Another Ameritron AL-811H Repair

Discussion in 'Amateur Radio Amplifiers' started by G3YRO, Oct 17, 2020.

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    Yet another AL-811H where one of the Valves has flashed over on switch-on.

    Fortunately it didn't take out the Relay this time (which is a real pain to replace) . . . but it did burn out the Grid Resistor and one of the MOVs on the filament circuit.

    I can't help thinking that the 811As simply can't take that high an HT voltage.

    (I tell everyone with one of these Amps to fit a pair of 572Bs instead)

    Roger G3YRO
  2. W1QJ

    W1QJ Ham Member QRZ Page

    I will say this again, the Al-811 and Al-811H both have 4 filter capacitors. The value is 270uf. If you do the math, the total filter C is 67.5uf. This is TWO TIMES the needed and recommended value of total filter C. Once you hit a total of 33uf there is a law of diminishing returns as far as what added capacitance will do for you. However, ANY added capacitance beyond what is really necessary just stores more energy. The extra stored energy is never really used and is just excess EXCEPT in a fault like when a tube arcs. When a tube arcs it will pull that extra energy out of the supply and all it does is support the arc in a vengeance. WIth 4 filter caps, the actual capacitance of each cap should ideally be around 130uf each. Using appx. 130uf you will have a total of around 32.5uf which is the ideal value. The next larger and readily available capacitor is 150uf. When I get these amps in for repair I urge the customer to have me replace them with 150uf caps. I have done this quite a few times and I have conducted BEFORE and AFTER tests. Tests show that there is no discernable difference in amplifier performance. I was not able to see any power output difference whatsoever. That being said, in a fault there is much less energy that can be released in a destructive manner.
    W2AI, K4AGO, W4NNF and 5 others like this.
  3. AE1N

    AE1N Ham Member QRZ Page

    "A couple of years ago I replaced all 4 811A tubes in my 811H with Taylor 572Bs. I was replacing the 811 tubes every year as they went bad. The 572Bs DO NOT produce increased output, their output is limited by the relatively low plate voltage in this amplifier. But they hold up much better. I recommend the conversion, it has worked very well for me. de KC9PFH"

    "My 4 572Bs are now 30 years old and fresh like the first day i used it."
    K1LKP likes this.
  4. KE0NSK

    KE0NSK Ham Member QRZ Page

    Does the W8JI grounded grid mod reduce the carnage after a tube failure?
    K1LKP likes this.
  5. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page




    73 - K1LKP
  6. W1QJ

    W1QJ Ham Member QRZ Page

    well maybe in the most extreme of faults, but what does the damage is extra power supply energy in the massive capacity of capacitance. Scroll up and read my post about that.
  7. K2XT

    K2XT Premium Subscriber QRZ Page

    When the valve flashes over it is a short circuit for a few milliseconds (microseconds?) as the caps discharge (as W1QJ points out).
    So, we want to minimize the current during this event, right? To keep from blowing components all over inside the amp.
    Louie says to be reasonable and limit the amount of energy in the caps. Good thinking.
    How about having some resistance in the B+ path, called a glitch resistor. It is a technique that has been used for probably close to 100 years. Does the AL811 have one? If not, why not install one? A few bucks; even fewer pounds.
  8. W1QJ

    W1QJ Ham Member QRZ Page

    Of course a glitch resistor is a great idea! Let me speak to that idea. Looking back at older amps that used 811 and 572b tubes all we need to do is look at Dentron amps. They actually did have glitch resistors. All they needed were 2 watt low value carbon resistors in the B+ line. No Dentron amp used more than about 28uf of total filter capacitance. Being around the ideal value needed without going over, even if a tube arced over the 28uf supply had the bare minimum energy stored up and a simple 2 watt carbon resistor was able to handle the momentary surge as the energy died down from the capacitors. Many tube arcs went unnoticed. The only thing is that the basic 2 watt resistor was not rated to a few thousand volts, but them seemed to have lasted to this day. The less energy the glitch resistor has to deal with the less "beefy" the glitch resistor has to be. THE WORST resistor to use for a glitch resistor is a "sand or cement" type. They have VERY POOR surge handling capability!! They are even terrible for step starting. Those type of resistors can't be beat for heat, so they are more suited fo things like slowing down a fan speed. In fact an old carbon type resistor can handle tremendous surges. The glitch resistor should hold together long enough for the line fuse to blow, before it does. To properly protect the 811 and 811H amps the MOV's have to go, ground the grids and add the gast tubes (as suggested by the designer) reduce the PS filter capacitance to no more than 33uf total and add a glitch resistor.
    WA6MHZ and K2XT like this.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The voltage rating normally doesn't matter as there's no voltage drop across the resistor in normal operation. If an arc occurs on the load side, the resistor sacrifices itself and you lose a cheap resistor that's easy to replace.

    Type BWH resistors are still available, and was the choice for low-R glitch resistors for many years. The 0.82 Ohm series resistor in lots of amps is almost always a BWH. In the old Drake amps, it's inside the power supply and not the RF deck.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    I appreciate the comments about the Filter capacitors . . .

    But WHY do the Valves arc over in the first place?

    I have worked on loads of FL2100s . . . and NEVER known the 572Bs arc over.

    It's not just the damage an arced Valve does to other components (which are a pain to replace in these Amps, due their poor physical design) . . . it's the fact that you need to buy a new Valve too !

    I see that the 811A is rated at 1200V . . . 1500V max . . . yet these Amps have 1700V on the Anodes.

    Roger G3YRO

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