Filament to Cathode shorts

[KL7AJ]

One common problem encountered by many novice "boatanchorologists" is a receiver with an intractable and very LOUD hum in the audio.....often accompanied by really low overall gain. All the normal test procedures....or even replacing the power filter caps....do nothing to relieve this ill.

Almost certainly this is caused by a Filament to Cathode short in some tube....generally in the audio stage. Unfortunately, it can take VERY little leakage to create horrendous amount of hum in this case. Sometimes you can see this with a tube tester....sometimes not. Certain types of tube are particularly prone to this problem...I'm thinking particularly of the 6BE6, which was used in a lot of product detectors, but elsewhere as well. You may encounter entire production runs of these tubes with insufferable amount of H-K leakage.

It may be necessary in some cases, to modify the offending stage to operate the filament on DC. I've had to do this on a couple of beloved Hallicrafters receivers. But it does work like a champ.

Eric

[KC8YLT]

Ive had problems with coupling caps in the audio drive being faulty and causing problems. Something you can try is to place a known bad tube with this problem in a tube checker and bring the filament voltage up 25 to 30% and leave it there for about 10 mins or so, this may cure the problem for a while. In the old days of radio this was called flashing. Sometimes it worked very well. Good luck, Carl.

[KM1H]

H-K leakage can occur in any tube that has been subjected to excessive dissipation by leaky paper caps and no amount of filter cap changing is going to help.
Thats why I very rarely have anything to do with used tubes, especially from an old Radio/TV shop as they kept just about everything hoping to pass it off in some other repair.

With NIB tubes such as the 6BA6, 6BE6, 6BZ6, 6SA7, 6SK7, 6SG7, etc being so plentiful, even the military versions, I see no reason to bother going to DC.

Carl

[N2EY]

Tiny, arcane point of semantics:

A "filament" is the electron emitter in a directly-heated tube. It is also the cathode.

A "heater" is the element that heats the cathode in an indirectly-heated tube.

So you cannot have filament-to-cathode shorts, only heater-to-cathode shorts.

73 de Jim, N2EY

[K8ERV]

Can such shorts be burned out?

TOM K8ERV Montrose Colo

[AF6LJ]

Tiny, arcane point of semantics:

A "filament" is the electron emitter in a directly-heated tube. It is also the cathode.

A "heater" is the element that heats the cathode in an indirectly-heated tube.

So you cannot have filament-to-cathode shorts, only heater-to-cathode shorts.

73 de Jim, N2EY

But we all know what is being discussed.
Good morning Jim.

[N2EY]

Can such shorts be burned out?

I don't think so. Such a "short" can have significant resistance so that "burnout" techniques don't work.

There's also the situation where a particular tube will be fine in one circuit (say, where the cathode is grounded or close to ground potential) but useless in another (where the cathode is high-impedance to ground).

--

Many years ago I encountered an HW-16 that had hum on received signals. The usual power-supply-filter tests revealed that the B+ was really well filtered. No wiring troubles either. So where was the hum coming from?

The HW-16 has full break-in (real QSK). You can hear between dits at 40 wpm. Really sweet! This is implemented by a transistor which keys the RF gain control line (no AGC in the '16) and by a TR system that uses the final tube.

What happens is that, on receive, the final amplifier pi-net is used as the receiver input circuit, coupled to the grid of the receiver RF amplifier by a very low pF capacitor. To avoid frying the grid of the RF amplifier when the key is down, a protective diode is forward-biased by the voltage at the final tube cathode. Simple and ingenious.

But in the particular HW-16 I encountered, the 6GE5 final tube had developed a bit of heater-to-cathode leakage. Not enough to mess up the transmitter, but enough to put some AC bias on the protective diode, which then acted as a modulator on all incoming signals, making a T9 carrier sound T7. This was confirmed by replacing the 6GE5 final tube.

73 de Jim, N2EY

[N2EY]

But we all know what is being discussed.

Of course, Sue. As I wrote, it's a tiny, arcane point of semantics.

But why not use the right terms?

A big part of the reason to do "boatanchors" is to preserve the know-how. Which includes using the right names for things. The fine points make it more fun.

Here are some more:

The correct name for the 11 pin connector found on Heathkit and some other rigs is "magnal", not "11 pin octal"

The correct term is "spark" not "spark gap". The gap is a component. Plus spark and arc are different technologies.

There is a big difference between a mercury vapor rectifier (such as the 866, 816, 872, 83) and a mercury arc rectifier (the kind with multiple arms and a pool of mercury at the bottom).

73 de Jim, N2EY

[AF6LJ]

Of course, Sue. As I wrote, it's a tiny, arcane point of semantics.

But why not use the right terms?

A big part of the reason to do "boatanchors" is to preserve the know-how. Which includes using the right names for things. The fine points make it more fun.

Here are some more:

The correct name for the 11 pin connector found on Heathkit and some other rigs is "magnal", not "11 pin octal"

The correct term is "spark" not "spark gap". The gap is a component. Plus spark and arc are different technologies.

There is a big difference between a mercury vapor rectifier (such as the 866, 816, 872, 83) and a mercury arc rectifier (the kind with multiple arms and a pool of mercury at the bottom).

73 de Jim, N2EY

I agree and it's good for newcomers to know what things are called.
By the way good find on the HW-16 that would have been a hard one catch for the less experienced.

[K8ERV]

Tiny, arcane point of semantics:

A "filament" is the electron emitter in a directly-heated tube. It is also the cathode.

A "heater" is the element that heats the cathode in an indirectly-heated tube.

So you cannot have filament-to-cathode shorts, only heater-to-cathode shorts.

73 de Jim, N2EY

Picky Picky.

I Gnu that.

TOM K8ERV Montrose Colo