[Amps] Drake L4B/8877 Retrofit Problem

[w2cqm at juno.com]

Perhaps some of the Amps Reflector experts out there can help with an amplifier construction project problem!  Recently completed the fourth in a series of  similarly constructed Drake L4B/8877 retrofits. All have upgraded,  Hipersil, step start,  higher voltage supplies.  All the projects utilized basically the  OEM Drake bandswitch tank circuit components and to this point have been well worth the effort making 2200w output.  Unfortunately, the last one is giving me grief. On the 40/80m band, I’m unable to load beyond the 600w output  range (10w of drive) without experiencing RF “fireworks” in the plate tune capacitor. This plate cap  was replaced with a wide spaced, heavy duty unit   because the original Drake cap  initially exhibited the same characteristics;  and I thought the replacement  would solve the problem. Didn’t help!  I’m able to Variac control the plate voltage and the problem appears not to be voltage initiated. Oddly, on 20m, the amp  (4+KV plate voltage) develops 1800w output (Bird Meter/dummy load)   without a whimper with only 30w of drive. To address the problem,  I tried a variety of parasitic choke combinations from no choke, “U” shaped, and the typical coil/resistor combination (all with 50ohm non-inductive  resistors). The latter  style did help only slightly until a higher level of output was tried.  I substituted a single 1000pFd blocking capacitor for a pair of parallel 1000pFd units with somewhat better results.  However, tuning remains critical  on the lower bands and output  remains about the 600 watt level with accompanying   flashing.  RF “fireworks”  manifest itself  within the confines of  plate tune  capacitor. The flashing is   either plate to plate or plate to ground and usually never in the same location. In this installation, I used the Drake HV plate choke since my custom wound  1”  porcelain units  have essentially the  same configuration and electrical characteristics.  Apparently the stable/unstable boundary line is very critical and really hasn’t changed much with any of the component changes. Incidentally, higher output can be achieved on the lower bands but only with fast  tune/load capacitor combinations and lots of unsettling flashes and noise. Not a comfortable scenario especially with the price of a replacement tube.  Meshing of the capacitor plates are all normal for the three bands tested. I have several spare tubes but did not make any substitutions as yet. When I regain some motivation, I plan to pad the plate tune cap with small increments of capacitance but the outcome is uncertain. I’ve already prepared a posting for a 0-300 pFd vacuum variable. That may or may not help but at this point I’m at a loss.  Incidentally the positioning of all the components are close clustered so the typical caveats have been addressed.  After reading my abbreviated tale of  tribulation, is there anyone out there who may  recognize where I may focus  next?  Any help would be appreciated. I sense I’m close to getting the old gal up and running.  For the Drake purists,  I exercised extreme care  to keep the amplifier  OEM so it appears to be a factory stock unit.   Regards, Ron W2CQM/3        
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