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4-400b tube in SB220

Discussion in 'Amplifiers' started by Jimbo165, Jun 19, 2017.

  1. Jimbo165

    Jimbo165
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    I was told today at a hamfest that you can replace 3-500z tubes with 4-400b tubes and it is plug and play in other words I do not have to do anything to the SB220 is this true??? I know the 4-400b tubes around here are easier to find used. Thanks Jimbo


     

  2. Captain Kilowatt

    Captain Kilowatt
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    The tubes have similar operating parameters but the 3-500Z is a triode and the 4-400 is a tetrode. IIRC they are not just a simple drop in plug and play.You need to tie the screen grid and the control grid of the 4-400 together when replacing 3-500Z 's with them. The 3-500Z has a plate dissipation of 500 watts while the 4-400 has a plate dissipation of 400 watts.
     
  3. nomadradio

    nomadradio
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    The 4-400 is a tetrode. Has two grids. The 3-500Z is a triode with one grid.

    Both tubes use the same cathode, 5 Volts at 14.5 Amps. It's connected to the pair of closely-spaced pins on one side of the socket. Easy to indentify, since these two pins are wired directly from the one socket to the other and to the cathode choke coil with the massive wires on it.

    The three widely-spaced pins on other three side of the tube socket all connect to the single grid in the stock 3-500Z tube. It uses three pins to make this one connection as a way to reduce the inductance of the circuit path between the tube's single grid and ground. In the SB-220, there are two grounds. One is the DC circuit, provided by the 1 mH RF choke. The other is the RF-ground circuit provided by a single 200pf capacitor on each of the three grid pins. Three caps per tube.

    The three widely-spaced pins on the 4-400 are wired differently. The one on the side opposite the two cathode pins is the 4-400's control grid. The other two both connect to the tube's screen grid. Simply adding a jumper wire that joins the three widely-spaced pins to each other will turn the tetrode tube into a triode.

    You will find that the power gain is not terribly impressive with this hookup.

    Technically, it can be done. The results are a separate issue.

    73
     
  4. wavrider

    wavrider
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    You will have to tune the input circuit to present an acceptable input impedance.

    The 4-400 will have less power output,,the 3-500Z is a higher gain tube.

    4-400 like a higher plate voltage but will work in the SB 220 with OK results.

    The receiving station will never know the difference between the 1kw a pair of 3-500 produce compared to the 700-800 watts the 4-400 will produce.
     
  5. Jimbo165

    Jimbo165
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    Thank you all for your advice and comments I picked up two of the 4-400b tubes at the Monroe Michigan hamfest I have 40 dollars in them and they are in boxes I will put them on shelf for now . Thanks
     
  6. nomadradio

    nomadradio
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    We adopted the habit of checking high-power glass tubes for gas before applying high voltage to them in an amplifier. The vacuum can degrade over decades on the shelf. If it does, even a tiny amount of gas in the vacuum can break down and cause an arc inside the tube. Generally not a nice thing to do to your power supply, or to the front-panel meters. Those meters are hard and expensive to replace, and there is NO protection circuit for them in the original design. Adding protection diodes is a good way to avoid expensive grief from blown meters.

    Most folks won't have a high-voltage breakdown tester to perform this test. Applying a few thousand volts between the grid and cathode will reveal any gas contaminating the vacuum. The tube should show little or no leakage current up to around 3000 Volts with the cathode cold.

    The gray coating on the tube's anode surface is a so-called "active getter". When the tube's anode gets hot enough, any gas molecules that get close enough to that surface become chemically bonded to it. Keeps the vacuum 'scavenged' of stray gas. But you have to operate the tube at a normal temperature to 'clean' the vacuum this way. A long shelf visit can allow tiny traces of gas trapped in the tube's materials to enter the vacuum and create this problem.

    Conventional wisdom recommends running old tubes with just the filament, and no high voltage for a few hours before trying to operate it at normal voltages. Simply disconnecting the plate cap will accomplish this in a SB-220. Just use EXTREME caution, and make sure the plate-cap connectors can't brush against anything else when they're disconnected from the tube's plate terminals.

    This amplifier CAN and WILL kill you if you fail to observe proper procedures.

    73
     

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