5A3 heater balance

[Leevi]

I'm building a Deluxe 5A3 and met an issue with heater balance. I didn't get rid of the 50Hz buzz before I grounded the second heater wiring. CT ground or balance resistors did not help. I know that Fender has used this method. What might be the reason for that, 6SC7 or 6SL7?


https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_5a3_schem.pdf


/Leevi

[Merlin]

Unbalanced heater supply was common in audio appliances at that time, people had low expectations about hum in the 1940s. Balancing is something the audio industry adopted over time as the quality of most everything evolved and improved.

[sluckey]

I would use the improved method...

[Leevi]

I would use the improved method...

Yes but that caused buzz. I tried even with a balance pot and the silent setting was when another wire was grounded.


/Leevi

[sluckey]

That's odd. Can you show us exactly how it was wired for the silent setting?

[Leevi]

That's odd. Can you show us exactly how it was wired for the silent setting?

Pls check the attachment


/Leevi

[passaloutre]

If it’s silent in that configuration, why mess with it?

[Leevi]

If it’s silent in that configuration, why mess with it?

You are right and I'm satisfied. I just want to get a clarification why it behaves like this. I have used to see either C.T. or balance resistors as a better solution.


/Leevi

[tubeswell]

Who knows why it hummed that way? Could’ve been the way you soldered things, or the way you attached the CT ground, or something else you haven’t disclosed yet. Did you actually wire it like the schematic you posted? (Or did you attach each tube heater separately to ground?)

[sluckey]

I suspect we don't have the whole story.   

[Leevi]

Who knows why it hummed that way? Could’ve been the way you soldered things, or the way you attached the CT ground, or something else you haven’t disclosed yet. Did you actually wire it like the schematic you posted? (Or did you attach each tube heater separately to ground?)

I wired the ground from one point (from the lamp holder lug)


/Leevi

[Leevi]

I suspect we don't have the whole story.   

Just like that, remains a mystery for now. Can be caused by many other things like PT, tubes, solders, etc. Is the reason for that balancing usually the 12A*7 dual triodes that require it?
 In 6SC7 and 6SL7, the heater structure is different.
/Leevi
EDIT: It's a great sounding amp, I also measured power and got ~12W, the sine wave is clean and symmetric.
/Leevi

[pdf64]

That's odd. Can you show us exactly how it was wired for the silent setting?

Pls check the attachment


/Leevi

Maybe one of your valves has poor h-k insulation, such that minimum hum occurs with that heater wire connected to 0V common?
How about if the connection is moved to the other heater wire?

[Leevi]

That's odd. Can you show us exactly how it was wired for the silent setting?

Pls check the attachment


/Leevi

Maybe one of your valves has poor h-k insulation, such that minimum hum occurs with that heater wire connected to 0V common?
How about if the connection is moved to the other heater wire?

If I connect the ground to the other heater wire I get hum.
/Leevi

[pdf64]

That's odd. Can you show us exactly how it was wired for the silent setting?

Pls check the attachment


/Leevi

Maybe one of your valves has poor h-k insulation, such that minimum hum occurs with that heater wire connected to 0V common?
How about if the connection is moved to the other heater wire?

If I connect the ground to the other heater wire I get hum.
/Leevi

I think that may support my hypothesis, ie that the greater hum with balanced heaters is due to an 'unusual' valve or 2, that have a lot of h-k leakage, such that the leakage path is close to one end of its heater filament.

Provided it's not a dead short or very low resistance (in which case the valve/s really should be considered 'bad'), balanced heaters that are DC elevated, eg to the output valve cathode node, should minimise heater hum whatever valves get fitted.

[Leevi]

That's odd. Can you show us exactly how it was wired for the silent setting?

Pls check the attachment


/Leevi

Maybe one of your valves has poor h-k insulation, such that minimum hum occurs with that heater wire connected to 0V common?
How about if the connection is moved to the other heater wire?

If I connect the ground to the other heater wire I get hum.
/Leevi

I think that may support my hypothesis, ie that the greater hum with balanced heaters is due to an 'unusual' valve or 2, that have a lot of h-k leakage, such that the leakage path is close to one end of its heater filament.

Provided it's not a dead short or very low resistance (in which case the valve/s really should be considered 'bad'), balanced heaters that are DC elevated, eg to the output valve cathode node, should minimise heater hum whatever valves get fitted.

Sounds feasible, thanks. Unfortunately I don't have other 6SC7 or 6SL7  tubes in order to verify that. The 6SC7 is a NOS (RCA) with metal shield.


/Leevi

[Carlsoti]

I've got a genuine '51 5A3 on the bench. The heaters for all the tubes have one side grounded, the other fed 6.3V from the PT. It's astonishingly whisper quiet. Sorry I can't be of further help on how you should wire your amp.
P.S. Before you go chastising me for the choice of components, I offered the customer the choice of F&Ts and MODs and explained the temp rating difference, and also how many people listen with their eyes, but the filter caps are more or less the same, but he'd have to pay $20 more for the F&Ts. I also explained the countless threads on innumerable forums regarding coupling caps, and that Mallory 150s do the job reliably, unless you're splitting hairs.  He stated it was gifted to him by his G'pa, who gigged this amp extensively through the 50's and 60s. The customer just wants it running safely, so they can use it at home. They have no intent of selling it.

[tubeswell]

genuine '51 5A3 on the bench.

Any more pics please? (Tube chart? Cabinet? Control panel? Xformers?, socket wiring? Control panel wiring?)

[Carlsoti]

Any more pics please?...

I'll start a new thread about it and let you know.

[Leevi]

Send here an update:
I replaced the 6SC7 with 6SL7 because of microphonic issues.
Unbalanced heater circuit worked still fine but if the mains plug was turned 180 degrees the hum level increased, not much, but it was remarkable.Then I tried to use CT instead but the hum was remarkable with both mains plug positions.Then I added a balance pot to the heater circuit and ended up an optimal balance point with resistor values 82R and 22R.That worked fine regardless how the mains plug was plugged.
/Leevi