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Re: [Amps] Failure of a pair of FU728F

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Subject: Re: [Amps] Failure of a pair of FU728F
From: Jim Garland <>
Date: Fri, 20 Sep 2019 10:05:21 -0600
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Two years ago, I completed my first tetrode amplifier (, a duo-band (80m/160m) design running three Russian GU74B tetrodes. The experience taught me a lot about the different design requirements between tetrode and triode amps.

From this experience, I learned that tetrodes require a stiffly regulated screen voltage (340V for my amplifier), benefit from tiered control grid voltage regulation (different operating bias for SSB and CW/digital modes, and higher cutoff bias), and need more sophistication in their safety trip and flashover protection circuits, than do triodes. In other words, tetrode amplfiers are harder to design, build, and debug that their triode equivalents, although this added complexity is offset somewhat by the simplicity of tetrode's passive grid-driven input circuitry. (Triode amps normally require bandswitched tuned input circuits, while tetrode amps use a simple 50 ohm resistive input.)

Despite the difference in design complexity between triode and tetrode amplifiers, however, their tuneup and operation are rather similar. During tuning, one has to repetitively dip the plate current and increase the loading, while slowly increasing the drive level, all the time keeping an eye on the grid current and, in the case of tetrode amplifiers, the screen current. For a tetrode amp, the screen current is usually a more sensitive indicator of proper tuneup than the grid current. Tetrode amplifiers normally run in class AB1 (zero grid current) and AB2 (a few mA of grid current). Since the grid current is usually about zero, there really isn't much point in monitoring it, except to see that it doesn't soar too high because of underloading or overdriving.

The important point in monitoring screen current is to keep it modestly positive when the amplifier is tuned for its rated key down maximum output. The screen current will then go negative if the drive power is reduced; a negative screen current is normal and doesn't signify mistuning. In my amplifier, I monitor the screen current with an LED, but don't measure it on a panel meter. The LED flickers red when the screen current goes positive, gets brighter as the current increases, and trips the amplifier off-line when it exceeds 50 mA.

It is possible to tune a tetrode amplfier at lower power levels than its full-bore rating, but I don't do that because it's too easy to overdrive the amp and exceed the screen and grid current ratings. The tuning just becomes too finicky. In my opinion, it's better always to tune the amplifier at its maximum design rating, and then reduce the drive to scale back the output power. (By the way, this recommendation also pertains to triode amps.) Doing so will assure that the amplifier is being operated with the proper tank circuit Q and plate load impedance. The downside is that the amplifier efficiency is lower when the drive is reduced, than it would be if it were tuned for maximum output at a lower drive level. In other words, my GU74B amplifier generates more heat if I tune it for 3kW output, and then reduce drive to get 1.5KW, than it would if I tuned it for maximum power output at 1.5KW to start with.To me, this added heat is a small price to pay to assure myself that the amp is running optimally with respect to all parameters, producing the cleanest signal, with large safety margins for the screen and control grids.


Jim W8ZR

On 9/20/2019 8:36 AM, Ron Youvan wrote:
  John KK9A, wrote:

I believe that the tuning of a tetrode is different than a triode - correct?  Are they tetrodes less reliable or more finicky?

  Tetrodes, if operated properly are a rugged as a tetrode, but they are
more sensitive to heater power, G1 drive power and screen current so
more care is required, in the design and operation, but gain and other
operating parameters are better.
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