>Rich you now said:
>
>"4. disregard the users' manual's ok of more than 500mA of anode current.
> No 3-500Z/8008 manufacturer has Ever rated this tube at more than 400mA."
>
>This 500 mA is a EIMAC IVS rating
There is no IVS rating on the Eimac 3-500Z/8008 spec sheet. One of our
"recognized amplifier experts" once stated that he had an official Eimac
letter approving his >500mA figure in the SB-1000 manual. My guess is
that he never produced the letter because it never existed. The
3-500Z/8008 is simply not capable of producing that much peak emission in
linear operation. .
> - the same as beeing used for the 8873,
>8875 and 8874.
The IVS rating only applies to some oxide-cathode tubes - such as the
8873/4/5 and 3CX800A7. . The 3-500Z/8008 does not have an
oxide-cathode, Hans.
> These tubes are rated 350 mA max anode current but have been
>used up to 500 mA in many amplifiers in the past. With no harm if you are
>careful
It is impossible to harm the epitaxially-bonded tungsten dicarbide
emissive layer in a filament-type tube.
>- my old HENRY K2000 still uses the original 8873 tubes with almost
>full output. But the question is, if the now avaiable emission, after more
>than 20 years usage, does give flat-topping?
In a new 3-500Z/8008, flat-topping begins at roughly 420mA. 400mA
appears to be a reasonable rating.
>
>The IVS system is a trade -off beetween tube life, distorsion and economy.
>If you are not on the air all the time this IVS system is cheaper.
>Eventually distorsion will increase to the point where the amp will be the
>dominating factor in the transceiver + amp system. Because, as far as I
>understand, 13 volt finals in transceivers have more distorsion than 28 or
>48 versions and may in most cases dominate the system total distorsion. If
>you on the other hand use one of the high voltage transceiver finals such as
>the excellent low distorsion TS 950 ( 48 volts ) or one of the old COLLINS
>rigs , you have to use negative feedback ( as in the SB220 )
Not really.
> and, as you
>say, limit your anode current to max 400 mA in a 3 -500Z. In order to match
>the low distorsion transceiver.
>
>Have not seen any distorsion measurments with 500 mA anode current- so
>maybe you are right in your statement? Better ask EIMAC, maybe.
>
I measured the distortion from a SB-1000 driven to 500mA. It was not
good news. The owner added 25- ohms of RF-NFB resistance to the cathode,
the anode current decreased to c. 400mA, the input match improved on some
bands, and the splatter was so small that it didn't matter. .
cheers, Hans
>
>See also Ham Radio 1/1971: " Intermittent Voice Operation of Power Tubes"
>by W6SAI
>----------
>>FrÂn: measures <measures@vcnet.com>
>>Till: "AMPS" <amps@contesting.com>
>>?mne: [AMPS] AL-80 / SB-1000 circuit improvements
>>Datum: mÂn 28 feb 2000 16.22
>>
>
>>
>>To owners of SB-1000s and AL-80s: :
>>
>>The B+ to gnd bypass cap. (C-23 in SB1000) at the base of the HV RFC
>>(RFC-3) is 1000pF. At 1.8MHz, 1000pF has -j88 ohms of reactance. It is
>>my opinion that 88 ohms is not a sufficient bypass. My unsolicited
>>advise is to:
>>
>>1. parallel more C with C23 to keep RF away from PS components during
>>160m and 80m operation.
>>2. add a glitch protection resistor in the B+ lead.
>>3. add 200a-pk glitch diodes to protect the meter movements.
>>4. disregard the users' manual's ok of more than 500mA of anode current.
>> No 3-500Z/8008 manufacturer has Ever rated this tube at more than 400mA.
>>
>>Might not the designer improve his credibility if he corrected his
>>misstatement?.
>>
>>good luck.
>>
>>- Rich..., 805.386.3734, www.vcnet.com/measures.
>>end
>>
>>
>>--
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>>
>>
>
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