New Amp owner questions

Discussion in 'Amateur Radio Amplifiers' started by KE0DMC, Mar 8, 2016.

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  1. KE0DMC

    KE0DMC Subscriber QRZ Page

    Hello I picked up an linear amplifier minus power supply at a hamfest for under $100. The owner said he hasn't used it in 10 years and is downsizing. He had already powered it up the night before and it worked fine. My club members told me he had a good reputation for the boat anchors he sold.
    The linear amp has 4 (4-250) tubes for output. An Elmer at my radio club told me it would provide 3kw easily with a 3.5kv power supply. He also told me each tube will draw 10 amps from the filament supply.

    Since the tubes are in parallel, could I simply remove 2 tubes and run it at 1.6kv?
    Another option, can I replace the 4-250 tubes with 4-100's and run 1.6kv?

    Is there any way to test the 4 tubes I have?

    Please help this lost soul out.
     
  2. KA9UCN

    KA9UCN Ham Member QRZ Page

    You could run all 4 at 1600 VDC and probably not get over 1.5 KW out with 100 wat in GG class AB operation.

    The 4-250 likes over 2000 VDC and works very well at 3000 to 3500 VDC but it will run at 1600 VDC. If you have a 1600 power supply some things to consider are run it at the 1600, use a doubler circuit to get 3200. If your supply is inductive filtered rework it as capacitive filtered for around 2400 VDC.

    1 tube or 4 tubes require the same plate voltage, only the current demand is different. 3 KW is not out of the question for 4 4-250 tubes but it is beating them. It would make a nice conservative legal limit amp with 4 tubes.

    Joe KA9UCN
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I wonder how the previous owner tested it without a power supply...or if he had a supply, why he didn't provide that with the amplifier.

    If the filament transformer was in the power supply and not right next to the tubes in the RF deck, that's an unsusal design; I always put the filament xfmr in the RF deck by the tubes, otherwise the interconnecting cable would need to be very BIG...four four tubes, probably at least 8 AWG for that.

    With sixteen hundred volts Ep, a pair of 4-250s can't possibly run a kilowatt. They need at least 3 kV to do that. Look at the data sheet for the tube. Four of them might run '3 kW PEP' maybe...at 4000 volts. At lower voltages...impossible.

    They're also not as easy to drive in grounded grid as power grid tubes like the 3-500Z. Four of them, even at 4 kV Ep, would require about 200W PEP to drive them to full power.
     
  4. KE0DMC

    KE0DMC Subscriber QRZ Page

    He had already sold the power supply early on and sold the matching transmatch n hour later. It was the end of the show and no one was interested in an amp so he let me have it for 1/2 the asking price of $80. All his other boatanchors sold.

    I could just sell the tubes and be ahead and purchase a Russian triode and rewire it.

    Thank you for the information. It has provided me food for thought.
     
  5. KA9UCN

    KA9UCN Ham Member QRZ Page

    You could do that but you might want to consider what the specifications are on a 4-250. They are quite a tough tube with a very long service life. To replace 4 already in service with a Russian tube might not be a good choice for several reasons.

    If I may ask. What is the build quality? Does it have a tuned input? Is it mono or multi band?

    If you answer the questions above and send some pics. Those on this forum will be able to tell you if you have a gem or turd.

    I personally would love to have a 4 tube 4-250 amp if no more than for the nostalgia let alone that it has 1000 total wat of plate dissipation with a very tough screen and grid. 140 and 40 wat total.

    Before I would even consider a Russian tube that will soon be unobtanium. I would build the proper power supply for the amp. I wanted. The Russian tube requires a plate power supply also.

    If you do build a power supply. That tube likes 3200 to 3500 VDC.

    Not to insult you in any way but with the questions you ask you will require updating your skill set to do this safely.

    If you build in the 3200-3500 VDC range you should have no problem with getting legal limit or very close with 100 wat drive and a tuned input.

    I often run a pair of 4-400 grid driven at legal limit in SSB with 3300 V on the plate.

    You can sell the tubes if you can find a buyer. Don’t expect to turn much if any profit. I put 4-400, a much more desirable tube up for sale on several occasions at $45 to $55 shipped and guaranteed and did not sell a one. Speaking of which the 4-400 is a direct replacement for the 4-250 and a lot more available should you decide to replace the tubes in that amp. One day.

    Post some pics.

    Joe KA9UCN
     
  6. G3YRO

    G3YRO Ham Member QRZ Page

    If I can give you an analogy, your question is a bit like someone asking if they should remove the 3 litre engine from their car and fit a 1.6 . . . because they don't want to drive at 130mph.

    So I'd suggest keeping it as it is, put around 3kV on the anodes, and run it at whatever power you want.

    Roger G3YRO
     
    KE0DMC likes this.
  7. KE0DMC

    KE0DMC Subscriber QRZ Page

    Thank you I am obviously new to this. I didn't realize the gem I owned. Seeing the prices of modern day amplifiers I should appreciate what I own. I guess Elmer misadvised me or thought of a day gone by. You are right, at the hamfest the fellow who sold it said he had 2 spare sets of tubes he would be willing to part with. There two for sale at the one I went to at $15 each that went unsold.
    My expertise with electronics so far is refurbishing boat anchors and building a stereo 6L6 audiophile grade amplifier with power supply. I am studying for my extra class exam.
    I admit my ignorance but appreciate your guidance.
     

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