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Re: [Amps] Drake L4B Transformation

Subject: Re: [Amps] Drake L4B Transformation
From: John Pierce <>
Date: Wed, 11 May 2016 11:09:49 -0700
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​>The Drake L4B RF deck easily turns into an Al-82 amplifier with the
> same power transformer in the power supply.  1500 watts output no sweat.

The Drake L4B doesn’t exactly transform into an AL82, since the L4B is
missing 160M, and missing the Ameritron electronic bias along, with some
much nicer metering, and great parts support - but I’m also running an L4B
at higher plate voltage with a replacement power supply that delivers 3750V
unloaded and 3300 volts loaded to 1500W output.

I read a comment in an old thread here asserting that this is a foolish
modification because the L4B wasn’t designed as a 1500W output amp, and a
beefier power supply just moves the problem to another weak area of the
amplifier.  But that hasn’t been my experience at all, except that I also
replaced the fan because IMHO the stock fan is marginal even for 1000W

This is the fan that I installed.  It’s very reasonable in price.

With this fan installed, I measured the static pressure under the tube
sockets at about 0.3 inches of water.  This fan is rated to put out 39 CFM
at that back pressure, which should be more than sufficient, even allowing
for some air leaks.  The 3-500Z are specified for only 13 CFM per tube at
the full 500W of plate dissipation, and most of us probably rarely push
things to that full dissipation.

The fan isn’t a perfect fit.  I had to grind off two side of the fan
housing discharge flange, and invert the motor, both of which are easy to
do.  The fan centerline is about ½” above the center of the fan inlet on
the back panel, but the fit is good enough that the fan functions properly.
The fan is quiet with no whine or rattle - only the reasonable sound of
moving air.

I also eliminated the Drake 120V ALC circuit, added my own ALC provision,
directly grounded all six grid pins, replaced the Drake plate choke with a
plate choke from Ameritron.  I added a small bias module and a pair of 10K
pots to the rear panel that allow me to adjust idle current in CW and SSB
modes while the amp is idling.

Until I directly grounded the grids and replaced the plate choke, I had a
lot of stray RF floating around when tested to high power into a dummy load
on 15M and 10M bands.  The RF would scramble the display of my Fluke DVM
that was monitoring HV, and cause my SS exciter to self-limit drive
power.  Directly
grounding the grid pins eliminated most of the stray RF.  Replacing the
Drake plate choke eliminated the rest.  Careful testing confirmed that
there were never any HF or VHF parasitic oscillations, so I don’t have the
theory to explain why those two changes improved matters the way they did.

Here are my typical numbers for 20M operation biased to 75ma idle current:

P out = 1500 Watts

Ep = 3280 volts

Ip = 680 ma

Ig = 203 ma

Drive = 75W

Efficiency = 65%

Any idea that this is a foolish modification to an old amplifier is sure
not correct in my experience.  I’ve had no arcing or smoke, although I am
careful to always peak the load control from the over-coupled side in an
effort to avoid excess voltage that might arc the irreplaceable band switch.

My goal was to assemble the most cost-effective, good quality, honest
1500W-output amplifier possible using 3-500Z - as I had a NOS pair of EIMAC
tubes on my shelf that I wanted to put to use.  I don’t operate 160M, and
don’t care at all about keeping the L4B in “original” condition.  By
shopping carefully, after buying the Drake components, and a lovely Peter
Dahl transformer, and selling off the L4PS and some other parts stripped
out of the L4B, I have only about $850 in the whole thing.  I’m very
satisfied with the result, and enjoyed a quite fun project.
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