So.. I'm dealing with a bit of hum on my latest amp.. It's a cathode biased dual 6V6 push-pull power stage. I'm using a single 350 ohm resistor bypassed by a 100uF cap for both 6V6's.
Volume doesn't seem to affect the level of hum.. It seems like it's coming from the 6.3V heater tap.
The 6.3V tap doesn't have a CT, so I'm using 2 100 ohm resistors from each side of the heater tap connecting them to ground. I've done this many times before.
Usually when I have a bit of extra hum I elevate the heaters - usually just attaching the ground point of the 2 100 ohm resistors to the top of the cathode resistor. In this amp the cathode resistor has about 18V on it.
Now the weirdness is that when I attach the 2 100ohm resistors to the top of that cathode resistor instead of ground. The voltage on the cathode resistor drops to only 6V, and actually the main B+ feed to the plates of the 6v6 drops from like 350V to 250V.. Something really doesn't seem right.
Any ideas?
Volume doesn't seem to affect the level of hum.. It seems like it's coming from the 6.3V heater tap.
The 6.3V tap doesn't have a CT, so I'm using 2 100 ohm resistors from each side of the heater tap connecting them to ground. I've done this many times before.
Usually when I have a bit of extra hum I elevate the heaters - usually just attaching the ground point of the 2 100 ohm resistors to the top of the cathode resistor. In this amp the cathode resistor has about 18V on it.
Now the weirdness is that when I attach the 2 100ohm resistors to the top of that cathode resistor instead of ground. The voltage on the cathode resistor drops to only 6V, and actually the main B+ feed to the plates of the 6v6 drops from like 350V to 250V.. Something really doesn't seem right.
Any ideas?
You are shorting your power tube cathodes to ground, probably putting the two 100 ohms in parallel with the 350 ohms resistor.
Follow the wiring and you'll find your mistake.
The 6.3V winding must be floating, not have any connection to ground (you didn't ground its center tap I hope), measure continuity to ground with amp off.
Post results.
Follow the wiring and you'll find your mistake.
The 6.3V winding must be floating, not have any connection to ground (you didn't ground its center tap I hope), measure continuity to ground with amp off.
Post results.
Okay.. Yea, you're right. Measuring from either wire of the heater tap to ground I read 2ohms.. I'm not grounding either wire though. I wired the amp up from scratch.. So I defininitely did not wire either leg to ground.
How could this be happening?
As far as I know there is no CT for the 6.3V tap. None that I can see in any case.
Maybe the heater winding is internally shorted to the transformer core? That's happened to me with a cheap Chinese transformer. Also check for solder whiskers or wayward copper wire strands between the valve heater pins and chassis.
I was wondering if it was something inside.. It's a power transformer from an old Webcor amplifier of some sort. So I don't think it's Chinese, but it's old. Before I used it, I measured all the windings, and everything measured good.
I've been checking for any solder or wire making contact that I don't want.. I've checked a dozen times now! driving myself crazy.
Agree.
To make certain, desolder both heater wires, keep them up "in the air/floating" and then:
1) measure resistance to ground again.
Remember to short multimeter probes first to check parasitic (wires/switch/probes) resistance so as to substract it from displayed reading.
As in: shorted probes: 0.8 ohms.
Displayed winding resistance 2 ohms
Actual reading 2-0.8=1.2 ohms.
Resistance to ground from each winding should be the same if there's an internal, hidden center tap grounded somewhere.
2) turn amp on, measure voltage again, end to end and from each end to ground.
If they are symmetrical, you have a real center tap somewhere; if one is 6.3V and the other 0V one side is grounded ; if random intermediate voltage, insulation is compromised (sometimes when E laminations are placed carelessly they can scratch wire insulation) and that transformer is a possible deathtrap.
If possible, draw a sketch of transformer windings.
EDIT: if winding tests insulated but when wired to sockets shows continuity, re re re check wiring or maybe a drop of solder or some metal shaving is shorting pins in a socket.
To make certain, desolder both heater wires, keep them up "in the air/floating" and then:
1) measure resistance to ground again.
Remember to short multimeter probes first to check parasitic (wires/switch/probes) resistance so as to substract it from displayed reading.
As in: shorted probes: 0.8 ohms.
Displayed winding resistance 2 ohms
Actual reading 2-0.8=1.2 ohms.
Resistance to ground from each winding should be the same if there's an internal, hidden center tap grounded somewhere.
2) turn amp on, measure voltage again, end to end and from each end to ground.
If they are symmetrical, you have a real center tap somewhere; if one is 6.3V and the other 0V one side is grounded ; if random intermediate voltage, insulation is compromised (sometimes when E laminations are placed carelessly they can scratch wire insulation) and that transformer is a possible deathtrap.
If possible, draw a sketch of transformer windings.
EDIT: if winding tests insulated but when wired to sockets shows continuity, re re re check wiring or maybe a drop of solder or some metal shaving is shorting pins in a socket.
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A real old Webcor amp?
In the 40's or late 50's (and maybe on some real cheap 60's stuff) winders "simplified your life" (read saved $$$$) by doing things such as this ... which I fear is your case:
That would explain the hum you hear.
And your 100+100 ohms "center tap" simply isn't so.
Only voltage checking as suggested above will confirm this.
In the 40's or late 50's (and maybe on some real cheap 60's stuff) winders "simplified your life" (read saved $$$$) by doing things such as this ... which I fear is your case:
That would explain the hum you hear.
And your 100+100 ohms "center tap" simply isn't so.
Only voltage checking as suggested above will confirm this.
A real old Webcor amp?
In the 40's or late 50's (and maybe on some real cheap 60's stuff) winders "simplified your life" (read saved $$$$) by doing things such as this ... which I fear is your case:
That would explain the hum you hear.
And your 100+100 ohms "center tap" simply isn't so.
Only voltage checking as suggested above will confirm this.
Ah.. Well that makes a lot of sense.. Damn.
Well maybe I should just pick up a 6.3V hammond transformer.. Run my own 6.3V line.. The only problem with that is that I have no space left! lol
Oh well. Thanks for the info everyone. I'm sure this is what's happening.
I'll do those measurements you mentioned though, just to get a bit more info.
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