[Amps] Drake L-4B Should I or Shouldn't I?

[Carl]

Consider how many of this very popular amp are still used regularly without 
making "Internet Changes" to join a few other lemmings.

Carl
KM1H


----- Original Message ----- 
From: "Don Jones" <ko7i at comcast.net>
To: <amps at contesting.com>
Sent: Saturday, March 29, 2014 9:12 PM
Subject: [Amps] Drake L-4B Should I or Shouldn't I?


> Last year I picked up a L-4B from an estate sale. About a month ago I 
> tried to fire it up. Looks like I got some problems to repair first. I did 
> hear a thump come from the transformer of the L-4PS as it charged the high 
> voltage cap bank. No biggie, also I did see the 3-500's light up in the RF 
> deck.
>
> What I did not see was any indication of High Voltage being present on the 
> L-4B's voltage meter. Fingers crossed I do not have a blown meter. What 
> should I see for pin to pin resistance of the High Voltage meter? Also 
> what resistance should I see for the plate current meter? This has me 
> puzzled, I would think that the meter should have shown that high voltage 
> was present. I presume that this is a proper assumption. Correct?
>
> Since I suspected there was problem, I stopped there and did not attempt 
> to "key" the amplifier and see if it would tune up. I pulled the 240VAC 
> power plug and let it set for a few days. After which I opened the high 
> voltage power supply section and inspected it internally. The power supply 
> has the original paper caps that came with it from the factory. I did not 
> see any blown diodes or smoked resistors, the paper caps still had their 
> guts which is a good sign.
>
> After some thought and having good service from Harbach Electronics with a 
> Clipperton-L power supply rebuild kit I decided to order one for my L-4PS 
> so I could get those old caps and diodes replaced. I also ordered a new 
> rebuild kit for the front panel power switches and a soft start kit for 
> the amplifier too.
>
> As for the Should I or shouldn't I question...
> I am going to operate this amplifier with an out board QSK switch and bias 
> control. I am thinking about hard grounding the grids and installing a 50k 
> ohm resistor in the B- lead to cut off the tube instead of relying on the 
> +150Vdc (or so) cathode voltage as designed from the factory.  I have 
> heard "internet stories" claiming that the L-4B can become unstable if the 
> grids are hard grounded. Something about the gain of the tubes becoming 
> higher and unstable. Is this true or not? I do want to use this amplifier 
> on all bands including 10 meters.
>
> My bottom line goal is to run this amplifier at about 750 to 800 W rms out 
> and enjoy it for many years to come. Looking for some technically sound 
> advice and objective criticism. Sorry for being long winded. Thanks!
>
> 73, Don KO7i
>
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