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Author Topic: Drake L-4B Amp  (Read 7038 times)

Posts: 101

« on: September 26, 2012, 05:40:17 PM »

Hi Everyone,

I just bought a lot of Drake equipment.  One of the items is the Amp L-4B.  However it didnt have a power supply and so I am trying to figure out what to get.  I have 120 and 240 available.  Is there a prefered power supply.  I saw some on ebay but they were very expensive. 

The amp itself appears to be in very good condition but without a PS I really dont know.  I have seen the L-7 power supply but I am not sure what to do.  Any advice as to which power supply to get and where or any other information would be very helpful. 


Posts: 144

« Reply #1 on: September 26, 2012, 06:58:19 PM »

Rob - either the L-4BPS or L7PS will work just fine with your L-4B. You really should run it on 220 VAC as a 15 A 110 line just won't cut it. When you find a suitable supply the first thing to do is rebuild it with either a Harbach:

or Heathkit Shop board]

(DANGER - only do this if you're comfortable working around potential high voltages as they can KILL you!). I've rebuilt three supplies and it's not that hard to do. You should also at least consider a Harbach Soft Start which is easy to install. Might be a good idea to replace the 50W bleeder resistors and 0.82 ohm in the HV line as well if they look flaky. Have had my L-4B since I bought it new in 1979 and it's still running the original 3-500Z's at full power out. There is also a 5K 7W resistor on the bottom side of the supply that lots of guys replace with at least a 10W unit 'cause if that fails it will take out the ALC pot and nowadays they are unobtanium (two 10K 10W resistors in parallel are a good substitute). A fan and a set of handles on the supply also make good accessories.

If you're going to keep the Drake gear you might want to join the two Drake groups, one on mailman:

and one on Yahoo Groups:

Good luck!

73 - Dino KL0S

Posts: 2747

« Reply #2 on: September 27, 2012, 03:30:44 AM »

Rob, yes either power supply the L4 or L7 PS will work, the L7 has bigger diodes but works the same.  The transformers work kind of weirdly.  I'm not a fan of the design at all, however, they do work.  I prefer to build my own supplies for Drake amps and remove the short comings of the design.  Originally it should have had a 3KV supply instead of 2500v.  But at the time the amp easily made the 1000wDC input mark.  Now that the output is 1500w you can build a bigger supply and run legal limit with the amp all day long.  The RF deck will handle it.  If you want to build your own let me know and I can tell you how and eliminate the shortcomings at the same time.  If you want to buy a supply I have a L4 supply with all the mods in it.  Lou

Posts: 2483

« Reply #3 on: September 27, 2012, 05:54:42 AM »

Unless you want to pay the "big bucks" for a Drake PS, about your only option is to build one or get somebody to build one for you.  As others have posted, the Drake L4B PS is not the best design.  IMO Drake cut a lot of corners.

However the L4B RF deck is (imo) the best built RF deck from that era.  Just look at it and compare it with other amps.  

The L4B was designed to run 1000W DC INPUT on CW (about 600 W OUTPUT) and 2000W PEP INPUT on SSB (about 1200W PEP OUTPUT), which was the legal limit during that time.   With the Drake PS the L4B will not run 1500 watts output.

I have a L4B with a home brew PS.  My PS runs around 3800V under load.  Some may feel that 3800V is a little too high for a L4B, but that is the transformer I had on hand.  Also some may argue that the LC ratio in the L4B tank circuit was designed for a lower HV, but I have not had any problems with arcing and efficiency is better than 60%.  I had to add a little cathode bias to keep the idling plate current within limits, but that is all I did.  The original tuning capacitor is a little close spaced for this voltage, but I have never had a problem with it arcing.  But then I have been tuning amps since the 1960's and I know how to do it.

Running the L4B on 3800V makes it a FINE legal amp.  In fact power output into a Bird dummy load easily exceeds 2000 watts.

As others have suggested you can get 1500 watts output of an L4B with a bit less HV.  

Dick  AD4U
« Last Edit: September 27, 2012, 06:21:04 AM by AD4U » Logged

Posts: 9749


« Reply #4 on: September 27, 2012, 08:24:57 AM »

The Drake L4B and L7, like almost all amplifiers of that age, were designed when the USA power limit was 1000 watts plate input power. This means they were designed when RF output was around 600 watts to 700 watts on CW, and about 1200 watts PEP voice SSB.

This also means tube dissipation was around 200-300 watts per tube, and that is what the cooling systems were designed for. 

While we may look at the Drake and other gear as under-designed, we should look at all that old gear in the context of what the law allowed for amateur radio power.

To REALLY have 1500 watts out with 100 watts of drive requires about 3800-4000 volts under load on a pair of 3-500Z. That is just the way the tubes work. With a funny meter or more drive you can see 1500 watts out at 3000 volts, but it takes ~130 watts of drive or a very loose power meter.

There isn't any magic to this. This is why Ameritron runs 3600 volts and why Viewstar tried running 3800 volts or so. Also, new Chinese tubes have less output, reliability, and life than Amperex or old Eimac tubes for the same voltage.

IMO, it is a little silly to beef up the supply in an L4 when the tank components are right at the voltage limit with ~3100 volts, and cooling is really right at 3-500Z seal thermal limits at 1200 watts plate power input ICAS on CW (~800 watts output).

If you put temperature sensitive crayon or paint on the 3-500 base pin seals, they run right around 250C with several minutes of normal Morse CW and 800 watts output. They are OK on normal SSB at 1000 watts out PEP.

Personally, I would not worry about a real heavy supply because it just kicks the weak spot down the road to the tube cooling or tank voltage rating. They are nice amplifiers but were never designed to be kilowatt + output like many people try to run them. They were nice 1200 watt PEP SSB, and 600-700 watt CW output amplifiers. I'd just build a supply that matches the RF deck power capacity and let it alone.

73 Tom

Posts: 149

« Reply #5 on: October 11, 2012, 01:23:07 PM »

The L4B is a very nice amplifier but, with age, do require some preventative maintenance.  Not done and you will probably have issues, sooner or later.  Case in point; I have 2 L4Bs, both in very fine condition.  I did replace the diodes and capacitors in the power supply and assumed that would be it.  Last week, one of the bleeder resistors failed and put B+ to chassis ground.  This failure worked back up thru the amplifier chassis, blowing out both meters (meter switch was in the GRID position), the 1 ohm B- return resistor and numerous bypass caps.  All parts were replaced via an order to Mouser... no problem.  Meters are another story and maybe the point of this post.  Alltronics has a series of rectangular panel meters that will work, albeit with some enlargement of the sub-panel openings and the rear meter shield.  The meters are $12 each, so no big thing.  The old Drake meter faces can be trimmed slightly and applied to the new meter face with super glue.  They look "factory"!  My real problem was that I spent 3 days troubleshooting a false B+ short that was, in fact, the new 1A meter!  Apparently, no internal shunt was installed at the Chinese factory and the meter was, in reality, only an 11ma movement vice 1 amp., so it pinned every time with idle current.  I checked it with a haywired panel meter and all now works fine.  A replacement meter is on the way, but we are back on the air! 
de K3HVG  Grin
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