Al 1500 with low output and few more questions.

Discussion in 'Amateur Radio Amplifiers' started by E75MC, May 14, 2020.

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  1. E75MC

    E75MC Ham Member QRZ Page

    Got it! Will tell you the results. Many thanks.
  2. E75MC

    E75MC Ham Member QRZ Page

    So here are the results: D17 was shorted off course. Replaced with the same type 1N4007 and now things are so better.

    14.020 MHz 30W in-500W OUT- 10mA grid current ,
    50W in-850W OUT- 15mA grid current,
    70W in-1200W OUT-25mA grid current,
    Plate current goes more that 750mA when OUT is about 1kW!

    On 21.02 MHz 45W in-1500W OUT- 30mA grid current
    On 28.5 MHz 50W in-1600W OUT-25mA grid current

    As I said, I dont have big dummy load to get precise results so I am very happy with this.

    So, since we are here anyway, can you tell me please few more things. I want to add those caps to get 4kV. I dont have 2 pcs of exact type of 210uF/450V but I have these at my desk:
    -330uF/200V x4
    -470uF/200V x2

    -If my math is correct I can combine these various values, since voltages are adding up. For example I can place those 4 of 330uF or these 2: 220uF/400V and 330uF/400V.
    -I also have found somewhere that another capacitor of 470pF ( as I can remember) should be placed somewhere ( around plate choke as I can remember)
    -Also there was posts about increasing warm-up time to about 5-6 minutes! That was something about changing resistor value on the board.

    If You can and when able, those information would be very good.

    Many many thanks !
    N0RU and K0UO like this.
  3. VE7DQ

    VE7DQ Ham Member QRZ Page

    If I read your post correctly, raising the plate voltage is not as easy as changing or adding capacitors. They have nothing to do with the voltage output of the power supply; they only smooth the output of the transformer and rectifier assembly*.

    *basically. ( for those that are gonna pick nits... );)

    The High Voltage tran$former must be changed to one that produces the higher voltage, and probably diodes added to the rectifier stack... then add more or higher-voltage capacitors. Then the plate tuning capacitor becomes more of a potential issue, as well as the increased voltages and currents induced around the bandswitch and the rest of the pi-network.

    Increasing the warmup time to 5-6 minutes isn't necessary, but won't hurt anything. Others may know about adding the 470pf capacitor. I have my suspicions but don't want to misdirect you, especially when working around voltages that can KILL you. Dead.

    It can be done, but it's not practical. I suggest you enjoy the amplifier as it is. It appears to be working well. Good Luck!
  4. E75MC

    E75MC Ham Member QRZ Page


    Hi there, thanks for input. :)
    The story behind my question about adding more el-capacitors in filtering board is because their total capacitance is very close to total high voltage they work on. ( 8x450V is 3600V) . Experts say this like "they work to often on very edge". Adding two same caps in series adds another 900V on total voltage and in that case you can use "green" tap from the transformer and that allows you to have about 4kV from that transformer. Higher voltage decrease grid current even more and also gives you a bit more output power. This explanation is what I learned on forums about this modification. Regard this subject, I have also find this article:

    The story about increase of warm up time is also found on forums and claim is that 8877 data sheet says it should be about 4 minutes! Here is one of the places :

    W9BR say there:

    "I would HIGHLY advise anyone owning one of these AL-1500 amps to look at the timing circuit of their HV relay. Ameritron has it set from the factory (by a cap) somewhere around three mintues. THIS IS NOT ENOUGH TIME. The EIMAC spec sheet calls for at least four minutes. I repeat, it should go at least FOUR MINUTES. I changed the timing cap and have set mine for 6 minutes. The 8877 better come up to full temp and all those little metal particles need to get off the tube components before you hit the HV!! Trust me as I have replaced the 8877 with a new Eimac on account of the poor timing."

    I can leave the amplifier right as it is now and make no mistake, as I can see. When I have about 1kW of output is enough for me. No practical difference between 1500W and 1000W in my experience.

    73 de E74D
  5. W1QJ

    W1QJ Ham Member QRZ Page

    I’ve been using the green wire for years on my 1500 amps. That’s why Jim W7RY and myself developed the 10 capacitor board. Not only for the ability to up the voltage but just to give some head room to the caps operating exactly or over 3600 volts. The extra 400 volts makes a huge difference. In my own amps which I use on a very limited basis running the green tap hadn't proved any ill results. I added a DPDT switch to easily switch. Jim sells the boards bare for those who want to build it themselves and I sell them fully assembled and ready to install. I do not recommend mixing types, values, and manufacturers in a string of electrolytic caps. In fact I generally match the 10 caps by actual value.
    N2AMM, AG5CK, E75MC and 1 other person like this.
  6. E75MC

    E75MC Ham Member QRZ Page

    All right, many thanks again. I will leave it as it is for now. :)

    73 de E74D
  7. K2XT

    K2XT Premium Subscriber QRZ Page

    We call him The King of QRO, The QRO King, or just The King for short.
    KZ3P, E75MC, W2VW and 1 other person like this.
  8. E75MC

    E75MC Ham Member QRZ Page


    One more thing please. Since I have this amplifier opened on the bench, is there anything else that I should check?
    What do you thinkg about need to increase warm-up time to about 4 minutes? Is that needed and how can I do it ? ( was reading about changing some resistor value)

    Thank you. :)
  9. W1QJ

    W1QJ Ham Member QRZ Page

    Normally Ameritron has it set for 3 minutes. That always worked for me.
  10. E75MC

    E75MC Ham Member QRZ Page

    I measured few times with stopwatch, in my amplifier it start after 2 min and 20 sec.

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