huuummmmm!

[billcreller]

Anyone here have any experience in trying to cut down on the hum in old vintage tube amps, with a field coil speaker ??  This particular amp has had some hum since it was new, in 1950.  Never paid much attention to the hum until I started building amps in recent years, and the clones I build of this amp have no hum.  Of course I don't do the rat's nest wiring like the original, and use Doug's board material & turrets.

[LooseChange]

Yes. Replace the filter caps. Increase the values a bit. Take a look close look at the grounding scheme.

[PRR]

Do not make large changes in filter cap values. There's hum in the amp and hum in the field coil. There is an optimum cap value which tends to cancel one against the other.

Is it possible the field-coil wires have been swapped? It works either way but one way will cancel PS hum, the other add.

[stratele52]

Same hum at any volume = filter caps

More volume more hum; it is not the filter caps. Check heater center tap at power transformer. Or wiring lead dress . And many other thing.

[billcreller]

The hum goes away when the PI tube is pulled, so I guess it's coming from the pre-amp section (?)  I replaced the filter caps a few years ago,  because one shorted out.
  There is a grounded center tap on the heater winding.  The mention about the possibility of the field coil wires being reversed (PRR) is something I'll check.  The speaker wires are just a twisted bunch, hard wired to a terminal strip on the chassis, and they have been off & on a few times.  The original filter caps were 40 MF on the first node, and 10 MF on the other two.  Maybe I'll try some other values.  Jumping a 22MF across the filters for the pre-amp section made no difference.
   And the hum doesn't change with the volume control.  Just the back ground hiss, or tube noise changes in volume.
    This old speaker has great tone etc, or I would swap it out for a new one, and run a resistor/choke in place of the field coil.  But that may not help the hum much anyway (?) 
  I just replaced a tube socket, and the volume pot, which were making a lot of noise, so I thought that looking into the hum thing is a good next step.

[stratele52]

The hum goes away when the PI tube is pulled, so I guess it's coming from the pre-amp section (?) 
   And the hum doesn't change with the volume control.  Just the back ground hiss, or tube noise changes in volume  
 

 You look to have many problem ; hum , hiss
Do you have an oscilloscope an signal generator ? I guess not,
Har to tell withourt seeing the amp.

Some close pictures ?

[PRR]

> goes away when the PI tube is pulled

Nothing to do with field coil.

Are the preamp heaters one-side-grounded? Cheap, acceptable in 1940, but today we prefer to balance the heaters.

[HotBluePlates]

As side note, an old MIT document, indicated that flat strips generate a different emf field than round wires, could this be the reason the PCB boards have less hum than the hand wired counterparts?

PCB amps don't have less hum, necessarily. I've only made handwired amps, and they don't hum. Others have bought or built PCB amps that hum or buzz.

Good technique is good technique, regardless of how you specifically build the amp.

[billcreller]

The heaters aren't grounded on one side.  But the wiring in this thing is helter-skelter
So I guess re-wiring part of it would help.  Heater wires aren't twisted etc, just run where they needed to be.
  The hum isn't real loud, and I have to be within about 7 feet from it to hear it.   Maybe I shouldn't get too excited about it.

[stratele52]

> goes away when the PI tube is pulled
Are the preamp heaters one-side-grounded? Cheap, acceptable in 1940, but today we prefer to balance the heaters.

As billcreller wrte the power transformer have a heater's center tap so it can't have a heater grouded on one side.

[stratele52]

   But the wiring in this thing is helter-skelter
So I guess re-wiring part of it would help.  Heater wires aren't twisted etc, just run where they needed to be.
 

Heater wire should be twisted for cancelling the hum. It look that this amp have a few wiring fault. The hiss is too long wire at each preamp's grids , pin 2 and 7 on 12AX6.
Too long wire is like an antenna . You can replace thes wires by a shieded one , grounded on one side only

[HotBluePlates]

The hum goes away when the PI tube is pulled, so I guess it's coming from the pre-amp section ... And the hum doesn't change with the volume control.  ...

What amp is this?

From what you're describing, the hum must be after the volume control, but before (or in) the phase inverter.

[billcreller]

 The amp is an old National, with the Valco 510-1B schematic.  I have to remember how to post it here...
Rewiring the heaters on the pre-amp and PI might help, along with some shielded wire in the volume & tone control circuits.  On the clones I built, I used shielded wire, and have no hum issues.   I'll likely do that before going any farther with this.
  Thanks for the replies....

BILL

[sluckey]

The schematic is right here in Doug's schematic collection.

[billcreller]

Today I added some shielded wire to the volume & tone controls, which did help some.  And ran twisted pairs to the heaters on the pre-amp & PI tubes.  Pretty hard to do much in there with the way it's built.  I think is the nature of the beast !!
  When it's being played through, or during normal conversation, it's not really noticeable. Maybe I'm just getting too picky with an ancient amp...
   It's better than it was, so I'll let it be.

  I built another clone of this, using loctal tubes,  with close specs to the octals, just to be different !  I'm currently mounting it in a tweed covered cab I built.

[billcreller]

If you do that, YOU will have to clean it up.........

[billcreller]

 Well, now that the hum is reduced a bit, today there is bacon frying in there    It does it even with the volume control off.  Must be some of those 60+ year old carbon resistors   I'm debating about replacing most of those with modern resistors.  Finding the bad ones might not be easy (?)  Likely since it's after the volume control, maybe in the PI circuit ....

[jojokeo]

check and/or replace only your load resistors

[stratele52]

Replace all plate resistors, often they are noisy when old

[jojokeo]

load resistor = plate resistor

[billcreller]

OK guys, I'll try that 

[billcreller]

OK !  When plate resistors were mentioned, I even remembered which ones they were.  Two 100K 1-watt resistors in the last node from the power supply, to the PI circuit etc.  I had planned on using some metal film etc types, but discovered that I had a bunch of new 2-watt 100K carbon types, so I used those.
  No more bacon frying in there !
Thanks for the replies

[stratele52]

Good to know your amp is ok now.  Enjoy it

[billcreller]

Well, the fun isn't over yet    Had some more noise, which turned out to be a 6V6, with apparently loose guts in it, which rattled & made racket when lightly tapped.  The 6V6s are Groove-Tubes, about 10 years old now.  Switched to another matched pair, and that part worked OK.......for a while.   Now I'm back to more frying.  One more resistor is suspect, a 27K-1 watt, that feeds the 3rd node of the power, from the second node.  Maybe this one is fubar too ! 

[billcreller]

 I finally won out, no more frying, and even the hum seems less since I replaced the last carbon power  resistor.  Should be good for another 10 years   

 Thanks for all the replies   

[HotBluePlates]

Sounds like you've pretty much cleaned that amp up!

It's good to hear your changes improve the amp's noise floor.