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Author Topic: 3-500 Filament Problem with Henry 2K Classic X  (Read 24622 times)
W8KR
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« on: February 23, 2014, 05:00:59 PM »

Problem began when one 3-500Z filament in rear socket of my Henry 2K Classic X would not light.  I assumed the tube had gone bad so I ordered two new matched tubes.  Put them in and noted that the filament of the tube in that same rear socket glowed a little different color (more orange) and then it began to flicker a little.  I put the old tubes back in to see what would happen and sure enough they lit up fine but the tube in the rear socket started to flicker, as well.

I went into the amp, carefully looked everything over in the filament circuit and resoldered all filament connections in both sockets.  For good measure, I replaced the .01 capacitor across the "rear socket", even though it looked just fine.  The filament choke, which is wound on a circular toroid, looks fine and all connections look good.

After doing all of this, I reconnected the amplifier to the power supply, but kept the RF deck outside of the cabinet so I could measure the voltage on the filaments.  The voltage looks right at 5 volts, but on the "rear socket" it varies to the upside about 1/4 volt, i.e. 5.25 volts, and each time it does that the filament of the tube in that socket flickers a little.  I measured it with each of the old tubes in that socket and each of the new tubes, and the result was the same for all four tubes in that socket.  There was no voltage variability noticed in the other, i.e. "front socket". 

It was hard to see the tube and the meter at the same time, but my impression was that the tube dimmed when the voltage increased, not vice versa, as I would have expected.  I couldn't leave them lying on their sides with no air flow, for very long, so I couldn't observe for long periods of time, but the problem was quite evident within about a minute of turning them on each time.

I am really at a loss as to what's going on here.  The only other component I can think of is the filament choke, but wouldn't that affect both filaments, if perhaps there was a winding breakdown?  I can't see all the way around the toroid but it looks good from what I can see.  If it was the transformer, you would see it in both filaments for sure.

I'm puzzled as to where to look next.  Any ideas please?
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W1QJ
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« Reply #1 on: February 23, 2014, 05:36:47 PM »

Sounds like a bad socket.
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KH6AQ
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« Reply #2 on: February 23, 2014, 05:57:58 PM »

Pushing on the tube with a wood stick should show if the problem is the socket.
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W8JI
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« Reply #3 on: February 23, 2014, 06:53:19 PM »

There are multiple things in the Henry that can cause that problem.

The filaments are connected in series. There is a voltage equalizing choke that runs back to the filament center tap. That path provides DC return and an "equalizing current" to hold the tube filaments more equal in voltage.

That is a terrible system for problems like this, and for tube filament stresses if anything goes wrong.

In a normal amplifier, everyone would be correct. The natural assumption would be something on or around that socket. This is an abnormal system. ANYTHING from the filament transformer to the socket can cause the problem you have.  The last Henry  I worked on for a friend had a bad plug pin connection on a Cinch plug that cost him a tube.

If I were in your shoes, I'd do a careful inspection of every single connection from the filament transformer to the sockets. Make sure all plugs and pins are tight and clean from the transformer to the socket, because anything anywhere between the transformer and socket can cause this. In my friends case, he lost the filament balancing connection. When he would turn the amp on, one tube would light like a flash bulb and the other stayed cold on power on. They would come up and eventually both work, but it ate the filament up in one of his tubes before he realized it.

The really painful part was when he brought it to me, the vibration temporarily fixed the connection.  So we just changed the tube and he ran it a year or so, and then it ate another tube.

That's why I'm telling you no matter what you find, inspect all the connections carefully from transformer to tube on all three filament wires, both of the ten volt wires and the center tap equalizing wire.

73 Tom
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W8KR
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« Reply #4 on: February 23, 2014, 08:33:04 PM »

Thanks everyone... I have tried pushing on the tube and I have carefully looked at the socket, which seems alright.  I will go to work looking at all the connections, as well as the equalizing choke, which sounds like a potential issue.
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G3RZP
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« Reply #5 on: February 24, 2014, 02:45:04 AM »

I've seen a similar problem on a tx using 4-250A  tubes (same filament as 3-500Z) when the lead from the 0 connection on 5-0-5 filament transformer came off. But check the clips on the socket.
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W1QJ
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« Reply #6 on: February 24, 2014, 03:20:18 AM »

Don't forget that unlike many amps this amp has a plug and socket type of connection in the filament supply line.  Any corrosion in that connection could be problematic.  Then there is the integrity of the wires connected to both the plug and socket.  Many points of interest here unlike other amps.
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W8JI
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« Reply #7 on: February 24, 2014, 08:02:30 AM »

I've seen a similar problem on a tx using 4-250A  tubes (same filament as 3-500Z) when the lead from the 0 connection on 5-0-5 filament transformer came off. But check the clips on the socket.

I am disappointed to hear G3RZP has series connected filaments, other than on his line operated filament radio from the 1950's or his Christmas lights.
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W8KR
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« Reply #8 on: February 24, 2014, 11:25:20 AM »

Thanks W8JI and G3RZP!  I've been licensed 60 years and have done some amp building, but I'm not much of a tech, without good mentors (all of whom are SK now).  I'll show my lack of experience, but here goes:

Is there any reason I could not re-build this series circuit, using the existing transformer (assuming it is 5-0-5), to make it a parallel circuit?  I would replace all components, including the filament choke, and eliminate the voltage equalizing choke.  I'm looking at a circuit in a 1981 Bill Orr handbook that is very familiar to me, given my earlier experience.

Before I do that, however, can I analyze this circuit with my meter, without the tubes plugged in?  In other words, turned on with no tubes and no B+ connection?  That would allow me to probe around to see if I can create the problem, while watching my meter.  It's pretty impossible to have the tubes in there for very long laying on their sides, without ventilation.  Yesterday, while watching the meter I could smell something cooking off and it turned out to be the solder on one of the filament pins of one of my brand new 3-500s!  ( I heated that pin up with my 40 watt iron and ran some solder into it, so hopefully it will be alright.)

With the 4-1000 amp I built years ago, I would have simply approached this problem without the tube in the circuit, so is my intuition right?

I'm going to tear into the 2-K rack to access the filament transformer connections and then follow every connection all the way to the tubes.  The way this thing is built mechanically, you'd think no one would ever have to get into it to service it!  Very hard to work on.

73,

Jack, W8KR  (in snowy Minneapolis)

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W8KR
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« Reply #9 on: February 24, 2014, 12:08:40 PM »

By the way, I bought this amplifier several years ago for $250, knowing that it had been used hard and thinking that I might want to rebuild it someday.  It has worked so well, I haven't had to do anything to it until now.  It's starting to look like it's time to begin the re-building process.
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W1QJ
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« Reply #10 on: February 24, 2014, 12:12:23 PM »

My money is on a bad plug socket connection or the wires connecting them.  Most chokes rarely go bad because it is only wire.  Perhaps the enamel is worn and there is some arcing going on.  That transformer is 10v at 15A so I doubt you can switch it to parallel the filaments.
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W8KR
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« Reply #11 on: February 24, 2014, 12:30:51 PM »

W1QJ... Thanks!  If you're right, I should be able to put my meter across the plug and jiggle things around to see if anything changes, correct? 
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W1QJ
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« Reply #12 on: February 24, 2014, 03:27:15 PM »

W1QJ... Thanks!  If you're right, I should be able to put my meter across the plug and jiggle things around to see if anything changes, correct? 

I suppose so with an analog meter.  I would clean the plug and socket pins anyway.  The flickering of the filament seems to indicate a poor connection or maybe there is a problem with the primary connection to the filament ransformer or even the transformer itself is flaky.
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W8KR
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« Reply #13 on: February 24, 2014, 04:13:42 PM »

W1QJ... I have an ancient Triplett VOM that will work just fine for this and I will indeed clean and/or re-solder all connections. 
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W8JI
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« Reply #14 on: February 24, 2014, 05:39:26 PM »

In the most polite and nicest way possible, using a series filament string on a directly heated tube pair is moronic.

It ensures the signal has some hum modulation, because one end of one tube or the other is always at 1.414 times filament RMS voltage above the bias voltage, while the other tubes is 1.1414 times filament voltage below dc bias. Few people do that unless there is no way around it.

Also, there is the voltage balancing issue and starting inrush and imbalance.

But it is what it is.

If you had a pin unsolder, I would look at that socket for tension. I forget the drill bit size, but use the chuck side of a smooth clean drill bit the same OD as a pin as a tension gauge. Check to see if the "drag" is the same on all the pin holes.


Generally unsoldering of one pin is caused a damaged clip or a weak spring, or corrosion. It also can be from inadequate cooling, or poor tube manufacturing.
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