heater hum dilemna

[shzmm]

I have an Acoustic g100T here at the moment with what I believe is a heater problem-- 50Hz hum. The amp is on a pcb where they have tied one end of the heater output to ground. In the schematic it shows two 100ohm resistors to ground as there is no center tap, although in end effect these are paralleled on one side.
My question then relates to what can one do to fix the hum in this situation?
I would like to avoid rectifying the AC if possible but I can't see what solutions there are beyond this when one leg is tied to ground.
Any suggestions are more than welcome.
Thanks-- schematic below, and fwiw all filter caps and tubes were recently replaced, although I have no idea when the hum started.

[txstrat]

Have you tried to use those 100 ohms resistors to ground (virtual CT) instead of one heater wire to ground?
That might stop the hum if it's from the heaters.

[Robert M. Martinelli]

My advice would be to disconnect the 6,3 VAC leg from GND and connect one 100 Ohm resistor from each leg to GND, following the schematic diagram, or, better yet, disconnect the 6,3 VAC leg from GND, solder the 6,3VAC legs at the end lugs of a 220 Ohm 2W potentiometer and the center lug of the pot ( wiper ) to GND. This way you would have a hum balance adjustment to be set for minimum hum.
Hope this helps
Best regards
Bob

Edit .... oops! Hi, Matt, seems like we just have a simulpost....

[txstrat]

Cheers Bob

[Robert M. Martinelli]

I forgot to mention that, since you stated that you don't know when the hum started, it could be a good idea to pull out the preamp tubes one by one to see if the hum stops ( one of the tubes could be defective ) - Also, bear in mind that hum in push-pull AB1 designs can also result from severely mismatched output tubes ( push-pull AB1s output stages are hum cancelling designs, provided that the output tubes are not severely mismatched ).

If all the tubes are OK, then my previous post ( and Matt's ) applies

Hope this helps

Best regards

Bob

[Enzo]

Are your 100 ohm resistors looking like they were installed at the factory? Or do they look like something someone stuck on there?

I cannot believe the factory made circuit boards that wire two 100 ohm resistors in parallel to ground from one side of the heaters. Especially when the schematic shows the very conventional virtual center tap arrangement everyone uses.

If you are using an ohm meter, the 6v winding of the power transformer will read as a dead short - more or less zero ohms - and the tubes sitting in the sockets will also read as very low resistance, and they are in parallel with the winding after all. So the ungrounded ends of the two resistors will measure on an ohm meter as though wired together but will in fact be on opposite sides of the 6v winding.

A simple test of this would be with your AC voltmeter. Measure AC volts between the two resistors. If they are wired in parallel, then there will be zero volts between them.

With power off, they should meaasure about 50 ohms across each.

With power off, measure resistance to chassis from pins 2 and 7 of the power tubes. Do you get 50 ohms on each or do you get zero ohms? If you don;t get zero ohms, your heaters are not grounded.


It seems that you have hum, and have decided the hum results from a mis-designed amp. How did you determine the hum is coming from the heaters and not a grounding issue or radiated 50Hz?

[Steve Conner]

While we're brainstorming, maybe one of your power tubes has a heater-to-cathode short? Try measuring again with the power tubes pulled.

[shzmm]

First off thanks to Enzo for calling me out. You're absolutely right, I should think twice before assuming the manufacturer made a mistake.
So here is what i have done:
I checked the power tubes' heaters and yes they both read 50ohm to ground. First, I pulled all the preamp tubes one-by-one. nothing...
When I pull the pi tube, the hum disappears. I replaced the tube just to be certain, but the hum remains.
Looking at the schematic, there is a 1uF/50V cap on the cathode of the inverter. I bypassed this with a similar value which seemed to help a little but I could be imagining it.
By the way, maybe it is interesting to note that all the preamp tubes have pin 9 routed to ground on the pcb-- i'm not sure how that works but maybe someone else has an idea. This is what threw me off in the first place.
Thanks to all for the suggestions, it is much appreciated. I hope to get this sorted out and learn a little along the way...

[Enzo]

When you pull a tube and the hum disappears, it means the hum is coming from that tube or something before it. So it may or may not be that tube itself.

Look at the schematic, the PI signal comes directly from the graphic EQ. SO run all those controls to zero. Does that lower the hum? ANy control that affects a signal is after the source of that signal.

The graphic stage in turn is fed from the Master volume. Does the MV affect the hum at all? And that is where the reverb recovery comes back in as well, turning the reverb down have any affect on the hum?

Just prior to the MV is the effects loop stage. Plug a guitar into the loop return jack but turn its volume control to zero. Any effect on the hum? Likewise you can plug a cord into the FX send and connect it to another amp for a listen there. Hummy?

Moving further back we see the tone stack. Do the tone controls affect the hum? And there is channel switching, but either way there are channel volume controls, so do they affect the hum?


This is systematic troubleshooting, and so far we haven't even got out the meter. The whole point of troubleshooting is to isolate the problem. Once you know where it is, then it is a lot easier to decide how to cure it.

Pulling the PI tube stopped the hum. V3 is the graphic tube, does pulling it affect the hum?

ANything that affects the hum is either after the source of hum or it IS the source of hum. ANything that does not affect teh hum is before the source.

[shzmm]

Thanks for the interesting thoughts. It seems tricky with the graphic eq. Pulling V3 has no effect on the hum. I thought about the inductors being a source of the hum as well. The second fader from the left-- so 120Hz or so-- does have a marked effect on the hum. Moving the fader up increases it, yet lowering it also increases the hum. I figured this was more a product of the equalizer doing it's job--increasing the gain will increase the harmonics of the signal while decreasing it would leave more of the fundamental tone.
I discounted the reverb and effects send/return as the master volume had no effect on the amount of hum, neither does plugging the guitar straight in. Nor does the hum leave the amp when running a cable to another amp. However, the relay for the eq, reverb and volume is directly situated next to the pi tube.
Should I start examining the power supply connection to the phase inverter?

[loudthud]

Enzo makes a good point about systematically isolating where the problem is. One thing he didn't mention: If the hum is 50Hz (sounds like an organ note) it probably comes from heater supply. If it's 100Hz, (more of a buzz) it probably doesn't. Without a scope or a frequency counter, it can be hard to tell the difference.

[Enzo]

You are right of course. 100Hz implies power supply ripple. 50Hz implies heaters/grounding/picked up fields.

[shzmm]

Ok a small update. Since the replies gave a lot of food for thought, I decided to work through everything again. Now, if I disengage the graphic eq by putting a cable into the external switch jack the hum is gone. That seems to point to the inductors picking up or inducing hum.
Knowing this is it something that I will just have to live with? There is a shielded cable to the inductor which I maneuvered around a bit, but this didn't affect the hum any.
I really appreciate everyone who has replied and for sticking with me on this one. I hope there is a solution, otherwise I guess it will be just eliminating the graphic.