Heater voltage 4cx250b - 4cx250fg

Discussion in 'Amateur Radio Amplifiers' started by N2EYE, Jan 13, 2020.

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  1. N2EYE

    N2EYE Ham Member QRZ Page

    I'm hoping someone here may have a quick answer for me as I'm starting to source parts for an amp build after more than a decade away from amateur radio. My old RF Amps group on Yahoo seems to have disappeared. Recently got an Eldico RF deck with Tetrodes and the screen circuit is new for me. I have previously only worked with Triodes.

    My question is: Will increasing the heater voltage in a Tetrode amp affect the rest of the circuit? I'd like to use 4cx250fg (26 volt heater) tubes instead of 4cx250b (6 volt heater) because the former tubes are much cheaper.

    Thank you,
    Last edited: Jan 13, 2020
  2. W1QJ

    W1QJ Ham Member QRZ Page

    Both tubes are identical except for the filament voltage. You can change the filament voltage on your amp and use that tube.
    K0UO, N2EYE and N2EY like this.
  3. N2EYE

    N2EYE Ham Member QRZ Page

    Thank you for your reply.
    It's been over a decade since I had my equipment out.
    Want to upgrade to General too. I'm glad they nixed the code requirement. I could never get beyond 5wpm when I tested for Novice and then Tech in the '80's. I never knew I had mild MD until 2009 diagnosis. Guess I'm lucky for that in retrospect.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    As Lou said, only heater voltage; nothing else needs to change. The 26.5V heater tubes are cheaper because years after almost the entire industry stopped using 4CX250Bs, VOR stations and various military gear continued to use them or at least need replacements and spares, so it seems thousands more than were ever really needed continued to be manufactured.

    That's bad for sellers, and great for hams.:)

    Even 40 years ago, the "F" tubes were cheaper than the "Bs," and I converted various amplifiers for the higher heater voltage without changing anything else.

    I don't know what the Eldico is, but remember for linear service both screen and grid bias should be regulated, and operating bias should be "adjustable" at least over a small range (like -50 to -60Vdc or so) to accommodate variations in tubes. Cutoff bias doesn't need any adjustment. And an important factor with tetrodes, especially ones having really no screen dissipation, is the Ep (B+) must be very considerably higher than EG2 (screen voltage) to prevent damaging screen current. With +350V on the screens I wouldn't go below about 1500V on the plates, and 2000V is a better choice.

    Have fun!
  5. N8CBX

    N8CBX Ham Member QRZ Page

    That's fine if one can find a 26.5vac transformer.
    Jan N8CBX
  6. W9FL

    W9FL Ham Member QRZ Page

    Just my two cents, but 24 VAC transformers are fairly common in industrial usage. They are used for control circuits.

    I don't know if 24V would be enough though, or if it could be boosted a little.
  7. W1VT

    W1VT Ham Member QRZ Page

    An old 115V to 25.2 volt transformer comes really close if your line voltage is 120 volts. Within 1%. Mouser still stocks transformers with these voltages.
    Line voltages have risen since the old days but for these old designs are still sold by distributors.
    Last edited: Jan 15, 2020
  8. K7JEM

    K7JEM Ham Member QRZ Page

    I would think if you couldn't find a transformer that a DC supply would work OK. Looks like it only needs .54A at 26.5v. You could even use a DC to DC converter and arrive at a regulated 26.5v from a 12v DC supply, or whatever. Those are pretty cheap on e-bay and other places.
    N2EYE likes this.
  9. W1QJ

    W1QJ Ham Member QRZ Page

    Filament transformers and control transformer like wall warts are unregulated. They all come with a voltage rating at a specific current. The transformer is designed so that when it's rated current is drawn then it will be that specified voltage. If you have zero load on the transformer it will read quite a bit higher than it's voltage rating. So you might be able to get a high current 24vac transformer say 10 amps maybe. The 4cx250fg only draws .54A. So you may find a 10 amp 24vac transformer might be around 26 volts to start with. I found that a 12.6v 4A transformer has a bit over 13v with no load. When a 3CX800 is hooked on to it the voltage drops perfectly at where it should be for a 3cx800. Something to consider.
    N2EYE and N8CBX like this.
  10. N2EYE

    N2EYE Ham Member QRZ Page

    Wow thanks for all the replies. Screen voltage transformer search has begun. I expect to use a Tetrode Board (e.g. GMSEK and IFWtech).
    I had wired a 4 x 572b heater string in series to reduce transformer current needs as voltage divides equally and that worked well but I'd rather use a prefab made control board. I think the controls boards provide for regulation.

    Another difference I'm pleasantly surprised to learn is that many Tetrode amps only use swamping resistors for input impedance or rather, no tuned input circuits like triodes require.
    I can't believe how simple the wiring is below the tubes on the Eldico SSB-1000 rf deck I have. It's all original never been touched.

    Searching for chassis/enclosure seems to be the sourcing problem. I'm not finding anything useful.

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