>
>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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