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Bringing an old tube amp back to life??

Discussion in 'Amateur Radio Amplifiers' started by W6BRY, Nov 26, 2018.

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  1. W6BRY

    W6BRY Premium Subscriber QRZ Page

    I need some guidance with a vintage Henry 2k amp running 3-500Z tubes. A friend of mine (non-licensed) purchased the amp at a garage sale and offered it to me since he has no use for it. I estimate it has not been powered up in 20 years at least. There is also a set of replacement tubes with it, but again no clue how long they have been sitting.

    I read this info on the W8JI website about bringing old tubes back to life:

    Under some conditions a glass tube can be restored to operation by running low anode voltages and positive bias on the grid. This will sometimes allow full operating anode temperatures to be reached, and the tube can be "cooked" for several hours. I've had about a 50% success rate restoring old 3-500Z's that have sat for years without use. Even though they initially arced severely at full voltage, by cooking them at low voltage and positive grid bias to show anode color vacuum was restored.

    If there is someone here that could explain in layman's terms how this is done, and if it is possible to adjust the amplifier to perform this procedure or do I need some other power supply or equipment to do this?

    I realize I don't even know if the tubes are good or bad yet (picking it up next week) but I want to make sure that I do this correctly. If there is a recommended procedure for powering up an old amp I would sure appreciate the info. Thanks!!
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    What Tom is describing in that quote is 'gettering' or re-absorbing any gasses inside the tube by bringing up to operating temperatures but at relatively low anode voltage. You may or may not need to try this as your tubes may be fine as they are.

    The bigger concern in an old amp of unknown condition is the condition of the power supply and the filter caps in particular. Bringing the supply up slowly on a variac is one approach to seeing if you have any issues with the supply without simply throwing the power switch and watching for fireworks. There are other ways like light bulb current limiters but it does get tricky for something like a big power amp that's designed to draw a lot of current.

    I'd focus on testing the supply and verifying that it works or determining whether things like filter caps might need to be replaced before worrying about the tubes. Even then I'd test the tubes in a controlled way (e.g. tuning into a dummy load) before I worried about gassy tubes and gettering.
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Henry made a lot of different "2K" models, with the old original ones being 55 years old and having "teardrop" meters, with a separate power supply beneath the RF deck. The more modern ones had rectangular meters and they made models both with the separate power supply and also with built-in power supplies (the "2KD" models).

    Which model do you have?

    The old ones used a pair of 3-400Zs, which were "zero bias" tubes; around 1970-ish they switched to 3-500Zs and added operating bias.

    Eimac had some very good runs of 3-500Zs which seemingly lasted almost forever and offered good long storage life; but then towards the end they made some very "suspect" runs which didn't last as long and didn't store as well, so this is a bit of a roll of the dice.

    The old 2Ks used oil-filled filter capacitors that can last 60+ years, but then eventually leak. When they leak, they'll let you know as it makes a mess. The "desktop" models used a string of regular electrolytics that don't last as long and do dry out -- and when they do, BANG!

    Which model do you have?:)
  4. W6BRY

    W6BRY Premium Subscriber QRZ Page

    I don't have the exact model number info right now. All I remember is that the upper cabinet is more horizontal, and sits on the power supply at an angle. I did a search for henry amplifier photos and id say it looks like a 2K Classic.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The 2K Classic has this printed right on the front panel:

  6. W1QJ

    W1QJ Ham Member QRZ Page

    I have a Henry 2Kd Classic in the shop right now. The "D" means it is a desktop model. The desktop models all had the individual 8 electrolytic caps in the power supply. Most of the floor models had oil filled cap and choke input. There are two phases to bringing an older amp on line after many years of not being used. One is to first verify if the PS is OK. Verifying if the PS is OK could be done rather easily. It requires the use of a Variac. You must first remove the tubes. After removing the tubes bring the line voltage up slowly observing the HV reading on the meter. You should see HV coming up immediately upon raising the line voltage. If you do not see any HV coming up rather quickly, DO NOT proceed any further. You must find out why there is no HV. It is a good idea to use a KNOWN good HV meter at this time in case there is a problem with metering. NEVER assume there is no HV just because there is no meter reading. This could get you killed. I always hang a known good meter when I do this. If and when you verify the PS is OK then you can install the tubes and once again bring the voltage up slowly. Try to safely keep the lid off so you can see the tubes and the same time observing grid and plate current. Watch for any plate or grid current UP or Down on the meter. ANY indication either way probably indicates a bad tube. STOP and determine which tube is causing any meter readings by using one at a time. Keep an eye on the tubes and check for gas looking for any blueish hue inside the tubes. The sooner any blue is seen in the tube the more gassy it is. Any sign of gas, you should STOP raising the voltage and consider getting good tube(s). If you can raise the HV to normal with no sign of gas then the next ste is to key the amp with no drive. Once again observe for gas. Some tubes do not show gas until excited. If you are lucky and everything goes well then load the amp up and check the amp itself for proper loading. If you can get this far with no ill effects then you are doing good.
    AC0OB, AI3V, K9ASE and 1 other person like this.
  7. WB2WIK

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    If it's a 2K Classic console as shown in past #5 above, in "CW" mode the HV is lower, in the 2000-2200V range (as opposed to "SSB," where it's well over 3000V) and using the "CW" position is a little bit safer when dealing with unknowns.

    The console units, as Lou wrote, normally have a big oil filled capacitor and a big choke in the power supply and the oil filled cans can last a very long time. I have some in a homebrew 2kW power supply I built in 1984 using caps that were from 1968 and they still work fine (8 uF @ 4000V each). Tubular electrolytics usually don't last that long.:)
  8. W6BRY

    W6BRY Premium Subscriber QRZ Page

    Hey Guys, sorry for the delay in getting back on this. I have a full time job that gets in the way of my hobbies :) so I had to wait until today to go pick up the amp.

    I went to my friend's place and this amp has been sitting in a dusty room for around 4 years... I thought it was longer but he corrected me. The amp is a 2K-4 and has 3-500Z Eimac tubes in it. It also has an extra set of tubes in a box that was included with the purchase. I initially tried to separate the upper console from the lower HVPS but it turned out to be a lot more work that I thought so I removed the tubes and chimneys and moved it in one piece. It fits perfectly in the back seat area of my crew cab GMC Sierra, so I kept it upright and it is so heavy it did not even tilt in the corners. I had to laugh when I unpacked the spare tubes and they have no markings other than a $45 price tag from Henry Radio. I wonder if they are an original set of spares that were purchased with the Amp when new? The serial is 01-595 and the Eimac tubes has 7715 on them so I am guessing they were made in April 1977.

    I got it home and blew the dust out with clean filtered compressed air. I then took some bore cleaning patches and moistened them in rubbing alcohol and then wiped out the interior as best as I could without disturbing anything.
    I cleaned the tubes and inspected them, they look to be in very good condition. I did see only one electrolitic capacitor, which is a 500uh unit so I think it would be wise to replace it. The caps in the PS are very large oil filled ones so I am assuming they are good to go since they are not leaking.

    I am going to try to post the pics here and hopefully I can get the links to work. I had my shop rewired a few months ago and had 220 run in there, but I didn't install the plug yet so I need to do that tomorrow.

    Link to pics:




  9. W6BRY

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  10. W6BRY

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