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Author Topic: flat topping on scope trace of filament voltage = hum?  (Read 6060 times)
WB4SPT
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« Reply #15 on: March 07, 2012, 10:27:36 AM »

OK, checked that, it is 60hz hum.

That leads me to think that either the AC line is getting in the audio, or perhaps the transformer is magnetically coupling into the audio tube.  I see the a twisted gray pair leading off of the AC switch, in back of the volume pot.  Make sure yours is twisted as well.  The switch itself, of course, puts 120vac right along side the low level audio preamp wiring.  You mentioned many or all the Twoers have hum?  Did you try reversing the ac line plug into the receptacle?  That's to get the grounded side of the house 120v on the switch.  Beyond that, if the transformer is not shielded (covered in steel), try putting a steel plate between the ac transformer and the 12ax7. 
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NZ5N
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« Reply #16 on: March 07, 2012, 12:50:46 PM »

Thanks, I am beginning to lose hope, it seems the hum is something I'll just have to live with.  There is a twisted gray pair of wires off the volume pot.  The 120vac is indeed close to the low level audio, but the hum persists even after detaching the resistor that runs between the volume pot and the 12AX7.  I did try reversing the ac line plug, no effect.  I'll try to figure out a way to put in a transformer shield, but the nearest edge of the transformer is 4" away from the 12AX7, so that would be some magnet. 

I know that many Twoers have hum.  Has anyone had one (or a Sixer or other lunchbox) that did not hum?

73, Bill 
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NZ5N
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« Reply #17 on: March 11, 2012, 08:32:57 PM »

This past weekend at the Palm Beach hamfest there was a Twoer and Sixer for sale, I turned them on, both had the exact same hum as mine.  I'm beginning to think it was a design defect and that all Benton Harbor lunchboxes hum.  One fellow on another board suggested that, since on the Twoer there is only one filament wire to each tube, with the other filament pin on each tube grounded to the chassis, the hum probably results from eddy currents in the chassis.  He suggested lifting the grounds on all 5 tubes and running twisted pairs of filament connector wires to each tube.  He may be right, but that would be a difficult job, with no assurance of success.

Also removed B+ and that stopped the hum, I was told that eliminates the possibility that the hum is the result of magnetic coupling from the power transformer.

Is there perhaps some way to filter out the hum at the speaker?  Something that cuts out everything below 300hz or so?

73, Bill  
« Last Edit: March 11, 2012, 08:51:38 PM by NZ5N » Logged
AC5UP
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« Reply #18 on: March 11, 2012, 09:13:07 PM »

Is there perhaps some way to filter out the hum at the speaker?  Something that cuts out everything below 300hz or so?

The Southern Engineering approach would be to install a junkbox electrolytic in series with the speaker... Something like 5 to 10uF @ 16 volts or greater, exact value is whatever you think sounds like the best compromise between Winchester Cathedral and the hum you have.

That's what a hi-pass filter looks like in a 2-way speaker system.
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NZ5N
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« Reply #19 on: March 11, 2012, 10:16:05 PM »

Thanks, that would be easy.  Does the positive or negative side of the electrolytic connect to the speaker? Or does it have to be non-polarized?
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AC5UP
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« Reply #20 on: March 12, 2012, 01:20:21 PM »

An audio purist would prefer a non-polarized cap, but considering the lineage of this particular piece I don't see that as mandatory. It's an AC circuit so there is no polarity, whichever way you want to wire it is OK. The larger the value of the cap the lower the pass frequency...........
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NZ5N
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« Reply #21 on: March 12, 2012, 05:58:59 PM »

Yes sir, a 10uf cap knocks out the hum completely.  Not sure yet how much of a Winchester Cathedral effect, my own voice does not sound bad.

Thanks for the tip,
Bill
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K9PU
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« Reply #22 on: March 20, 2012, 03:38:25 PM »

Usually hum in audio amps is a bad bypass cap.  Did you check you ceramic disks?

Scott
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NZ5N
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« Reply #23 on: March 20, 2012, 09:16:04 PM »

No, replaced the electrolytics but not the disks.  Had been told disks do not often go bad.  AC5UP's idea has been working great, the 10uf cap knocks out the hum with little effect on audio quality.

73, Bill
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KB3HG
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« Reply #24 on: March 23, 2012, 06:24:06 AM »

You liked 5UP's solution, Now for the esoteric solution. Maybe I'll jar someone's memory, Some Radios had a resistor balance pot arraignment.  I read it on this site it was in another forum. Not to long ago. By adjusting the pot it decreased the hum. apparently it was not on radios, audio equipment also. But Swan rigs may have had it in the example I read. Try the  boat anchor forum.

Tom Kb3hg
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NZ5N
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« Reply #25 on: March 23, 2012, 07:20:04 AM »

That's interesting, did a forum search but nothing found.

I recently tried the Twoer squelch mod, it involves a bridge of two 4.7 ohm resistors and two #47 lamps, see
http://qsl.net/nz5n/TwoerGonsetRestoration.htm

Wonder if what you are thinking of is similar? The squelch mod attenuated the audio quite a bit so I took it out, although I did notice that it seemed to stop the hum.

73, Bill
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AC5UP
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« Reply #26 on: March 23, 2012, 02:21:56 PM »

...Some Radios had a resistor balance pot arraignment.  I read it on this site it was in another forum. Not to long ago. By adjusting the pot it decreased the hum.

That post was one of mine. Wherever there is hum I'm usually not too far away.........

Anyway, vintage audio gear often used twisted pair for the filament string that was wired flush against the chassis. In some cases the 6.3 VAC transformer winding included a center tap that went straight to chassis ground. In others a pair if 33 Ohm 1 Watt resistors were wired in series across the filament line with the center junction straight to the chassis. The adjustable version featured a 100 Ohm 2 Watt wirewound potentiometer across the filament line with the wiper grounded to the chassis. In this case the user would adjust for minimum hum while listening to the most hum-prone circuit, like the mag phono or tape head pre-amp. I've seen this in Scott & Fisher gear and it really does work.

In the case of the Heathshkit Twoer, one side of the filament line is on the chassis. Possibly to bleed stray RF on the line to ground. So forget the previous paragraph... Unless... Someone re-wired the filaments twisted pair / balanced line just like damn near everything toobular that has ever been made and bypassed the filament line with .001 uF ceramic discs on both sides to ground.

Then a hum pot could be added. I suppose.

There could be a good reason not to do this. That I don't know, but judging by the response from NZ5N there might not be much motivation for trying it........  Grin
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