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Re-purpose old amplifier for 10m - Help/Suggestions Needed

Discussion in 'Amateur Radio Amplifiers' started by KD9MDB, Sep 28, 2019.

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  1. KD9MDB

    KD9MDB Ham Member QRZ Page

    Hello all,

    First time posting here, getting into ham radio and I really enjoy tinkering on RF equipment (mechanical eng. by trade, but enjoy the challenge of electronics as a hobby). So I have this old amplifier from the black market CB days, an "ABC Electronics 150" that I'm wondering if it's worth using on 10m for SSB use. I converted an old 23ch SSB CB to 10m using a DDS and an arduino in place of an original crystal oscillator. So to keep with the theme of re-purposing old CB equipment, I'm thinking this old clunker amp on my shelf might make a good learning project. End goal is for a 100-200 watt amplifier.


    It is a bit rough inside to say the least. At some point the main tube was changed to an Eimac 8875 crossflow cooled tube. I can't find any information on what this thing had originally, so no idea if this swap has been done well or not. The amplifier also has a 6KV6A tube to drive the 8875. That tube may be the original in that location.

    I replaced the HV filtering capacitors and bleeder resistors so that I could at least find out the B+ level. It runs about 890 volts.

    I cleaned it up quite a bit from this photo below, but if I decide to use it, the chassis needs to be stripped and painted. the plating on the top of the chassis is toast.

    It has also seen some abuse at some point, the plate is a bit misshapen in the 6KV6A, so it's seen some overheating for sure. I worked through the circuits and checked the tubes for shorts, and not finding any, I went ahead and powered it up and used an old radio to see if it would come to life. It puts out over 100 watts after being tuned up (into dummy load of course). I have a larger wattmeter on my wish list for the near future....

    I then tested this thing with a new 6KV6A tube I had, and this thing has self oscillation issues with the new 6KV6A.....

    I also made a rough schematic, but I still need to do some disassembly to measure values of some components in the tank circuits.

    Any thoughts on whether or not this old pile is worth saving and re-configuring?? Maybe it's better off being a parts chassis....... I do find it odd that somebody put an 8875 in here, with such a low B+

    I'm not in a big hurry on this project, I still have to button up my 10m conversion and build a filter for it.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    An 8875 is certainly a weird choice, and without sufficient air flow (these were normally cooled by a pretty powerful fan using ductwork to steer the air where it needed to go) may not run much power or last long; at such low B+, the "not much power" is probably a given.

    I didn't evaluate your schematic much, but I didn't see any cathode bias source for the 8875. Looks like the cathode is just pulled to ground through a 4.7 Ohm resistor on transmit. That won't be very linear. Also looks like it's "RF actuated," so an input signal is what makes it switch from RX to TX and that's a really bad idea.

    If it works at all, I'd at least add a bias source and a method to "hard key" the relay from the transmitter so it doesn't chop in and out with a drive signal. That old "RF keyed" system can sorta work with a carrier mode like AM (or FM) but for SSB or CW it's terrible; and on any mode it means "hot switching" the relay, since RF drive will be there before the relay activates.

    But with respect to "tuning," if it tuned up on 27 MHz, it should tune up on 28 MHz fine; such amps normally didn't require any modification for that, the frequency is close enough.

    Never seen or used that particular amp, but many of these "CB" amps were not very linear and could easily cause "splatter" that will bother other users on frequencies near yours. Adding proper bias could help. The ZSAC for the 8875 is probably in the 50mA range.
  3. KD9MDB

    KD9MDB Ham Member QRZ Page

    Gotcha, I was figuring I would remove the RF keying, it does work currently with an AM carrier, but I'd rather tie it to the PTT on the radio i'll use it with or setup a foot pedal. It has the sketchy "SSB" mode that just adds a large capacitor in circuit to keep it keyed in SSB. I'll see what I can find to read up on biasing methods for the cathode.

    I did put a near field probe next to it hooked up to my spectrum analyzer. It is very good at splattering....... But the source radio isn't that great either.

    The other issue I came across is, the new 6KV6A tube i have for it, has it's manufacturing vacuum port on top, and not on bottom. So it would be too tall to slide the cover back on this thing......

    I'm wondering if I would be better off just harvesting the parts from this, building a new housing, or re-purposing a housing, to get some more real-estate. If that were the case, I wonder if it would be better to find a different main tube to replace the 8875 that makes more sense? I was looking at some 811A tubes, being pretty readily available from overseas now and a max plate voltage much closer to what I have for B+, but they require twice the filament current as the 8875, and I have no idea what the transformer I have is rated for.
  4. W2VW

    W2VW XML Subscriber QRZ Page

    Most likely no need for cathode bias at 1000 anode volts.

    The idea may have been borrowed from here:

    The 8875 is unobtanium these days and would be worth a few bux if not trashed.

    The driver tube in your amplifier is not necessary. A CB rig makes 16 peak watts on AM or 12 watts on ssb if it works properly. Plug that in to an expected gain of 16 dB with the 8875 working properly and that takes the 8875 right to the specification for plate dissipation.

    What's needed is another 1000 anode volts and then bias.

    I've seen a number of otherwise great ceramic triodes killed by hobby radio people who don't use enough anode voltage. Then they drive the pants off the poor triode trying to get the output the tube is expected to make.

    Afterwards they post on the www saying this or that tube has a "fragile grid".

    Grid current metering is absolutely necessary with these tubes. They will overdissipate when underloaded.
    KK4RSV likes this.
  5. KD9MDB

    KD9MDB Ham Member QRZ Page

    Thanks for the input, the more I think about this, I'm leaning away from using this amplifier in it's current configuration. I'm thinking that it has a few components to rob for parts to build an amplifier with a tube made from obtainium, I'm not much for unobtainium. Spending 600 or more on an 8875 just isn't feasible. I have no idea what shape the one I have is, based on how this thing looks to have been treated, it might not be too great.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I see "CB amps" sorta similar to that at the local ham swap meets all the time for about $75 or so. One of them (sold under various brand names) uses a sweep tube driving 4 sweep tubes to run 300W PEP or something, and at least the sweep tubes may be reasonably available.

    Most of those are pretty "dirty" only because they're not biased properly for AB2 operation, but that's easy to change.
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    Palomar 90A is 40 thru 10 meters, operates at 150 watts out and is about $75 to $100 on eBay. 15 watts peak drives it well, 10 watts will work for about 125 watts out.

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