12AX7 hum on Twoer

[nz5n]

Hi,

My Heathkit Twoer has a hum on receive that has been traced to the 12AX7 audio driver tube.  All electrolytics have been replaced, all connections have been checked for bad grounds.  The odd thing is that the transmitter uses the same circuit to amplify the microphone output, but my TX audio is good and clean with no hum at all.

Audio comes to the grid of the 12AX7 from the 6BS8 detector through the volume control and a 470k resistor.  Grounding the grid of the 12AX7 has no effect on the hum.  The audio  output from the 12AX7 goes to a 6AQ5, and grounding the grid of the 6AQ5 stops the hum, so we are assuming that the problem is in the 12AX7 stage.  Replacing the cathode bypass electrolytic capacitor reduced the hum by about 50%, but it is still quite bad.  Removing the cathode bypass capacitor stops the hum, but then audio output is greatly reduced.

Any thoughts on what the problem might?

Thanks and 73, Bill NZ5N

[KM1H]

Start by tightening all screws and replacing missing external toothed washers.

[K5WLF]

Is the hum still there with another 12AX7?

[nz5n]

All screws tightened, tooth washers in place, tried a different 12AX7 and 6AQ5, no joy, the hum persists.

It looks like the output from my Twoer's power supply is rather low. On the 12AX7, I am reading 89v on the plate (should be 135v), 0v on the grid (should be 0v), and 0.9v on the cathode (should be 1.2v). On the 6AQ5, I see 260v on the plate (think it should be 295v, but the print is unclear), 285v on the screen (should be 335v? - unclear), 0v on the grid (should be 0v), and 15.5v on the cathode (should be 13v).

Not sure why the B+ voltage from the p/s is so low, the AC input voltage is over 120v. Maybe the rectifier diodes are not up to par? The electrolytics are new. Could the low voltage be a cause of the hum?

Google research indicates that hum in a 12AX7 first stage audio amp is common and not easy to eradicate, at least in guitar amps. Apparently has to do with the a/c filament voltage causing the filament to radiate an electromagnetic field that induces hum on the grid or anode. Suggested solutions include changing the filament supply from AC to regulated DC, changing to a center-tapped filament transformer, and adding DC voltage to the filament. See http://www.freewebs.com/valvewizard/heater.html.

73, Bill

[Ken - K2UPI]

Very interesting .....out of several Heathkit HG-10 VFO's I have one with a similar problem. Chasing down the obvious causes, even replacing the power cable and changing tubes had no effect.  I finally applied d.c. to the 6CH8 filament and the problem is gone. Always wondered why....now after reading this thread I have a clue.

Thanks to everyone for posting.

73 Ken / K2UPI

[nz5n]

Still have the hum issue, finally borrowed a scope to check things out. 

All DC voltages look OK, no ripple seen on the scope.  The filament voltage looks odd on the scope, instead of a nice sine wave there is some flatness on the top and bottom of each cycle.  Could this have something to do with the hum?  If yes, is there any cure?  Some have suggested changing the filament supply to DC.

Any thoughts or ideas appreciated.

73, Bill NZ5N

[W2WDX]

Since you have checked the screws and mounting of the tube socket, a common source for hum, I will move on.

The hum could certainly be caused from AC on the filaments. Try applying a DC source and see what happens.

As far as your voltage on B+, did you use the same values on the electrolytics you replaced?  If not this could change the voltage coming from the supply. Could this cause the hum? Unclear. If the values are too low, it could cause hum and might also cause your DC voltages to be lower than expected.

The problems may be related though. What supply are you using? The internal 117v or an external? You may want to check to see if the multi-pin power connector is wired correctly with the proper jumpers (and resistor in the 12v DC variant.)

I would also look at the caps going to ground on Pins 3 & 4 of the 12ax7. If these are bad it could be the source of your hum, since they are used on the filament voltage.

John, W2WDX

[The Slab Bacon]

Check the rivets or screws holding the tube socket down, a high resistance connection could cause the problem.

Also check the plate load resistors on the low level audio stages. If they have drifted to very high values, it will dramatically increase the stage gain of the tube possibly causing a possible hum pick up.

[nz5n]

Thanks for the replies.  I did further testing this evening, lots of new info but still no fix for the hum.

* Tube sockets and ground terminals all check nice and tight.

* Removed tubes one by one, no change in the scope display of the heater voltage, some flat topping.  There's a photo of the heater line scope display below and at http://qsl.net/nz5n/Twoer%20heater.jpg.

* Replaced the heater voltage with regulated DC.  Somewhat to my surprise, no change, the hum persists even with DC.

* The hum persists even if the grid of the 12AX7 is grounded.  The hum goes away if the grid of the 6AQ5 audio output tube is grounded, so it sure seems like the problem relates to the 12AX7.  With the scope connected to the plate of the 12AX7 or the grid of the 6AQ5, we get an odd-looking display pictured below and at http://qsl.net/nz5n/12AX7%20plate.jpg. Does that tell us anything?

John, I am using the 117v supply and the replacement electrolytics are quite close to the original values.  Slab, I have not yet checked those plate load resistors but will do so.

I am attempting to find another 12AX7 or two, just in case both of the two I have already tried are bad.  Both check OK in the tube tester.  Another than that, I am about out of ideas.

Any further thoughts?

73, Bill

[n1ps]

More ideas....Hum in these vintage radios is quite common.  The 12ax7 especially as it is a high gain tube.  Some guys like W1VZR replace them with a lower gain unit like the 12au7. 

The usual issue is eddy currents within the chassis.  The 60 cycle filament returns all tie into the chassis at the tube sockets on most transmitters.  On many guitar amps they run wires back to the transformer for ground returns.  So we fight this right from the start.  I found hum in a Gonset where the power xformer ground return was less than an inch from the mic connector. 

Another area to watch is magnetic coupling.  Many of these 50s rigs used non shielded transformers.  Some radios locate the power xformer right next to the mod transformer  .  I have made some shields to reduce the coupling.  In the case of 60 cycle...make sure to use ferrous (sp?) metals....copper will not work.  A way to tell if you are getting transformer coupling...pull the rectifier(s) and listen for any hum still present.

A few ideas....
Peter
~ps

b

[nz5n]

Very interesting and makes sense, Peter.  Sounds like the first and easiest thing is to try is a 12AU7 in place of the 12AX7.  I'll buy or borrow one and try that.

On the eddy currents, are you saying the grounds on the filament return pins of each tube socket should be lifted and replaced with wires from these pins to the filament transformer?  Guess one way to test if this would help would be to lift just the ground on the 12AX7 heater and connect wires to a separate 6.3v source.

On the transformer shield, I don't quite understand how this would be shaped or placed, do you perhaps have a picture?

73, Bill

[n1ps]

On the eddy currents, are you saying the grounds on the filament return pins of each tube socket should be lifted and replaced with wires from these pins to the filament transformer?  Guess one way to test if this would help would be to lift just the ground on the 12AX7 heater and connect wires to a separate 6.3v source.

On the transformer shield, I don't quite understand how this would be shaped or placed, do you perhaps have a picture?

Yes on the eddy currents.  The heater ground returns are through the chassis and that leads to our humation issues.  Its not a big deal at the modulator or RF circuits, but in a low level high gain amp, especially with a 100mu 12ax7.  Read some of Tim's (Wa1HLR) tech papers in the AM window. 

On the shield, I found the problem with the Gonset G50....on just about every one I looked at.  I was bound and determined to conquer the hum at one point.  When I removed the rectifiers...it was still there   The cause was indeed coupling between the xformers.  I added a steel plate between the xformers and repositioned the modformer to null it out (using a scope).

Good luck...

Peter

[nz5n]

I disconnected the rectifiers, that stopped the hum immediately.  Tried replacing the 12AX7 and a 12AT7, no difference with the hum.

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, perhaps due to eddy currents as suggested.  Peter, you and someone on another board suggested lifting the grounds on all 5 tubes and running wire to each tube instead.  He also said there should be 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.  It's not just a matter of listing 5 wires, some tubes have the pin wired to the big pin in the center.  Some use the grounded pin as a terminal, with several components soldered to the pin. I'll have to think hard before attempting this hum fix.

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

73, Bill 

[KB2WIG]

B,

Have you considered using the 'twisted pair' method to just the first af amp? Maybee youse wud get a reduction in the hummm. And it sounds like it wud be  'o bit easier to do.

FWIW, I did the Tron ground lift for my Ranger audiio section. It seems to work FB, but I only have 2 megs of z in fer the 1st audio amp.

klc