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

[Jim W7RY]

A Cantana is a very poor dummy load. Especially when it gets hot from 1500 
watts at high duty cycle. It drifts horribly!  Try it on the antenna or 
better quality load.
10 and 15 are pretty dead these days.

73
Jim W7RY


----- Original Message ----- 
From: "Karl Brandt" <nd8dx at yahoo.com>
To: <amps at contesting.com>
Cc: "Gus VO1MP" <gussam at nl.rogers.com>
Sent: Sunday, February 01, 2009 6:28 AM
Subject: [Amps] Alpha 78 Grid current anomily (possible problem)


>
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>
>
> 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
>
>
>
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>


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