Drake L7 Amp

Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 17, 2012.

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  1. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    I now have a Drake L7 amp. I see the 10M mod isn't done yet. Found the data on that, but I need two 68puff caps to make that mod. Nice to see it tunes 160M and the WARC bands (no 30M though).

    I have heard the power supply in it is weak and won't give the full 2KW it is designed for. The manual says 2KW PEP SSB and 1KW CW. So this won't be that much more than my SB-200. But any extra dBs in a Pileup will help.

    Currently this is wired for 120VAC, but I see that requires 30A. My AC line in the house is probably on a 20A circuit breaker. The nearest 220VAC is in the garage about 50-75ft away where there is a dryer outlet. Maybe I can juryrig up a cord to go from that outlet, in and around the house and into the radio shack window (where all the coaxes are going) to put an outlet in the room. I don't know if that long a run on 220 will be any better than the stock house 120VAC wiring. NO, I am not going to hire a professional Electrician to put it in. That would be ASTRONOMICALLY EXPENSIVE! We are talking here basically about a 220VAC Extension cord. Will check out the Home Depot to see what kind of money I am looking at. SO FAR, I am getting away very successfully using the standard 110V into the shack to run the SB-200. So maybe I can try the L7 and see if it blows the breaker. If it does, then guess I need the 220VAC Extension cord.

    Wonder if this L7 can have a tap on the coil put in for 6M?

    I see that an L7 went on Ebay for $1475!!!! WOW!

    Might be a good idea to redo the HV Capacitor bank. This amp isn't that old, and was in frequent use up until the Previous owner passed away. So its not like it has sat for 30 years. I see it was new in 1981 and last serviced by the Drake factory in October 1997.

    Looks like a wonderful amp! Nice to have QRO on 160!
  2. W9GB

    W9GB Ham Member QRZ Page

    Pat -

    Yes, the Drake L7 will be much happier with a 240 VAC circuit in the ham shack.
    You may be price SHOCKED at the 240 VAC connector prices (usually > $10 each) and
    you will have to look (at least): 10-3 w/ground drop cable --
    to handle the voltage drop for that length -- which is not a cheap cable ($ per foot).
    I believe that the Drake L4 and L7 HF amplifiers use 200 mFd @ 450 VDC electrolytic capacitors with AXIAL leads.
    Axial lead electrolytic capacitors at 450 VDC ratings can be difficult to find as direct replacements.
    The standard replacement value (mFd) produced today is: 220 mFd

    Just Radios does carry an Axial electrolytic capacitor: 220uF at 450V axial electrolytic capacitor. Size: 1” by 1 3/4” ; Temp rating: 85C

    You can substitute the Snap-In or Screw terminal versions and fabricate and solder appropriate leads to fit the installation.
    The preferred method by many restorers is to use a replacement PC board -- so that you can use common Snap-In versions.

    1. Jeff Weinberg, W8CQ at Harbach Electronics offers a replacement HV board (with new capactors) for the Drake L7 (as well as L4).
    Jeff's new board replaces both of the old diode and capacitor boards.

    2. Mike Bryce, WB8VGE at The Heathkit Shop also offers a Drake L7 HV replacement board

    Last edited: Jan 22, 2012
  3. VK4TUX

    VK4TUX Ham Member QRZ Page

    Is that not an input rating?

    Adrian ... vk4tux
  4. W9GB

    W9GB Ham Member QRZ Page

    Drake L-7 provides 2000 watts INPUT on SSB and 1000 watts on CW or RTTY.

    It covers 160, 80, 40, 20 and 15 meters. This amp employs a Pair of the Eimac 3-500Z triodes.
    There is a by-pass switch. There is a built-in RF wattmeter (calibrated with 300 and 3000 watt scales) with forward and reverse switchable readings.
    Power requirements are 115 VAC 50-60 Hz 30 amps or 230 VAC 50-60 Hz 15 amps.
    The power supply is housed in a separate enclosure.
    This amp features a temperature controlled, two speed fan. 1
    3.7 x 6.75 x 14.25 inches 27 lbs. plus 43 lbs for the power supply
  5. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    I have new caps, diodes and bleeder resistors on the way. I am going to use a Perfboard and busswire to make the HV board, taking into account where it might spark over. Wish those suppliers would just sell the PCB! The caps are radial types like used in the Harbach and WB8VGE boards, so I will rob the way they laid out their boards. Should work fine. Not still sure what I am going to do about the 240VAC cable. That sounds like just the cable and connector are going to be towards $100!! And I have to worry about rats chewing the insulation in the garage. I have heard that smearing chili powder on the cable makes them not chew it, so thats an option. Will have to drill a hole in the garage wall to get the cord out, and lash it to the house where the pit bulls can't get at it to chew. Wonder if some 3 wire Romex cable would work. Most of those are 12 or 14 gauge, so 10 gauge is going to be outrageous! I will have to go to Home Depot to check it out. I might run out SCREAMING!!!! Also have to see what kind of plug the dryer uses. 2KW would SURE be nice in the shack! That will break pileups for sure!
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The amp will run probably 1200W PEP output, which is indeed 3dB more than an SB-200.

    It's a really pretty amp and wish I had one now to match my TR-7, just because they'd look really nice together. One of these days...

    I have put these on six meters, but in every case tore out all the HF components and just put it on 6m only. I used the TUNE cap for the LOAD cap on six, replaced the TUNE cap with a 3-30 pF vacuum variable, and wound the tank inductor 3-1/2 turns of 3/16" copper tubing on a 1-1/5" diameter form. For the input network, I rewound the 10m inductor and substitured smaller mica caps. For the plate RFC, I used the same form but removed about 3/4 of the turns until it dipped at about 60 MHz.

    The original filament chokes work. The original T-R relay works, but has some loss on 6m so I'd usually sub a pair of Dow Key relays to pick up that dB or so that the original transfer relay and wiring loses.

    It's not a difficult conversion, but it is a single-band conversion.

    Never tried to "add" 6m along with the other HF bands, I'm not sure that would really work.
  7. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    The parts came in from MOUSEYWOUSER yesterday, so, with the boss being out of the office, it was time to MODIFY and REJUEVENATE the AMP!

    First procedure was to make it work on 10 meters. This was a very difficult process.

    Step one is to replace the Low Pass filter (designed to prevent any sigs on 10M) with a coax jumper. Not too hard.


    I lifted the coils and caps connected to the input and output coaxes; and installed the RG-8X jumper. This is located in a hidden compartment on the bottom of the amp.

    Now it was time to go after the REALLY hard mod. That is to remove the stop on the bandswitch so it will rotate to the 10M position.

    The stop is on the Ceramic switchdeck only accessible through this narrow slot from the bottom.


    somewhere up there in the darkness is a little bolt and nut. You have to use a screwdriver as a chisel and loosen the nut and then rotate it off. Then work the screwhead out of the switch. Very very VERY Difficult! This took the longest time to accomplish.

    Next, you add 2 caps to the phenolic bandswitch accessible throught the little compartment. You can see the two 68 pf mica caps added. This was a painless step


    Now the access plate could be reinstalled on the amp completing the mod. U are supposed to readjust the 15M coil for low SWR but I skipped that step.


    it is time to go after the power supply now.
  8. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    The L7 power supply has these enormous 200uF/450V caps. Time for those to go!


    Harbach and the Heathkit store make a nice PCB to use, but I made one out of Radio Shack perfboard. I reused the mounting brackets to one of the original PCBs and mounted them on the RS board. The perfboard is cut to the exact size as the Drake board.

    In the commercial models, they used shorter caps, but mine were too tall to install in the cabinet. So I laid them down on the perfboard and used the bleeder resistor leads to go down into the holes. Then all the caps were held down with lacing cord. The 14 1N5408 diodes were also mounted on the perfboard.


    The bottom of the board was wired up and the old boards yanked out. Then the new PCB was mounted in the chassis.


    The power supply was tested and worked great.

    Finally, it was mated up with the Drake L7 and tried out. Works great, plenty of voltage at 110VAC

  9. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Today, I tried out the L7 with the Kenwood TS-830S it came with. Plenty of RF out on all bands including 10 meters! But, alas, the Plate current meter does not work. I tore apart the amp and tried to find out why,. The meter was disconnected and tested. Works perfectly. I tested all the associated resistors and stuff, but could not find any problems. I don't know WHY it doesn't work. Maybe something to do with the mod I made to the Power Supply.
    I do have Grid current measurements, so that says the relay associated with it works fine. No reason why it shouldn't work, but it doesn't. Well, I can still use it. Just tune up the amp on a wattmeter for maximum SMOKE!
  10. W9GB

    W9GB Ham Member QRZ Page

    Drake L7 Manual -- [i[WB4HFN Drake Resource page[/i]
    Schematic diagram on page 28

    Pat -

    It sounds like you took the proper first steps of troubleshooting this issue:

    1. Disconnect the plate current meter and measure its internal resistance with an ohm meter.
    This meter would test either an Open circuit (meter is bad) or some value of Resistance (meter is good).
    Your testing said meter is GOOD.

    2. Since your L7 plate meter tested good, then tropubleshooting moves to tracing the associated wires (and circuits).
    Possible problem sources are: Open multiplier resistors or bad solder joints, broken wires, bad switch/relay contacts, etc.
    These are only 4 components in the immediate area of the Plate meter -- and C38 at the J4 connector --
    which your testing said were GOOD.

    3. Assure that you have continuity from the Plate meter to Pin 6 on the J4 connector (rear panel of L7) as well as that
    cable that routes HV from J4 connector to the L7 power supply you just rebuilt. TRACE this connection.
    When you rebuilt the L7 power supply, was this wire reconnected in that power supply - OR -
    did the copper foil on the old HV board supply (PC board) provide this connection via mounting hardware??
    Since you used standard perf board (no copper), this may have been a missed connection in the retro-fit.

    4. Next check switch S5, the HV Interlock for the L7 amplifier. Previous owner disable, disconnect?
    IF you operated this amplifier -- then this likely is OK -- BUT I ALWAYS check Interlocks on HF amplifier acquisitions - SAFETY FIRST.

    5. Next check the Relay contacts on RLY 1(A), corrosion or damage to these contacts is possible.
    Your earlier tesing said these contacts were GOOD.

    Last edited: Jan 25, 2012
  11. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    The amp works great with plenty of RF OUTPUT, so that is why this is a hair tearer! I examined the Relay but if it was open, there would be no plate current. There HAS to be plate current or I wouldn't get RF out. Same with the rest of those steps. Any break in the path to ground would cause no RF Output from the amp. I measured the resistors, and the path to ground. Also measured the meter with the Shunt in line with 200mA to ground so that works, and says the 1 Ohm resistor and circuit is good. Think I am going to have to try and run the amp with the cabinet off so I can measure currents while it is operating. This is very DANGEROUS as lethal voltages are there when powered up, and I don't want to be a silent key quite yet. The interlock shorts the HV to ground. The only variable is that I ripped out all the wiring in the L7 PS and rewired it, as I didn't like how Drake used 24 gauge thin wire to pass the HV and high current paths. I was ASTONISHED to see 24 gauge wire connecting the 50W resistors to the 7W resistor! WHY would they need 50W if their wasn't any current? SO I used 18 gauge wire to replace all that.

    THis is indeed a HAIR TEARER! But the amp works Fine otherwise!
  12. W9GB

    W9GB Ham Member QRZ Page

    Power Supply Schematic diagram is on page 27 of the L7 manual (electronic verison).

    That usually means a Homer Simpson moment -- DOH !

    I just had a funny thought -- L7 Plate meter wired backwards?

    Last edited: Jan 25, 2012
  13. W0UZR

    W0UZR Ham Member QRZ Page

    Is your house a single phase or 2 phase electrical? Is there a dryer in your garage? is it being used? If not, then there would be a dryer in your house then. Well, what I would do is take and trace that line from your 240v in the garage, and since it isn't being used anymore, then find the breaker that is for the 240 in the garage. Now drill a hole going down in the basememt just below the breaker box, run some house wire from the breaker down in the basement and then drill a hole to where your radio equipement is. Mount an outside wall outlet at just above the baseboards, and put a 220/240 outlet in there. Get a plug to match the outlet on your amp and you are set.
  14. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Thats what I am looking at doing, but it is a long ways to the dryer outlet, and my wife does occasionally use the dryer so I can't just unplug it and hope she can. I have to make an adapter box where her dryer is always plugged in and then the 240 tapped off and run to the radio shack. But it is around 50ft to this outlet, so I am looking at a very thick cord to make it there. I am working on that.
  15. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I think to do it legally you'd need to run conduit, but if you plan to die in that house and never sell it, most homeowners do whatever they want...

    #12-3 heavy duty (thick insulated) outdoor extension cord would probably be sufficient for a 50 foot run. Whatever you do, don't use 120V connectors -- that's a bad accident just begging to happen. Use 240V connectors no matter what.
  16. KO6WB

    KO6WB Premium Subscriber QRZ Page

    And Pat, do not run Romex unless the entire run is enclosed and usually indoors the whole lenght. Like behind walls and through the attic. Running Romex through conduit is a bad idea. Running Romex outside is also a bad idea.
    Hope this helps
  17. VK4TUX

    VK4TUX Ham Member QRZ Page

    Love the meters on that L7, I have seen the meters lit with blue back-lighting on some. Really one of the finest looking fascia's on any amp I have seen.

    Adrian ... vk4tux
  18. VK4TUX

    VK4TUX Ham Member QRZ Page

    I'd would start with replacing R20 1ohm @12w resistor, C51 & C61 0.01uf caps.

    Adrian ... vk4tux
  19. WA7PRC

    WA7PRC Ham Member QRZ Page

    I don't think conduit is necessary for a "temporary" feed, as long as the correct components are used. For a temporary "extension cord", use SJ or SO cord. Most dryer circuits are 30A, requiring 10 AWG or larger wire and connectors rated 30A @ 250V... NEMA #6-30, etc. To use 20A-rated cord & connectors (12 AWG & NEMA 6-20, etc), I think you'd need to fuse/breaker it at no more than 20A at the point where you plug into the dryer circuit.
  20. W9GB

    W9GB Ham Member QRZ Page

    Adrian -

    Good suggestion, I think Pat (MHZ) said he tested R20.

    The HV Safety Resistor used in the Drake L-4/L-4B/L7 Power Supply should also be checked.
    Harbach sells (part #RES-400) a direct replacement for the original IRC SPH-style safety resistors used in the Drake L-4/L-4B/L-7 remote power supply.
    These resistors open under conditions of high plate current draw from the power supply (tube arc or power supply short) to protect tubes and power supply components.
    Harbach sells for for $ 6.00 USD in packs of 5.

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