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Re: [Amps] 4 KW?

To: Adrian <>,
Subject: Re: [Amps] 4 KW?
From: Aleksandar Petkovic <>
Date: Fri, 24 Jun 2022 12:57:34 +0800
List-post: <>
The main one is that you cannot get a good new tube in this country.

Horror stories abound of people spending hundreds of dollars again and again, only to have tubes fail in very short order. Even ones that they bought from OM Power.

All FU728F tubes that you see advertised on Ebay are factory rejects. They should never have found their way onto the market.

I friend of mine went through 2 tubes in 2 months. He now uses the 4CX1000A in his DX-2SP and we know he'll never have another tube fault in that amplifier. Thankfully Rudi  built that model such that 4CX1000A tubes can easily be substituted. He had the socket modified and had a 6V tap on the transformer for the heater. A quick tickle of the pre-bias and bias and you are on the air with a proper tube that will last forever.

I bought 10 x FU728F tubes ten years ago. They all failed very quickly, except for one, which has been in daily use for the past couple of months now. That was when the tubes were US$100 each landed here in VK. I sold them for virtual cost price. I lost a lot of money on them. I still have 2 of the failed/returned tubes left. Each year, I manage to give one or two away as ornaments at our local hamfest.

So, as I said, don't ever go anywhere near a FU728F amplifier unless it has already been retrofitted with the 4CX1000A.

73, Alek, VK6APK

On 24/06/2022 10:00 am, Adrian wrote:
Alek, What faults / problems did you find or hear about with the 728 ?



On 24/6/22 10:14, Alek Petkovic wrote:
And those FU728F tubes are junk. It is all but impossible to get a good one these days. The supply has completely dried up.

I would not touch an amplifier that uses that tube.

73, Alek, VK6APK

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