High Gid Current

[NE7X]

I acquired a very cosmetically clean Henry 2K Classic HF amplifier for a really below market price, it uses two 3-500Z tubes in grounded grid configuration. However it does has a strange problem which I can’t find the cause.

The original problem, the 10V bias zener was shorted and the two tubes were both gone. I replaced the 10V zener and the two tubes. Now the amplifier puts RF output and the Ip reads normal. However the grid current meter pins full scale when I give anything over 30 watts drive.

I checked all the other resistors in the BIAS circuit, disconnected the ALC circuit and validated and replaced al the grid meter shunt resistors and finally replaced all the .01 bypass caps. Still have the issue.

Could the filament winding on the transformer be shorted? If so, how do I check? The tubes appear to light up OK. The filament winding are part of the same HV transformer.

Any suggestions what might be causing the high grid current?

Thanks.,

73s, Thomas NE7X…
http://ne7x.com

[WB2WIK]

3-500Zs shouldn't really fail if the zener shorts out. That's weird in itself, since these tubes can operate at "zero bias," even at 3500V. They'll draw more idling current, heat up the shack, waste power...but they shouldn't get the tubes so hot that they'll fail, unless maybe they're run that way a very long time. The zener is there really to reduce the ZSAC (idling current), saving heat and power while still maintaining linearity. It's not even very critical.

What might be happening is you're just "underloaded." Grid current is very much a function of amplifier loading. You can't "tune up" and 30W drive and then just add more drive -- grid current will soar. More drive = more loading required.

As a check: What happens if you tune up at 20-30W or whatever, then increase drive a bit, watching grid current closely, then when you see the Ig going up, turn the LOAD control -- possibly quite a lot, and usually clockwise in most amps (clockwise usually reduces the LOAD capacitance, thus increasing loading -- although some amps might have this backwards). If turning the LOAD control has an obvious effect on Ig (it should), keep turning it while keeping an eye on both Ig and also output power, then increase drive and do it again. If the amp is working properly it should easily deliver a kilowatt output power while the Ig is under 200mA -- IF it is loaded heavily enough.

[NE7X]

Sorry, my error, the original 10V zener "opened" not shorted. It has a 220 ohm 2W resistor in paralle. It was good, did not fail. The orginal 3-500Z tubes which were bad, had burnt black spots on the glass. This tells me the tubes got extredmely hot before they died.

I have several other amplifiers here in the shack and I never noticed any of them "pinning" the Ig meter full scale when driving with 30 to 100 watts. Let me play with the loading and see what happens.

Thanks.

Thomas NE7X...

[WB2WIK]

Sorry, my error, the original 10V zener "opened" not shorted. It has a 220 ohm 2W resistor in paralle. It was good, did not fail. The orginal 3-500Z tubes which were bad, had burnt black spots on the glass. This tells me the tubes got extredmely hot before they died.

I have several other amplifiers here in the shack and I never noticed any of them "pinning" the Ig meter full scale when driving with 30 to 100 watts. Let me play with the loading and see what happens.

Thanks.

Thomas NE7X...

Wow, it's unusual for a zener to fail "open," that's a bit weird, too.

But anyway, the Henry tunes a bit differently from some other amps since it has a rotary inductor that track tunes and is controlled by the PLATE TUNE knob, and most have a switch that switches in fixed amounts of capacitance on various bands. Definitely crank the LOADing up when you increase drive, go back and re-check the TUNE for an output power peak, then add more loading again.

Of course, maybe something's still wrong with the grid current monitoring. With 30W drive it should achieve probably 400W or so output power...is that what you're getting?

[NE7X]

I did as you suggested, no luck. Actually I made neg progress.

I was driving it with 80-90 watts, the amp was putting out about 1100 watts into dummy load, and the Ig meter was pegged, no mater where I placed the loading. I did have a very nice Ip dip. Then after about 3 mins I head an arc and the amp stopped putting out any power and the tubes started to turn red, Ig was still pegged. I unkeyed, and keyed up again, no RF out, Ig meter full scale, and tubes are now turring red. HV is good.

Thomas...

[WB2WIK]

If HV is good and you have grid current but no output, maybe the plate RFC opened or the T-R relay lost contact on the "output" switching side. But if the tubes are turning red, then the HV must be making it to their plates, so it wouldn't be a bad RF choke. Amps are pretty simple when they don't involve microprocessors and stuff (and the Henry did not).

Henry amps are mostly pretty "beefy" and it's not common for them to fail. They do have one potentially weak point, which is the connection between the sliding contact and the edge wound roller inductor. If the contact loses connection with the coil, or gets wedged in between turns of the coil (can happen from mechanical shock) then it's possible for the amp to draw lots of current and have no output since the tank is far out of resonance.

Maybe something like that occurred. You have to remove all the covers and look carefully.