[Amps] QB3-300 Amplifier

[Peter Frenning, OZ1PIF]

Alberto,

Pls note that g2 always must be at same or higher potential as g1, g2 
potential can be used as a very convenient gain regulation. If tied together 
the gain will be approx. 6-7dB at 2.5KV

Vy 73 de OZ1PIF, Peter

**********  OZ1PIF **************
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Peter Frenning
Ternevej 23
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***********************************
----- Original Message ----- 
From: "Alberto" <re349 at yahoo.com>
To: "R. Measures" <r at somis.org>
Cc: <amps at contesting.com>
Sent: Friday, October 29, 2004 11:33 AM
Subject: Re: [Amps] QB3-300 Amplifier


> Thanks Alek, Richard and Peter for your answers.
>
> Richard, thanks for the notes about high voltage
> capabilities of these tubes. I'll use a HV supply of
> 2.500V since this output is available in my shack.
> More than 2.500V means constructing a new PS, which is
> quite difficult for me since I have no access to cheap
> HV transformers right now.
>
> Anyway, reading your experiences, now I think G-G
> operation will be a good option. According to FL2000B
> mod by Peter (OZ1PIF), QB3-300 should work in G-G
> service as follows:
> Va=2.500V
> Vg2=0 (tightly grounded)
> Vg1=between 0 and 10 volts to get apropriate Ia.
>
> Could you confirm this data, please?
>
> Thanks again!
>
> Alberto
> EA4PL
>
> --- "R. Measures" <r at somis.org> wrote:
>
>>
>> On Oct 28, 2004, at 4:44 AM, Alberto wrote:
>>
>> > Hello!
>> >
>> > Recently, i got some spare QB3-300 tubes
>> (equivalent
>> > SRS455, 6155, 4-125), so I´m planning a new SSB
>> > amplifier using them. I've seen lots of
>> configurations
>> > for these tubes in PA operation. Watching tube
>> > datasheet, it seems like grid driven class B
>> operation
>> > would work: Va=2500V; Vg2=350V; Vg1=-40V aprox.
>>
>> Alberto --  This will produce so much IMD on SSB
>> that you would do well
>> not to give your call-sign within 15kHz of a band
>> edge.
>> >
>> > Some PA builders say also g-g configuration would
>> > work; that is grid and screen connected and
>> grounded
>> > together. (I don't think so)
>>
>> I think they know what they are talking about
>> because 4-125As are not
>> ideal for grid-driven linear  service.  If I was
>> building a SSB linear
>> using 4-125As, I would use a 200w transceiver, drive
>> 6 to 8 of the
>> little suckers in parallel. and use more than 2500V
>> on the anodes for
>> SSB.  The reason for more PS V on SSB is that the
>> 2500V rating is for
>> plate modulation, which means that the 4-125A has no
>> problem with a
>> peak anode supply potential of 5000V during 100% AM.
>>  Thus, my guess is
>> that you could use a 4000v anode supply for SSB --
>> provided you did not
>> use AØ/NØN during tuneup, but instead tuned up with
>> 50wpm dits or tuned
>> up with a tuning pulser..  With AØ at 4000vdc, one
>> should expect to
>> burn a few holes in the tantalum anodes.
>> >
>> > I'd like to know the usual ratings for amateur SSB
>> > operation. Has anybody experience using QB3-300 in
>> PA?
>> >
>> > Thanks!
>> > Alberto
>> > EA4PL
>> >
>> >
>> >
>> > __________________________________
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>> >
>>
>> Richard L. Measures, AG6K, 805.386.3734.
>> www.somis.org
>>
>>
>
>
>
>
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