Monday, December 15, 2008

QRO a go-go

I had some spare cash and with the pound falling fast I decided it would be a good time to invest in a reasonable HF amplifier for my station while my money was still worth something.

I already have a Tokyo Hy-Power THP-200BDX which has given me three years of good service. This is really a mobile amp and delivers 200 watts. Although its got good gain I thought something that could provide UK full legal was in order.

I had my eye on the Tokyo Hy-Power THP-700B for a while and at £800 seemed like the best bang for buck out there. Tokyo Hy-Power advertise this amplifier for mobile operation but I think the remote control of the 200BDX and small size are more suited to this. The 700B can be run in two configurations giving either 400 watts out for 10watts in or 600watts for 100watts in. The amp contains four parallel amplifiers each containing 2 THP120 devices.

In the 400 watt configuration one of the amps is a pre-driver with three delivering the required output. For increased resilience I decided to drive mine in the 600watt configuration but only to 400 watts (legal limit in the UK) which gives me plenty of overhead for safety.

The amp arrived much sooner than I anticipated and I was soon wiring it up to see how it performed on RTTY and SSB.

Unlike valve amps solid state devices present a different set of issues to successful use. The amp requires about 72 amps at 13.8v for full output so most supplies aren’t man enough for this job. Luckily I run the shack off a single supply 13.8V supply capable of 120amps, the PSU was made by Manson and is a prerequisite if you intend to run lots of power at this voltage. I found the cable THP provided with the amp was getting warm on transmit so I shortened it and doubled it up to ensure there was very little voltage drop.

The photos below will give you some idea of how well this amp is constructed. I think this amplifier design has in service for a few years now and it doesn’t have automatic band switching but I can live with pushing a button. Once open the main amplifier boards on their heatsink hinge upwards for easy maintenance

In service I found the two fans a bit noisy so I have modified them to run at 8 volts on receive. Full overload, VSWR, and thermal protection is built into the amp. This all works well but I really value my Palstar AT-Auto antenna auto tuner which ensures the correct antenna and tuner settings are always selected.

The amp easily provides 400 watts continuous for RTTY on all bands so even those KW Russian and US stations will now copy me. I have set up my FT-2000 to give the correct input power on each band which is between 35watts on 3.5Mhz to about 42 on 21MHz. If I run the FT-2000 at 100 watts output I can drive it to the full 600 watts for SSB but of course that really isn’t necessary. I just keep that option for busting those big DX pileups ;o)

So now its back to improving the antennas…!


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