hum balance

[mic67920]

Good morning all ,
I am French and my name is Michel.
rare are the forums of pationnés of electronics and all the more of tube electronics, it is admirable.
Audio and video technician by training, I had the chance to study the tubes up to the processors in my career, yes I have exceeded 50 years, but I am not here to tell you about my life which will not interest anyone (- :
I have always been fond of music, a little musician and, in search of sound, good, of course permanently.
I have electronic tubes and transistors and my question concerns one of my LM508IA magnetic line tube amplifiers which has a hum balance settings that I do not find effective.
there is a slight snoring that is present in my focal electra 1038be.
do you know a solution to reduce this snoring?
being rarely present on forums, I hope I have posted in the right place.
thanks for reading.

 

[zapbuzz]

do you reach null with the hum balance or does it end up all the way to one end of the adjustment to get the lowest hum?

 

[mic67920]

I fine-tune the setting as closely as possible and there is still snoring

 

[6A3sUMMER]

Snoring?
Do you mean hum?

Mains power is either 50 Hz, or 60 Hz.
Certain amplifier topologies have hum at the mains power frequency; and also have hum at 2 times the power mains frequency (100 Hz or 120 Hz).

The 300B and 805 have directly heated filaments.
Your amp probably uses AC to power those filaments.
That has multiple problems.
It seems that you may have discovered one of them.

It seems that one amplifier (or one channel) has the snoring, and the other amplifier (or other channel) does not.
Turn the amp off, and wait for it to cool down.
Very carefully label each 300B and each 805 tube, and the exact location of each.
Swap both the 300B and 805 to the other amplifier (or other amplifier channel).
Now you have swapped all 4 tubes.

You need to know if the tube bias is automatic or needs to be set manually.
If the bias has to be set manually, you need to do this when you power-up the amp.

Now re-adjust the hum balance pots.
Did the snoring move to the other amplifier (or other channel)?
Some 300B tubes and some 805 tubes are more sensitive to AC filament power, and do not balance very well.
Now go through the procedure again, but only swap the 300B tubes. If you need to adjust the 300B bias, do so now.
re-adjust the hum balance pots.
Did the snoring move, or stay in the same channel.
The question is, is a 300B or a 805 causing the snoring?

You may have to get some more 805 tubes and/or 300B tubes.

 

[mic67920]

Hello ,
thank you for this very interesting answer, from a point of view settings, I have only the bias of the 805, the filaments must actually be supplied with ac but it is to be verified. I have already done the manipulation of crossing the tubes which gives the same result. in France we are in 50hz and the snoring looks like 50hz that I would have to measure with the ossiloscope. the amplifier being polarized in class A that may be to its topology and there after I must immerse myself in the technique of the tubes that I have partially left aside after decades of troubleshooting tv audio and video transistor.
I looked for the schematic diagram which I did not find but I have hope.
I will do more investigations.
supplying dc filaments could correct the problem?
thanks again in the beautiful day

 

[6A3sUMMER]

AC filaments on Directly Heated Triodes (DHT) can cause hum.

But there are many more sources of hum:

1. Power transformer and/or choke magnetically coupling to the output transformer.
Spacing and rotational orientation of the power transformer and/or choke to the output transformer.
Magnetic Steel Chassis conducts the magnetic field from the power transformer and the choke to the output transformer.

2. Ground Loops, primarily 2 different types:
Input circuit connectors, potentiometers, and input tube circuits.
B+ ground loops.
There are other ground loops.

3. Not enough B+ filtering

4. Filament circuits that do not refer to AC ground and signal ground.

5. Chassis that are not grounded to the amplifier ground circuits.
Caution, depending on your power mains connections that include ground or not, and your amplifier, this needs to be checked and according to your countries safety requirements.
An amplifier from one country that is used in another country could be a problem.
Safety First!

In order to convert an amplifier from AC filaments to DC filaments requires lots of changes.
The AC filament secondaries do not have enough current capability to work with the transient and RMS currents of a DC supply that will power the same filaments.
Changing power transformers, or adding other filament transformers usually involves needing more real estate (a bigger chassis).

 

[mic67920]

Hello ,

with a little delay, thank you for your answer, I'm afraid the topology of the amp is like this.
I have no other tubes to test at the moment, but as soon as possible I will test to see if there is an improvement.
have a good day