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@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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