Bill,
> Does anyone have any further thoughts or comments on this? Is it
> OK to put the amp into WSJT service at 400 watts?
WSJT is a linear mode (multiple tone frequency shift keying) - it
does not tolerate clipping/compression well. Since the 3-500 is
specified for for 400 mA MAXIMUM you are running it well into
compression (current starvation).
Further, if you look at all of the specifications (typical operating
conditions), you will find that ALL of the cases are limited to
400 mA per tube no matter what the plate voltage. It would seem to
me that the continuous carrier (one minute key down, followed by one
minute receive) nature of WSJT would result in premature tube failure
at a sustained 520 mA due to filament depletion.
Some other considerations ... 500 mA at 2000V represents 1 KW input
but you say you're only getting 400 W out. The efficiency is too
low (less than 40%). That indicates potential issues with tuning,
the design of the output tank or possibly an accuracy problem with
the plate current metering. You should be seeing 60 to 65%
efficiency from a 3-500Z.
I would not drive the tube to more than 400 mA single tone plate
current - no matter what the power output. If you are not happy
with 300 - 325W output in your amplifier and have checked all of
the other items, I suggest replacing the plate transformer with
one from the Ameritron AL-80B (Ameritron will sell transformers).
>> Guess I had interpreted the spec sheet as meaning that, if you
>> configure the tube in class B grounded grid and apply HV of 2000v,
>> you should expect to see 400ma of plate current and 500 watts of
>> peak "useful" output.
The key is in the "Maximum Ratings" section. 400 mA is the
maximum allowable DC "key down" plate current. You might be
able to "get away" with short duration overloads but sustained
excess current like one would get in RTTY or WSJT are likely
to damage the tube.
73,
... Joe, W4TV
On 5/23/2010 8:25 AM, Bill Dzurilla wrote:
> Does anyone have any further thoughts or comments on this? Is it OK
> to put the amp into WSJT service at 400 watts?
>
> 73, Bill NZ5N
>
> --- On Sat, 5/22/10, Bill Dzurilla<billdz.geo@yahoo.com> wrote:
>
>> From: Bill Dzurilla<billdz.geo@yahoo.com> Subject: Re: How to read
>> the 3-500Z spec sheet? To: amps@contesting.com Date: Saturday, May
>> 22, 2010, 5:54 PM Thanks for the reply, but I don't think I
>> follow.
>>
>> Guess I had interpreted the spec sheet as meaning that, if you
>> configure the tube in class B grounded grid and apply HV of 2000v,
>> you should expect to see 400ma of plate current and 500 watts of
>> peak "useful" output.
>>
>> I am just trying to figure out if this homebrew amp, which does not
>> have any manual or detailed info, is working as it should. If the
>> spec sheet means what I think it means, it seems the amp is in the
>> ballpark.
>>
>> 73, Bill NZ5N
>>> Spec sheets don't know anything about load or no load.
>> If it says for
>>> example, 3000 volts at 500 mA, that's what it means.
>>
>>> 73, Bill W6WRT
>>
>>
>> --- On Sat, 5/22/10, Bill Dzurilla<billdz.geo@yahoo.com> wrote:
>>
>>> From: Bill Dzurilla<billdz.geo@yahoo.com> Subject: How to read
>>> the 3-500Z spec sheet? To: amps@contesting.com Date: Saturday,
>>> May 22, 2010, 10:39 AM Hi,
>>>
>>> I'm looking at the spec sheet for the 3-500Z at
>>> http://www.scottbecker.net/tube/sheets/088/3/3-500Z.pdf
>>>
>>> At the top of the second pages, it gives numbers for various
>>> parameters at different levels of high
>> voltage
>>> (3000, 2500, 2000, and 1500). Are these voltages at load or
>>> no-load? And how do Single-Tone DC Plate Current and Peak
>>> Envelope Useful Output Power
>> correlate with
>>> key down plate current and output?
>>>
>>> The reason I ask is because I picked up a homebrew 6m
>> amp
>>> with a single 3-500Z, which uses a power transformer
>> from an
>>> SB-200. Key down power output is 400w. Assuming the amp's
>>> meters are accurate, no-load plate voltage
>> is
>>> 2300v, and plate voltage under full load is 1950v. Plate current
>>> with no drive is 100ma and plate current
>> with
>>> full drive is .52a. About 40 watts of drive
>> produces
>>> the full 400w out. Further increases in drive, up
>> to
>>> 100w, do not increase output.
>>>
>>> Judging from the spec sheet's 2000v table, my guess is
>> that
>>> the amp is performing as well as it can with this
>> power
>>> supply. I know that a converted AL-80B with a
>> single
>>> 3-500Z can produce 900w or so on 6m, but it has a
>> 3000v
>>> p/s.
>>>
>>> Do you think it would be OK to run this amp at the
>> full
>>> 400w in WSJT (continuous carrier) modes?
>>>
>>> 73, Bill NZ5N
>>>
>>>
>>>
>>>
>>
>>
>>
>>
>
>
>
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