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Author Topic: Ameritron AL-811H - Plate Voltage Drop  (Read 912 times)
VE1XOP
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Posts: 4




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« on: July 01, 2019, 11:53:17 AM »

When the amp is freshly turned on, the plate voltage on the front of the amp reads 1600v.

No power input and amp keys on a dummy load. 100V drop. Not sure if that is ok.

20 watt CW carrier applied - voltage drops to 1400v.

So I am guessing... transformer issue or something else? As more carrier is applied, the voltage continues to drop.

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K6BSU
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Posts: 39




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« Reply #1 on: July 01, 2019, 12:09:13 PM »

Looks like you should look at the HV for ripple.  Normally, you have to build a resistor network to reduce the voltage input to a 'scope.  Then, look for ripple.  Sounds like your HV filter capacitor bank is going bad, with the ripple making average DC lower.  The transformer used in all of the 811-type amps is the same one.  With four 811 tubes, loading on the transformer is maximum, and (in my opinion) more than the trans can handle.
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K6AER
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« Reply #2 on: July 01, 2019, 12:11:55 PM »

What is your AC supplly voltage at the wall plug?
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VE1XOP
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Posts: 4




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« Reply #3 on: July 01, 2019, 01:18:18 PM »

I checked the voltage and also conf that the wall outlet of 120V is working as it should.

I will check for the ripple, but I am also guessing the power transformer is maybe having it's days. I got it used, and only have tested it recently on a dummy load.

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AD5X
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« Reply #4 on: July 01, 2019, 01:33:14 PM »

Is the 120VAC wall outlet constant when the amp is keyed, and when driven? I had a 10VAC drop when keying my ALS-600 when running it on 120VAC. I've since converted to 240VAC and that solved my problems.


Phil - AD5X
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VE1XOP
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« Reply #5 on: July 01, 2019, 01:45:04 PM »

240 Would be nice - but not possible. I wish.

I will make some double checks on the source voltages and such, good points.
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N8FVJ
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Posts: 867




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« Reply #6 on: July 01, 2019, 03:13:26 PM »

A HV diode in one leg of diode bridge rectifier is open (bad).
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K7RJB
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Posts: 185




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« Reply #7 on: July 01, 2019, 04:29:20 PM »

It is normal for the plate voltage to dip some.

A dedicated 220v circuit will take some load off the transformer.
and fresh caps in the power supply is about all you can do unless the transformer is failing.

I think you can increase the MFD (to a point) of the caps to a point but a softstart should be used.
I raised the MFD to 330 in my SB-220, I've heard tales of others going to 450's.

The manual shows a hundred volts more in the spec sheet but that may be optimistic.
1700v no load 1500v full output (@ 700ma.) ~15% of 1500 is 1275v, so your 1400v is in spec.
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KM3F
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« Reply #8 on: July 01, 2019, 10:39:24 PM »

You have to test and see where any possible issue may be.
120 volt line source may be soft from a long run, AC may be down, other loads in the house.
In tune, the plate voltage will be at max drop with a steady drive signal.
When on SSB mode the power supply assuming it is in top shape won't drop that far if the filters are good and won't lose there full charge on voice peaks.
Powered from 120 draws 2 times the AC line current therefore losses will be higher than at 240 volt line supply.
Test line voltage with amp in stand-by then at full tune.
If you see a large drop there, that's the first cause. You can even test piolet lamp supply voltage as an indicator that would be off the secondary of the transformer, but do it in a safe manner with the cover off.
Filters and rectifiers won't fix it unless they are causing to much load/voltage drop  on the power supply.
There could be multiple issues at the same time with a pre owned amplifier.
If you re-cap the power supply, a somewhat larger capacitor value will hold the plate voltage up better (harden the supply) on voice peaks, but won't show it in tune position. Don't let that fool you, it would still be better from a voice peak stand point.
Were talking about a discharge time constant function.
The power supply charges the filter network 120 times per second assuming a bridge rectifier source. On SSB, your voice peaks are slower than that, hence an improvement if the caps are increased in value some amount but don't overdo it.  Beyond a reasonable point it become harder on the power switch, transformer and rectifiers at initial power up to charge the extra capacity. A time delay circuit is normally used to slow this initial current surge.
Good luck.
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N8FVJ
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« Reply #9 on: July 02, 2019, 03:52:47 AM »

My AL-811 (same power transformer as AL-811H) uses three Cetron 572B tubes. I have a (wimpy) 15 amp rated circuit at 118 volts (measured). The amp idles at 1700 volts. Keyed up at no signal input the voltage drops to 1630 volts. On 75 meters it drops to 1500 volts at 800 watts PEP out. It drops to 1450 volts at 1000 watts PEP out on 75 meters. Three Cetron 572B can easily make 1000 watts PEP out.
A normal AL-811 or AL811H should never drop below 1500 volts at 800 watts PEP out using 118 volt @ 15 amp AC line and 800 watts PEP is beyond what four 811A tubes can reasonably produce on 75 meters (less on 10 meters) and have a long life.
« Last Edit: July 02, 2019, 03:57:15 AM by N8FVJ » Logged
N8FVJ
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Posts: 867




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« Reply #10 on: July 02, 2019, 04:19:10 AM »

My AL-811 (same power transformer as AL-811H) uses three Cetron 572B tubes. I have a (wimpy) 15 amp rated circuit at 118 volts (measured). The amp idles at 1700 volts. Keyed up at no signal input the voltage drops to 1630 volts. On 75 meters it drops to 1500 volts at 800 watts PEP out. It drops to 1450 volts at 1000 watts PEP out on 75 meters. Three Cetron 572B can easily make 1000 watts PEP out.
A normal AL-811 or AL811H should never drop below 1500 volts at 800 watts PEP out using 118 volt @ 15 amp AC line and 800 watts PEP is beyond what four 811A tubes can reasonably produce on 75 meters (less on 10 meters) and have a long life.
I should have reminded the original poster the amp acts like it is 1/2 wave (not full wave) at the bridge rectifier indicating a diode either opened up or has a bad or missing the solder joint on one diode.
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KM1H
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Posts: 5106




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« Reply #11 on: July 02, 2019, 06:31:51 AM »

Collins rated the 30L1 with 4 RCA 811A's at 500W and they had a long life when tuned fast. The Chinese 811's, not real A versions, have less plate dissipation and should not be run over 600W in the AL-811H in SSB or CW if long term life is important to the user.  Less in other modes.

Quote
A HV diode in one leg of diode bridge rectifier is open (bad).

There is nothing wrong with the PS from those reported indications. The voltage drops are due to the resistance (I squared R losses from entry level math) in the AC line from the main panel and the transformer drop is normal.

An "economy" amp will have around 12% (sometimes worse) regulation of the DC from no to full load. Dynamic regulation will be better on SSB due to the energy storage effect of the filter caps.

From the manual:
Power Supply
Circuit type: full wave bridge
Full load current: 550mA
Regulation: 12%
Maximum draw at rated output: 8A

Carl
« Last Edit: July 02, 2019, 06:48:14 AM by KM1H » Logged
KM1H
Member

Posts: 5106




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« Reply #12 on: July 02, 2019, 06:47:07 AM »

It is normal for the plate voltage to dip some.

A dedicated 220v circuit will take some load off the transformer.
and fresh caps in the power supply is about all you can do unless the transformer is failing.

I think you can increase the MFD (to a point) of the caps to a point but a softstart should be used.
I raised the MFD to 330 in my SB-220, I've heard tales of others going to 450's.

The manual shows a hundred volts more in the spec sheet but that may be optimistic.
1700v no load 1500v full output (@ 700ma.) ~15% of 1500 is 1275v, so your 1400v is in spec.

All that increasing the output C will accomplish is to place more of a surge load on the power switch. The total of 52 uF is much more than even most high end amps and AC measurements are highly dependent upon the meter used....most do not display true RMS readings and the meter circuit in the amp has its own accuracy tolerances.

Anything over 330 uF in a SB-220 is asking for switch failures and considering the switch is unobtainium a step start should be considered or the input redesigned for a readily available switch.

Carl
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N8FVJ
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Posts: 867




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« Reply #13 on: July 03, 2019, 05:22:55 AM »

Quote
There is nothing wrong with the PS from those reported indications. The voltage drops are due to the resistance (I squared R losses from entry level math) in the AC line from the main panel and the transformer drop is normal.
Carl

Really, nothing wrong? 20 watt carrier input to amp will have a voltage drop not less than 1600 volts on the weakest 15 amp @ 120 volt AC line. 600 watts PEP out never has less than 1500 volts on the plates. 800 watts PEP out briefly dips to 1450 volts. I own the AL-811 amp.
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KM1H
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Posts: 5106




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« Reply #14 on: July 03, 2019, 11:04:07 AM »

Quote
There is nothing wrong with the PS from those reported indications. The voltage drops are due to the resistance (I squared R losses from entry level math) in the AC line from the main panel and the transformer drop is normal.
Carl

Really, nothing wrong? 20 watt carrier input to amp will have a voltage drop not less than 1600 volts on the weakest 15 amp @ 120 volt AC line. 600 watts PEP out never has less than 1500 volts on the plates. 800 watts PEP out briefly dips to 1450 volts. I own the AL-811 amp.

Your comments make no sense. No further reply needed until the OP returns; the major unanswered question is what the RMS AC voltage is AT THE AMP during the various steps
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