I hate it went projects go BOOM!
Phil K5SSR
A wise man once said nothing....
On Feb 20, 2014, at 10:24 AM, Jim Garland <4cx250b@miamioh.edu> wrote:
> For the past two years, I've been woking on a duo-band 80m/160m amplifier
> using three GU74Bs. I've debugged and bench-tested the control grid and
> screen grid regulators, QSK circuitry, plate and grid-trip circuits, and
> various interlocks (e.g., to kill the screen voltage if the HV fails). The
> amp uses a pi-L circuit with Q=10 (bandswitched with vacuum relays), with
> 2500V on the anodes (although I can change taps on the plate xfomr to raise
> it up to 3000V if desired).. The plate trip circuit is set at 2.1A. and I've
> initially set the operating bias voltage at -58V and the screen voltage at
> 300V. The tubes are fused with three 900mA HV microwave oven fuses, each
> rated at 5kV. The input network is a passive 50 ohm 6dB power divider rated
> at 150W. For initial testing, I removed all but one of the fuses. Before
> applying HV, I conditioned the tubes by running them with just filaments
> (and cutoff bias) for 50 hous.
>
>
>
> Yesterday, was the initial checkout of the amp with HV and RF drive. My
> initial test was just to key up the amp with no drive and look at the
> resting plate current. It was about 250mA (600W), which strikes me as fine
> for CW but a bit low for SSB. Any comments on this point will be very
> welcome. I used an IR thermometer to measure the airflow temperature and
> found that two of the tubes were matched to within a few degrees F, but one
> tube was clearly running hotter, by 15-20 degrees F. I've got extra tubes,
> so I plan to swap them around to find three that are more closely matched. I
> have no way to measure their transconductance, but matching the resting
> current is probably good enough.
>
>
>
> With drive applied, the amplifier tuned easily and in accord with the Pi-L
> network calculations. Everything seemed very stable, although with the top
> cover off the amp the dimmer for the track lighting in my station went
> crazy. Power output rose to about 2500W with 60W drive (15W after the 6dB
> pad), but with no indication of any grid current at all. I don't measure
> screen current directly, but have a red LED that lights when the screen
> current goes positive. Tuning, therefore, consisted of peaking the power
> output and adjusting the loading until the screen LED barely lighted. The
> plate current dip seemed very shallow to me, possibly an indication that the
> screen voltage is a bit low.
>
>
>
> The tests stopped when there was a boom and the primary circuit breaker
> tripped. The amp had been running for about 60 sec at about 2200W output (I
> have a 2500W Bird dummy load), with about 1.5A plate current. At that
> point, I remembered I'd removed two of the HV fuses, leaving just one 900 mA
> fuse in the HV lead. I'm not surprised the fuse blew, but don't know why
> there as a boom and the circuit breaker tripped. I'm guessing I may have
> flashed over one of the tubes, probably by not nudging up the plate current
> slowly enough to give the news tubes a chance to burn in under load.
>
>
>
> So that's the status of my project. I don't know why I'm not getting any
> grid current at full power, but I'm guessing that's probably normal. With
> the tubes' bias tilted more toward AB2 (i.e., with a lower bias) I'd
> probably see some current. I've not got any experience with the GU74Bs,
> however, so I don't really know what to expect.
>
>
>
> So that's the status of my project. One of these days, hope to you work you
> all with it on the LF bands.
>
> 73,
>
> Jim W8ZR
>
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