[Amps] Alpha 78 Grid current anomily (possible problem)

[Karl Brandt]

Hi All, 
 
I recently acquired a 78 and am just getting used to its characteristics. One thing that I've recently noticed, which to me seems like an indication of a possible problem, is that under 100%duty cycle transmissions ie tuning amp in RTTY etc. the Ig will drop some 20 or 30ma,  as things heat up. Also at times the Ig will fluctuate 5-10ma. This occurs in both manual tune and band-pass mode.  The decrease in Ig occurs faster and is greater on the higher bands. For example 15m, to get to l500 out requires abt 70 watts drive and some Ig "swamping", tune amp and wait a minute or so to let things cool down. Key amp, Ig starts at 90ma with an Ip of 1.28A ( meter doubler installed) over abt 10 seconds Ig has dropped to abt 70ma with some fluctuation of maybe 5ma.   While this is occurring there appears to be no change in Ip, plate voltage, or power output. I have changed to the spare trio of tubes (used), switched rigs, bypassed the wattmeter and
 everything else going straight from the amp to a Cantenna using a know good coax jumper and got the same results. On thing worth noting is tt when I first fired the amp up I discovered that idling currents were too high and subsequently determined tt the biasing zener was bad, replaced it and tt took care of tt issue. I hv "bounced" this question off N4UQ whom suggested possibly a problem with the bypass caps before and after the plate choke or "soft" tube or tubes. I have yet to try changing the bypass caps and figured before I did anything I'd see if anyone else has any ideas. Any thoughts, input, etc.  will be greatly appriciated. 
 
73 GB  Karl ND8DX