer and/or oven while I'm operating. I didn't look at cost, though, but it would probably be more expensive for a buck/boost transformer.
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WX7G
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Posts: 6220




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« Reply #12 on: May 18, 2012, 05:02:03 PM »

You are correct that I should have labeled both as LINE.

The bucking voltage can be in one leg only. The Amp cares only about the differential voltage as NEUTRAL is not used.
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K4RVN
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Posts: 794




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« Reply #13 on: May 18, 2012, 08:21:12 PM »

If you need a little more info here is a link which explais how it works and how to wire it. http://sound.westhost.com/articles/buck-xfmr.htm#s30

Frank
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WX7G
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Posts: 6220




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« Reply #14 on: May 19, 2012, 01:53:53 AM »

Frank thanks for the great link.

I like the circuit in Figure 4. It reduces the voltage across the transformer 240 VAC primary thereby helping to ensure that the transformer doesn't saturate from the application of 246 VAC.
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