[AMPS] ZSAC for a pair of 3-500ZG's?

[Tom Rauch]

Hi Eric

> I am noticing that the ZSAC current when set to the SSB position is around
> 300ma Ip for 4000v Ep, and 150ma Ip for 3400v Ep when set to the CW
> position. These levels seem hi for a pair of 500Z's, and when keyed with
> no drive, the tubes get orange within 15 seconds or so. The zener diode is
> rated @ 10v, and seems to check OK. Is this a result of using Chinese
> tubes, or normal for a 10v bias diode ?

Red is good for metal anode 3-500Z tubes, because the getting 
agent is coated on the anodes. The gettering agent absorbs 
different gasses at different temperatures, and so to be fully 
effective the anode must show some color.

If you never operate a 3-500Z with color, it will gas-up and arc.

Since you have Chinese tubes, you can expect short tube life. 
They are very poor life tubes, and especially prone to gas ingress 
through seals and emission loss.   
 
> I know that too low of a ZSAC results in splatter, but other than excess
> heat, what is the downside of having a higher ZSAC ?

ZSAC means very little in IMD, as long as the tube is biased into 
conduction by at least 5% of the peak anode current. If you watch 
IMD in a two tone test (which does not tests for poor supply 
stability by the way), you will see very little change is IMD over a 
wide range of anode current. You will also see some IMD products 
are reduced as bias is reduced, while others increase.

As long as you have anode current of at least 20 mA, the typical 
SSB exciter will be many times worse than the PA. Typical 
exciters are only down -25 to -30 dB below one tone of a two tone 
test. A 3-500Z properly loaded and using 40 mA of quiescent 
current will be about -35 to -40 dB down in the same test. If you 
move quiescent current up 400%, IMD only improves about 1 dB.

> BTW, filament voltage is around 5.2v , measured at the point the
> transformer enters the RF deck. When measured on the transformer side of
> the choke, how much higher should I be looking for to result in 5.0v at
> the tube ?

Measure it at the tube. I do that by wrapping small teflon wire (wire-
wrap wire) around the pins and plugging the tube in. Be sure you 
disable HV for the test.






73, Tom W8JI
w8ji@contesting.com

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