grounding a 6.3V CT to reduce hum

[vaughandy]

Hi,

I recently replaced the power transformer in an old Ace Tone 601 as part of my first tube amp repair process. That, combined with re-capping and replacing some diodes, has resulted in a pretty sweet sounding amp.

As I think is pretty common, though, I'm having some pretty annoying hum issues. I'm running the amp off an extremely clean power source (filtered through a furman balanced power conditioner), so I can at least (mostly) check that off as a potential hum source.

Ok, so dumb question number one: I'm currently using the CT and one of the 6.3V taps (there are two) of the secondary windings of the power transformer for all the heaters. The CT and one of the leads are connected to the tube heaters, while the other is floating and not in use. I've read that maybe I want to ground that CT as well. Does that make sense? (see EDCOR Electronics Corporation. XPWR058-120)

Also, I've had to use an additional 15V secondary transformer to provide that voltage source, as the original PT had that winding as well. It also has a center tap. Should I consider grounding that as well?

I know these are really silly novice questions, but if someone wanted to take pity and help me out, I'd be very appreciative. I've thrown up some of the schematic for the power supply and surrounding circuits if that's helpful. BTW, if somone could inform me as to what exactly that 15V circuit is all about, that's be cool.

[vaughandy]

oh, and also, is there any reason I might want to run the preamp tube heaters off one of the 6.3V taps and the power amp tubes off the other? just looking at the original Power supply schematic there, where it looks like they did something like that.

[Chuck H]

If you have the amp wired as per the schematic then there could be a few things that could be going on. One thing is that you may be taxing the one 6.3V tap beyond it's rating. If the PT is designed to use two 6.3V taps then either one by itself may not be rated for enough current to heat the whole amp. How do you have the heaters wired??? One 6.3V tap shows a hum balance control but no CT is shown on the either tap??? Also, it's important that you differentiate which heater lead goes to what side of the filament on each tube to achieve hum cancellation.

DO NOT GROUND THE CT FOR THE 15V TAP. The 15V tap is a bias supply. Not a really adjustable supply either. The amp needs -14 volts and the supply is -15V. What if you need -16 volts??? with modern wall voltages and tubes it's entirely possible your bias is incorrect. Have you measured idle current?

[vaughandy]

Ah, interesting. So the schematic shown is how it was wired up with the original power supply. It was busted, so I replaced it with the one linked to above. The current transformer is rated for 4 amps in the 6.3V secondary, and it's one winding with the two taps plus a CT (I think, right?). Also, I didn't wire up those heater leads with any kind of scheme in mind other than connecting them to one of the 6.3V outer taps and the CT. When you say differentiate, I assume that would mean I need to make sure the same 6.3V lead is connect to the same pin of each tube, same for the CT connections?

Haven't ventured into looking into the bias yet. Outside the limits of my current working knowledge I'm afraid I'll have to do some learnin' for that.

here's the transformer diagram:

[Chuck H]

A "tap" would be an auxilary lead connected to a dedicated winding. Like the center tap on the 6.3V winding for your transformers. You do not have two 6.3V taps. The transformer diagram is probably incorrect as it shown what amounts to a 6.3V-0-6.3V winding (or a 12.6V winding with a center tap). It is probably a 6.3V wind with a center tap which would properly be indicated as 3.15-0-3.15. You should measure the filament secondary and see what's really going on though. So how do you have it wired now?? What leads of the filament winding are you using and where exactly do they go? A diagram with voltage readings would be best for evaluation.

[vaughandy]

I've measured the voltage from that 6.3v secondary. so yeah, it's a 12.6V winding with a center tap, as I thought. sorry about the misuse of "tap" there. I should have said leads when refering to those connections except for the CT, yes?

The way it's currently wired is that I have the all the heaters hooked up to one of the 6.3V leads connect to the one of the pins of the pre and power tubes, and the CT connected to the other. The CT runs into that little "hum adjust" resistor, which is itself connected to ground. I tried connecting the CT directly to ground as well (as I initially asked about). no difference in the hum level. messing with that hum adjust pot makes only a very slight difference in the hum.

Not sure what to try next. I've tried turning on the amp while it's open and nudging the wiring with a big wooden stick, and I can't find any offending wires, so it doesn't seem to be an issue of proximity with EM fields interfering with each other or anything like that. I've also tried swapping all the tubes. If I take out the preamp tubes completely, most of the hum is gone, but of course this would make sense as most of the signal isn't being amplified. The hum doesn't become really bad until I put the third preamp tube back in. I suspect this doesn't have anything to do per se with that part of the signal path, but rather that that's just the threshold gain stage where we really push that hum into the annoying volume range. Or not?