Ameritron AL-84 GI-7B modification

Discussion in 'Amateur Radio Amplifiers' started by VE2GCE, Oct 1, 2019.

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  1. VE2GCE

    VE2GCE Ham Member QRZ Page

    I recently bought a Ameritron AL-84 modified for the GI-7B tube.
    I was told that the tube was weak and it needed to be replaced.

    How much power can I expect from a new GI-7B ?
    The original 4 x 6MJ6 were good for 400 watts.
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    How high is the plate voltage?

    With a puny power supply that puts out around 1500 VDC under load, I get over 850-watts out of my home-brew 160-meter amplifier that has a pair of the GI-7bT tubes in it. With a good power supply, like putting out 2200 VDC, I expect easily over a kW. That would mean over 500-watts per tube instead of the like 425-watts per tube that I am getting now.

    The GI-7bT (about the same price as the GI-7b but has a larger ceramic body - same electrical specifications), is available all over eBay from like the Ukraine (very good sellers, I have bought both GI-7bT and GS-31 tubes from several without any problems), Russia, etc.

    Glen, K9STH
     
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  3. VE2GCE

    VE2GCE Ham Member QRZ Page

    Glen, thanks for the feedback.

    There is about 1300 volts (no load) on the plate, so I figure about 1100 volts under load in this amplifier.

    I also bought audio tubes from sellers in Lithuania, Ukraine and Russia with no issues on my end.
    There is some good prices ranging from 15$ (no heatsink) to $35 (with heatsink) from Eastern European sellers on Ebay.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Just a guess, but unless someone modified that AL-84 power supply with a different transformer, it never delivered 1300V. I owned one "brand new" back in about 1980 or so and HV was about 1000V, and about 850-875V under load. Can the filter capacitor string even withstand 1300V? I don't remember, would have to look that one up.

    The GI-7B is a serious tube and if it's "weak," either someone beat the hell out of it or possibly it was defective from the start.

    Most users of that tube report they're very, very good and last forever assuming they're properly cooled. Russkies made a lot of really good tubes.
     
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  5. MM0IMC

    MM0IMC Ham Member QRZ Page

    Some people are Russiophobic when it comes to Soviet era valves (tubes), unfortunately.

    I've got a small collection of various ones I bought from Ukraine. I'm gathering parts to homebrew my own amp based around these lovely pieces of kit.
     
  6. W1GCI

    W1GCI Ham Member QRZ Page

    It seems that the AL-84 is similar to the Dentron GLA-1000 in many regards. W4EMF did a retrofit to the GLA1000 described here - http://w4emf.gs35b.com/GLA-1000-retro-GI7/ .
    The key is to change the power supply over to a voltage doubler, which gives you about 2200 V. I've built a few GI-7B amps, both one and two tube versions with good results. The one tube amps put out about 450W and the two tube versions, as expected are about 900W. I've newer run lab style IMD tests on these - there are those who predict that IMD will not be great. But, I expect the IMD will exceed that of four sweep tubes. Also, I noticed that the AL-84 does not have input filtering. You might comment if that's been added to yours. On a one tube GI-7B I built, I placed a 3 db power pad (50W) at the input to reduce the drive power and also provide a resistive 50 ohms to the driver. The GI-7B can be easily over driven by a 100 Watt transmitter, which will lead to bad distortion. I like the tube because it's cheap. :)
     
  7. VE2GCE

    VE2GCE Ham Member QRZ Page

    WB2WIK,
    I am going by the front panel voltmeter. It says 1300 volts, but I will confirm with a real voltmeter.
    My line voltage is 127 volts , so it is possibly a correct reading (1300 volts).

    The power supply has 3 x 450 volt capacitor totem pole, so that gives us 1350 volts.
    The previous owner, told me the was used by a CB'er that did not bother to match the amplifier to the antenna (i.e. high SWR).

    W1GCI,
    It does have a 50 ohm 50 watt non-inductive resistor on the input.
    I remember my Elmer telling me that using VHF /UHF tubes (like the GI-7B) in HF applications results in a higher IMD.
    This would have to be confirmed with actual measurements.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I just checked the manual and it states clearly on page 3 the unloaded HV should be "1000V." I remember under load it's in the 850-900V range (full power).

    If your meter is reading 1300V I suspect the metering resistor R6 has drifted in value and should be replaced. They used a 1.5 Meg, 2W resistor there and such resistors aren't actually rated for 1000V (normal rating is 300V), so it's better to use two 750K 1W resistors in series to avoid overstressing the resistor itself. I suspect if you remove R6 and measure it with a good Ohmmeter you'll find it's pretty far out of tolerance.
     
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  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The GI-70b is the same as the GI-7b except that it doesn't have the plate heat sink attached. The heat sink is just attached with a machine screw and it can be removed from a GI-7b or GI-7bT and put on the GI-70b.

    You do need to get the plate voltage higher. Otherwise, you won't get all that much power out of the tube.

    Glen, K9STH
     
  10. VE2GCE

    VE2GCE Ham Member QRZ Page

    Glen,

    You were right. I was going by the front panel meter which read about 1200-1300.
    When I opened up the unit, powered it up and measured the plate with a high voltage probe, it read 2000 volts.
    So the bridge rectifier was converted to a full wave voltage doubler along with the GI7B modification.
     

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