This is not for hmmm issue....for right condition between pre and output stages, that I told in my post 35.
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Make a tube shaped shield out of aluminum foil, clip ground it and drop it over a tube. Might tell if the tube is picking up EM from the supply tran. Next, if you haven't done it three times already, verify all grounds in the circuit especially the plate. Maybe ground the plate at a different point. You may have a ground loop with that cl60 and the OTs grounded directly to the plate.
if properly understood - the hum is even if the tube is not installed and not connected to B +..only the transformer is connected to mains. I think that it is induced hum from power transformer to output trafos. I have similar problem with may PP amp, where i have the same transformer placement like member needtubes and I have a hum of 1.5mV rms without tubes and B+. My 6c33c with toroid power transforms have hum of 0.5mV Rms.
I would try to re-route the heater lines away from the OT. Also, remove the heater selector switch and run only one heater line to each side (use 18 aw not 22 aw). The 6c33 should have both the filaments lit. Replace the CL-60 with a piece of wire. The CL-60 is basically a 100 ohm resistor. Run a piece of wire between your two star grounds (aluminum has a higher resistance then copper).
I do need to shorten those heater wires... The star grounds are connected together through wires. Only the transformers make their ground connections through the top plate.
UPDATE 3/17/2015
I finally got around to electrically isolating the power transformer from the top plate. This means the electrostatic shield in the 370MX is currently not connected to anything. The amp still has 60Hz hum with only the heaters on and still has 120Hz hum with B+ switched on, but I feel like both are now quieter. And, it sounds like the hum with B+ on is more pure 120Hz, rather than an even mix of 60+120Hz. Perhaps I can tame that by adding a little more filter capacitance...
The unnerving thing is that with the new arrangement, there is a ~55VAC differential between ground (measured from the top plate) and the unshielded screw on the 370MX. I shudder to think about what could happen if this screw is suddenly grounded to the top plate...
Should I do anything to get these at the same potential?Normal, perhaps, but not desirable.
Yes, it is very low level. I would say I now have to have my ear about 2" to hear it. My crusade against this hum issue is mainly due to the fact that it was absent in the prototype amp, but appeared suddenly in the final build.
Normal, perhaps, but not desirable.
Yes, it is very low level. I would say I now have to have my ear about 2" to hear it. My crusade against this hum issue is mainly due to the fact that it was absent in the prototype amp, but appeared suddenly in the final build.
How much of anyone's listening time is done 4" from the speakers?
Yes. Very nice looking build.
hum that can only be heard 4inches from the speaker that appears with only the heaters on and no B+ ? ....But you never heard it when breadboarded?
I had this with very inexpensive OPT's a few times - because they were cheap! stray capacitance.. poor shielding... questionable laminations.... etc. but your OPT's look like NICE ones. so....
If you don't want to move the power transformer, unbolt the OPT's and move them away from the amp just to test. or just unbolt one of them and rotate it 90 degrees to see what happens. Be careful when doing this though! Don't risk a shock - I don't see any shunt in your PS... 2 watt 100k Ohm across one of the caps might make it safer...
I would also put a 1k grid stopper before the 6c33, lose the 47 ohm voltage drop resistors in the power supply, rectify with tv damper diodes instead of silicon diode, seriously reduce (or get rid of) the input cap as well if your choke won't rattle...
but if it sounds great and you love it then ignore all the stuff I write.
hum that can only be heard 4inches from the speaker that appears with only the heaters on and no B+ ? ....But you never heard it when breadboarded?
I had this with very inexpensive OPT's a few times - because they were cheap! stray capacitance.. poor shielding... questionable laminations.... etc. but your OPT's look like NICE ones. so....
If you don't want to move the power transformer, unbolt the OPT's and move them away from the amp just to test. or just unbolt one of them and rotate it 90 degrees to see what happens. Be careful when doing this though! Don't risk a shock - I don't see any shunt in your PS... 2 watt 100k Ohm across one of the caps might make it safer...
I would also put a 1k grid stopper before the 6c33, lose the 47 ohm voltage drop resistors in the power supply, rectify with tv damper diodes instead of silicon diode, seriously reduce (or get rid of) the input cap as well if your choke won't rattle...
but if it sounds great and you love it then ignore all the stuff I write.
There is a resistor across the last cap, although I forget the value. Somewhere around 100k, though. I may try to move the OPTs as their wires have a bit more play than the power tranny wires and they are so much lighter!
The power supply is probably due a rebuild some day...
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