I have no definitive answer to this Jeff as my only experience with
Russian tubes has been the 4CX400A used as replacements in commercial
amps for expensive 8122's for instance. In a NCL-2000 they have
substantially better IMD due to improved internals.
Pulse service as used by the Russians is as Class C switches in radar
modulators and in the USA we currently think in terms of ultra linear
medical applications. Huge difference!
It is rather simple to determine an acceptable B+ by using the very
basic Eimac/ARRL 2 tone test procedure. While it is far from adequate
for human voice it can be used to compare apples against published data.
Any tube that cant make -30dB 3rd order IMD at a reasonable B+ should be
used for target practice along with the ham that puts it on the air (-;
>From stories Ive heard (unconfirmed) some Russian tubes are in the 6LQ6
sweep tube range of high teens IMD. A few homebrew amps I know of
roughly confirm this on the air.
The much vaunted 4CX800A tetrode as used for a decade or more have to be
biased to high ZSAC levels inorder to produce fair IMD.
Since RTTY is steady state there is no IMD concern, it could just as
well be Class C to reduce wasted heat. The primary concern is keeping
tube seals intact as well as not pushing beyond its capability if life
is of importance.
If you want legal limit with IMD in the mid -40's run four 3-500Z's at
1500-1750V. Even Chinese tubes should last for decades at that voltage.
Carl
KM1H
----- Original Message -----
From: "Jeff Blaine" <keepwalking188@yahoo.com>
To: "Carl" <km1h@jeremy.mv.com>
Cc: "Ulf Tjerneld" <star@frizon.org>; <amps@contesting.com>
Sent: Saturday, May 16, 2009 11:25 PM
Subject: Re: [Amps] first time questions
> Carl,
>
> Righ you are. Yet here it's hard to know in the case of these tubes.
> They are pulse rated. So what is the "proper" high voltage? The GI7B
> range could be argued to run between 1800-9KV. Pretty big spread.
> The ham application of these tubes, especially in RTTY type use
> (abuse?) conditions is just about as far as you can get away from
> pulsed radar duty. I could not find a reliable source of data on the
> best operating sweet spots - and with 10 suggestions from the
> community, I had 10 different answers - there is not much data-based
> rules of thumb on the operating domain and implications.
>
> In my case, I ended up using a variac arrangement that would let me
> move the plate from 1500-3400v. And it took a long time to work out
> the "sweet" spot on the GI7b. For the GS35, I think the similar thing
> would be needed there.
>
> These are more esoteric comments though. Clarly the tube works and
> works well, almost without regard to the B+. Amazing well.
>
> These old tubes would be a good PhD project for an EE student who
> knows how to use the Tagucci DOE methods technique.
>
> 73/jeff/ac0c
>
> Carl wrote:
>> To add to that IMD also gets worse as Ep is increased especially when
>> it is operated above its published ratings for any class of
>> operation.
>>
>> Carl
>> KM1H
>>
>>
>>
>> ----- Original Message ----- From: "Jeff Blaine"
>> <keepwalking188@yahoo.com>
>> To: "Ulf Tjerneld" <star@frizon.org>
>> Cc: <amps@contesting.com>
>> Sent: Saturday, May 16, 2009 8:59 PM
>> Subject: Re: [Amps] first time questions
>>
>>
>>> Ulf,
>>>
>>> To make the pi calculations, you should consider using the LOADED
>>> plate
>>> voltage. Idle plate voltage is not a factor. The difference,
>>> however,
>>> is not very much. Maybe 10-20%...
>>>
>>> And on #3, you are right. The voltage and current are unimportant
>>> when
>>> considered SEPERATELY. Unfortunately, there is not very much
>>> emperical
>>> data published on the Russian tubes so picking the optimal B+ is
>>> hard to
>>> do. Generally speaking, higher B+ is better as the efficiency and
>>> gain
>>> is better. But in my experience with the GI7b, at a certain voltage
>>> point, the efficiency of the amp starts to drop. At low B+, the PO
>>> is
>>> fine - the trouble is the Ig gets high due to the low gain of the
>>> tube
>>> at that lower B+.
>>>
>>> 73/jeff.ac0c
>>>
>>>
>>> Ulf Tjerneld wrote:
>>>> Hi all,
>>>>
>>>> I'm collecting parts for an amp-project. And I have som newbie
>>>> questions:
>>>>
>>>> 1, Maybe this question is silly: When calculating the Pi network
>>>> for
>>>> an amplifier with GI7-B's I'm aiming for an unloaded plate voltage
>>>> of
>>>> 2500 volts. With one tube this means that the plate current would
>>>> be
>>>> 350 mA for an input power of about 875 watts and not dissipating
>>>> more
>>>> than 350 watts. If I'm instead going to use a pair of tubes, this
>>>> means that I would input 700 mA in the Pi network formula. Right?
>>>>
>>>> 2, I have a perfect capacitor for the Pi-tank in the above example.
>>>> But is rated exactly 2500 volts (Johnson something...). Would that
>>>> be
>>>> to little for an unloaded plate voltage of 2500 volts?
>>>>
>>>> 3, Most people that builds amplifiers based on the GS35B use a
>>>> plate
>>>> voltage of around 3500 volts. This would mean a plate current of
>>>> about
>>>> 700 mA for 2500 watts input. But let's say I only have 2500 volts,
>>>> but
>>>> enough iron in the transformer. Would I achieve equal results if I
>>>> let
>>>> the the tube draw more current? I.e. let the tube run with 2500
>>>> volts
>>>> and a plate current of 1 A? And to extrapolate further, even go to
>>>> a
>>>> plate voltage of 2000 volts resulting in a current of 1.3 A? To
>>>> summarize: does it matter what combination of voltage and current I
>>>> use as long as it is within the specs of the tube?
>>>>
>>>> All the best
>>>>
>>>> Ulf/SM0NOR
>>>>
>>>>
>>>> _______________________________________________
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>>>> Amps@contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>
>>>>
>>>
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>>
>>
>
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