Re: [Amps] QB3-300 Amplifier

[R.Measures <r@somis.org>]

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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