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Re: [Amps] QB3-300 Amplifier

To: Alberto <re349@yahoo.com>
Subject: Re: [Amps] QB3-300 Amplifier
From: R.Measures <r@somis.org>
Date: Thu, 28 Oct 2004 08:06:47 -0700
List-post: <mailto:amps@contesting.com>

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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