I agree with your thoughts on this subject and thanks for some good info!
I currently have a transformation in progress. Using an AL-82 transformer
with a populated Far Circuits 4KV power supply PCB, a K4POZ bias board, & a
Harbach soft key board. Also directly grounding the grids and incorporating
a plate voltage relay and step start in the power supply. The relay will be
controlled by the on/off switch which will reduce the current load on the
switch.
I am eliminating the 120v bias and ALC voltage as well.
I had not planned to change the plate choke, but probably will now that I
have read your post. I also have been wondering how to beef up the cooling.
Your info on the blower/fan is most helpful. I may go in this direction.
Overall I should end up with about $800-900 dollars in the whole project.
So far it has been a fun project.
On Wed, May 11, 2016 at 1:09 PM, John Pierce <lumin9@gmail.com> wrote:
> >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
> output.
>
>
>
> This is the fan that I installed. It’s very reasonable in price.
>
>
> http://www.amazon.com/Dayton-Model-1TDN4-Blower-4C761/dp/B000BK8614?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage
>
>
>
> 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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