QUOTING (******* indicates redacted text)


February 2, 1995


George Wilson, W4OYI (the then President of the ARRL)

I just received an info copy of correspondence to you from Jack Najork, W5FG concerning Rich Measures' article "The Nearly Perfect Amplifier" which appeared in January, 1994 QST. At the time, I too felt that the mass of deliberate criticism of Measures' original article (9/94 QST) bordered on unethical journalism. I have just re-read both the original article and the rebuttals by Telewski et al, and while some of Measures' statements cannot be fully substantiated, that is not really the point. I agree with Najork that closer scrutiny by the publication review committee should have been made prior to publication. If those members on that committee are technically unqualified with regard to vacuum tube HF power amplifiers, then that should have been the time to ask the "hit team" to review, not after the fact with the September rebuttal.

However, I believe Measures' work contains a lot of good points. Having probably built as many home brew amplifiers in the last 45 years as all of those "experts" combined, I know a little about the subject. Several years ago, KD9Q and myself suffered the same identical problem on separate occasions as Measures describes. This is where UHF/VHF parasitics destroy a tube. We both lost perfectly good 8877s due to internal flashover within the tube itself most probably caused by an improper load (wrong antenna) being switched to the amplifier. In my case I had a guest operator who inadvertently switched in a 15M Yagi while operating on 20M. There was a loud "bang", and the 8877 along with the bias circuitry, RF plate choke, and both meters were destroyed. I didn't need a network analyzer as is suggested by Telewski to understand the problem! The wrong antenna presented the tube with an unrealistic plate impedance prompting the parasitic VHF or UHF oscillation. KD9Q's amplifier, which was commercially built, experienced a similar situation.

My amplifier was basically my copy of a similar unit published in Eimac's Application Notes. It uses Jennings vacuum variables for both tune and load, and with a Jennings vacuum output relay sequenced with a slower frame type relay for the input.

After we both lost expensive 8877s, I installed Measures parasitic suppression kit in both amplifiers. I am happy to say that the problem has never recurred to either of us. For Telewski to categorically state that it is only the external circuitry is at fault is nonsense. The loop which causes the oscillation includes the tube itself as well as all surrounding external circuitry.

With one of my amplifiers I use 1000 volt PlV avalanche diodes without equalizing resistors or capacitors in a bridge HV supply consisting of 7 each per leg. This represents a total of 7 KV PlV. The transformer is a hipersil from Peter DahI rated at 3500 V @ lA ICAS. The oil-filled filter capacitors are a combined 18 mfd at 5 KV. The supply delivers 4200 volts under load to a single 4-1000A in grounded grid. I have never experienced any rectifier problems with this power supply. In the rebuttal, several of the experts go to great length to say it can't work.

As far as filament inrush current is concerned, for years I simply applied full voltage to a pair 3-5OOZs at turn on. From that experience I have a box full of shorted grid to filament pullouts! The rebuttal by Telewski and Rauch discounts this phenomena as rare in amateur applications. Since the late 70s,l have used a stepping sequence which slowly (3 seconds) allows the tubes to receive filament power. I have never had a subsequent failure. (Knock on wood!)

All in all, I feel the September 1994 rebuttal was vindictive. I hope that there was not a 'secret agenda" underlying the rebuttal. In the interest of journalistic fairness, I believe that Najork's request for an apology is in order. I also believe that Measures' rebuttal should be printed in QST.


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