High Gid Current
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 canít 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?
73s, Thomas NE7XÖ
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.
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.
Wow, it's unusual for a zener to fail "open," that's a bit weird, too.
Originally Posted by NE7X
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?
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.
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.
Good work you guys are doing. Good troubleshooting, using your evidence and your brains and Steves experience.
Impressive. Please keep going.
The arc, followed by no output, seems to suggest a failure in a component in the tank circuit (or bad connection as Steve mentions).
Unlikely it was a flashover in a tube, and I'm assuming the tubes getting red just means due to idling curent.
When you said you were getting 1100 watts out, you had a nice dip with the tuning cap, but the loading cap had no effect - check the loading cap, any padding caps across the loading cap, and the bandswitch contacts that are involved. In fact pay very careful attention to any burn marks or carbon spots on the bandswitch. What band have you been testing on? If one of the low bands (160 or 80) go to 20 meters and try it, where there won't be any padding caps.
I haven't used every Henry amp in the world, but I've used several and owned a few. Most are unique in that the wafer bandswitch is only engaged to select the tuned input; there's no wafer at all in the output tank switching. Most I've had use a camshaft activated plunger mechanism for that, and the plate TUNE is a big rotary inductor, with selected doorknob caps to resonate it. The LOAD control is usually a variable capacitor, with selected doorknob caps to set its tuning range, also driven by a camshaft.
Originally Posted by K2XT
The advantage, of course, is that without a wafer, the switch can handle tremendous current (and voltage!). But the disadvantage is it's mechanically complex and if anything gets out of alignment, you could have the wrong caps selected for the band you're on. The advantage of the big edge-wound rotary inductor is that it does a great job as long as everything is perfect; the disadvantage is, if the roller wheel doesn't make good contact or gets wedged between turns (it's happened to me), all bets are off and you can crank until the cows come home and maybe never really resonate the plate tank. That can results in lots of power dissipated in the tubes, and little to no output power.
I think I've seen it all...
Rick, Many of the Henry amps to not have a variable cpacitor for the plate tuning! They used the variable inductor for plate tuning and used lumped capacitance for each band that was switched in. Sometimes one of the doorknob lumped capacitors opens up and throws evrything off. He didn;t say of the plates on the tubes are getting red in the standby mode or in idle. If the amp is in standby and the plates are getting red then he lost cutoff bias. This happens to the Drake L4B alot when one of th 50K bleeders goes open. Open blocking cap?
An open blocking cap would usually blow the fuse since there's an RF choke across the output.
Originally Posted by W1QJ
I "suspect" something else is wrong. A wedged roller contact on the rotary inductor is a common problem in Henry amps...it's happened to me, and you crank the TUNE control thinking you're doing something, but you're really doing nothing.
Henry mostly used ceramic doorknob caps for padding in both TUNE and LOAD sections, and these can crack and go bad also, throwing the tank out of resonance. I only had that happen during a lightning event, it's not so common, but it obviously can happen. I was operating right through a local lightning storm in NJ once years ago, and suddenly the amp output dropped way off. I thought there was electrical damage to the amp, but it wasn't realliy that: It was a failed doorknob in the tank circuit. $5 later I was on my way, and Henry offered to replace it for free. Unfortunately, "Henry" as we knew it is really no longer around, but they were great at servicing stuff back when they were building HF amps.
A case in point is my 3K Premier fell off my operating bench in the 1994 Northridge Earthquake. The RF deck literally hit the floor, it was a big quake. That cause multiple things to go wrong. I drove the deck down to Henry one morning at 9 AM, they told me to take a break and go have breakfast at the local diner. I did that. I came back at 11:00 and they had replaced everything that failed and already tested the amp. No charge.