[Heath] Heath SB 220 problem

[John King]

I need assistance in diagnosing an  operational and loading problem with one of my two my SB 220 amplifiers.I have owned this amplifier since it was new. It has always performed extremely well although I have changed the bias and metering board three times over the 50 years I have owned. I replaced the bias and metering board with the W7RY board. The amplifier has proper plate voltage and I have even replaced the two 3-500 Z Eimacs with a NEW pair of Machlett 3-500 ZG tubes.
The question is: How is it malfunctioning??
Using a 100 watt exciter the SB 220 load control will not advance the plate current.  The C1 does not make power by dipping the tank capacitor. The SB 220 is connected to a 50 ohm dummy load and a wattmeter and only 150 watts is produced by the SB 220 with 100 watts drive. The load control does not increase plate current or power as if the C2 was not even the circuit. The power that is produced by peaking C1 when the power should develop with a dip. The new 3-500ZG tubes do not show any color.  I have checked the output relay and it is making contact as it should.
One unusual  factor is  when the amp is keyed the idling current is very LOW even with almost 3,000 vdc on the plates of the tubes. Does that indicate that the tubes are over biased ? The metering and bias board  has been replaced and the low idling current problem has existed since the metering and bias board has been installed. The board was replaced due to a malfunction in the former metering and bias board. It seems like a bias problem because the tubes do not draw additional current, as in loading, when the C1 is not dipped or when C2 is advanced as in loading. 

I have been tuning and using RF amplifiers for over 60 years and most want to "run away" if not kept in resonance whereas this SB 220 acts like it has been tranquilized!!
Your input based on your experience with the SB 220 will be greatly appreciated !
73, John, K5PGW







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