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

[measures]

>
>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.   

There are several manufacturers in China who make 3-500Z/8008s.  Rumour 
has it that one is better than the rest, however, there is no way to tell 
who made what.  
> 
>> 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. 

The peak anode current for linear operation in the 3-500Z/8008 is c. 
1.2A.  5% is 60mA.  Eimac recommends a bit more ZSAC.  In tests at 60mA 
of bias  in my amplifier, listeners reported hearing more feculence in my 
signal than they did at 75mA.  

> 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. 

This is apparently not the case.  Try this yourself to see if Tom's 
pronouncement holds water.  

>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.
>
[chortle]  

>> 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.
>
good idea when you can't get to the pins.  .  
....

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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