Another Hum Question

[rockybottom16]

I finished my 2nd 5e3 build a week ago and have almost eliminated the hum thanks to this forum. The thing is is that my first build didn't hum at all from day one. This second one started out humming quite noticably. And I did everything the same on both amps as far as grounding. I'm thinking the first one was a fluke. Anyway, on this latest one I have the main filter cap and the power tube cathode resister grounded along with the center taps and mains ground all at the PT bolt. Everthing else grounded via buss on pot backs to a bolt a couple inches from the input jacks. It's not bad but still seems to be a little more hum that my first build and the only other difference is that my first build didn't have a heater CT, so I used 100 ohm resistors from each pilot light tab to ground. My question is this: Is the heater center tap always grounded, or is there anything to be gained by hooking it to the power tube cathode instead?

[guitician]

If you know about connecting the heater CT to the cathode, then you must know the benefits. Have you swapped the tubes from your 1st build over to this one to see if there is a difference?

[loudthud]

Running the heater CT to the 6V6 cathodes will make a difference. Most of the time hum has two components. A 60Hz organ note like tone with a 120Hz buzz on top of it. Elevating the heaters will reduce the 60Hz component. An interesting experiment is to make a voltage divider with a 220K 2W resistor to B+ and a parallel combination of 22K 1/2W and 10uF @ 100V cap to ground. Connect the CT to that. Now connect a jumper lead from ground the CT. Allow the amp to warm up and flip the Standby on. Listen/measure the hum, then disconnect the jumper allowing the cap to charge up. In one of my amps it reduced the Hum by 9dB.

[rockybottom16]

I don't know the benefits first hand - my knowledge of this comes solely from reading the forum. Maybe I'll try connecting the heater center tap to the cathode first, since I don't have the components on hand to try loudthud's experiment. I'm assuming the CT gets connected directly with no other components? I'm curious as to why none of the layouts I've seen show this being done with the heater CT. They all show it going to ground.

[guitician]

You've never seen it probably because it is not needed in a properly laid out and constructed amp. It is just a "Work Around".

[rockybottom16]

Then you're saying that Weber's layout of the 5e3x2 is not "proper"? I just came across it, and it has the heater center tap going to the cathodes through a single 100 ohm resistor. I'm admittedly a novice builder, but my lead dress is tidy and virtually identical on both amps. Thanks for your critique of my skills, but I still need to know if the CT gets connected directly to the cathode or if it should go through a low value resistor first.

[guitician]

I'm not commenting on your skills or any specific amp. That resistor is probably a precaution in case high frequencies get back to the input stages through the heater/cathode capacitance?

[rockybottom16]

So if I decide to do it, the 100 ohm resistor would probably be a good idea? The hum is pretty mild, so it's not really a pressing issue. I was just curious as to why it's more pronounced than my last build (which I don't have anymore, by the way). One thing I noticed is that when I have my vintage P12R in the amp, it's whisper quiet. It's only when I have my Celestion G12H in there that I notice the hum. Of course the G12H is a whole lot louder across the board.

I'm in the process of building a 5F2A and it will be my first endeavor using the brass plate under the pots. The Weber layout shows everything but the center taps(and wall supply ground) getting grounded to this plate. I'm curious to see how this grounding method works, since it seems to go against the conventional "keep power supply grounds separate from signal grounds" wisdom.

[Bruce / Mission Amps]

I'm not commenting on your skills or any specific amp. That resistor is probably a precaution in case high frequencies get back to the input stages through the heater/cathode capacitance?

What... no.
The two 100 ohm resistors are to allow a point of reference that supposed to be half way between the 6.3vac line and at its center in phase.
Think of the 6.3vac wave form and the center of that wave being 3.15vac but the center is like the fulcrum or pivot point of a teeter toter.... it never moves but all the other voltages move around it.
On my 5E3X2 design, and all other amps for that matter with a power tranny with a 6.3v filament winding and a center tap... then the resistor can be used in series with the center tap lead but mostly as a form of a lousy fuse in case there is a dead short from lug 3 of a power tube socket (Hi-V B+) to lug 2 (a heater filament lug) of the same socket and, because the filament supply can be referenced to the DC voltage of the cathode, on to ground through the cathode biasing resistor.
That kind of short (lug 3 to lug 2) puts all the high voltage right to ground because the filament supply is grounded too... albeit held above ground by either the 47 to 100 ohm resistors and, in the case of the cathode biased amp, the cathode biasing resistor.
Those 47 through 100 ohm 1/2 watt resistors have nothing to do with any high frequency feedback or any other reason other then stated above.

Now with respect to the "never have seen" comment....
This is a method for filament hum/noise reduction that is, well, as old as the hills and very common.... seen in many old tube amps from before WWII even!!
Sometimes they would use a 100-200 ohm 2 watt linear pot (or rheostat) with the center tap grounded and the two outside lugs across the two legs of the filament supply... then it was called hum balance or a similar name and probably still is.
Regardless, the effect is the same and it is always done for the same reason.

[rockybottom16]

Thanks for the explanation, Bruce. I'm obviously not an engineer, so I appreciate the plain language. If I connect the filament CT to the cathodes, I'll definitely include the 1/2watt 100 ohm resistor in series with it.

[loudthud]

A guy I know decided to change the pilot light on his silverface Bassman. He tried it while the power was on. The bulb slipped from his fingers and the bulb shorted one side of the heater supply to ground and smoke immediately started coming from the amp. On later versions of some silverface amps you see that they added small resistors (1.8 ohm 1/2W IIRC) in series with the pilot light to act as fuses to prevent this. Now if the CT for the heater winding was connected to the 6V6 cathodes, it would short out the cathode bias which would not be good. So a resistor in series with the CT does serve a purpose. Be aware that high gain amps can oscillate using the heater supply as a parasitic feedback path.

[guitician]

Bruce, that single 100 ohm resistor doesn't balance hum.
Were does the cathode short too?
The filaments don't return to ground anywhere. The schematic that he mentioned "5E3x2" has the 6.3v secondaries floating, CT to cathodes through 100 ohm. One 100 ohm resistor wont change the "short" to ground situation in that case.

[Bruce / Mission Amps]

Bruce, that single 100 ohm resistor doesn't balance hum.
Were does the cathode short too?
The filaments don't return to ground anywhere. The schematic that he mentioned "5E3x2" has the 6.3v secondaries floating, CT to cathodes through 100 ohm. One 100 ohm resistor wont change the "short" to ground situation in that case.

I could be wrong, it wouldn't be the first time.
Using one 100 ohm resistor on the center tap isn't to balance anything... the center tap itself is doing that.
The two 100 ohm resistors acts as a virtual center tap with a PT that has no center tap. You would not use both.

I'll have to look at the most current 5E3X2 layout, the original one I gave Weber has to be four or five years old now... but if the center tap of the filament supply is connected to the cathode of the cathode biased power tubes, then the entire AC filament supply is now also referenced to ground through the power tube's cathode biasing resistor... 150ohms or so, even though it has a positive DC "bias" voltage on it.
If a power tube or tube socket shorts out from pin/lug three (Hi-V B+) to pin/lug 2, (one of the filament lugs and legs, with lugs 3 and 2 being right next to each other and the most likely candidate for a flash over), then there is a low resistance path to ground for the B+... right through center tap, to the cathode resistor and the cathode bypass cap, which could also explode if it is only a 50v cap and onto chassis ground.
With the 47 to 100 ohm 1/2w resistor there is as sloppy fuse link, which will burn open with high voltage and enough current...
the idea being to burn the resistor open before the output transformer does.

[guitician]

OK, yeah right. A short of the B+ to a side of the filament winding. But isn't that a 1/4 amp B+ winding vs. a 1-2 amp filament winding. The cap may put out a surge, but then the fuse should blow to protect the windings.

[Bruce / Mission Amps]

Gettng too wordy over a small issue... sorry

OK, yeah right. A short of the B+ to a side of the filament winding. But isn't that a 1/4 amp B+ winding vs. a 1-2 amp filament winding. The cap may put out a surge, but then the fuse should blow to protect the windings.

Well you would hope so but after over 40 years of playing with and working on vacuum tube radio and amp stuff... I've seen a ton of blown 100 ohm resistors across the filament leads and lots of burned up tube sockets from flash over from lugs 3 to lugs 2 and many of these with no blown primary fuses.
Especially from non-techies would keep feeding bigger, higher current rated fuses in them to keep the amp going.
I just recently had a Princeton Reverb here with simply a bad power tube and a 25 12v car amp fuse in the holder! ha ha. The guy was very lucky the PT or OT wasn't fried.

For those that are still reading:
Look at it like this... B+ 450vdc... connected to a big ass 47uf-450v filter cap and a power tranny that can probably deliver 200% to 400% or more of it's constant current rating for a quite a few seconds before overheating.
Now in the scenario I described above, (shorted plate voltage from lug 3 to lug 2 of the octal socket) the entire 450v B+ rail will be across that 47 ohm 1/2 watt resistor in the center tap, which is grounded into say a 150 ohm to 450 ohm 5 or 10 watt cathode biasing power resistor.
Add up the resistors in series to ground.
Put the 450v across that resistance to solve possible surge current...
now put that same current across the 47 to 100 ohm resistor.
I^2*R=P

EX:
Use a single 1/2 watt 100 ohm balancing resistor in series with the center tap to a single 200 ohm 15 watt cathode biasing resistor of four 6V6s.
Dead short from lug 3 to 2.
400v across a total of 300 = 1.3 amps! Ok not likely to do that and that is just a giant current surge which should blow the primary fuse... but if it doesn't and the PT can shove 400% of it's rated current for a while...
With all that B+ on the cathodes of the power tubes, they will probably shut down so the tubes will be OK.

You mentioned a 250ma winding but say you have an PT in this amp with a min of a 150ma rating.
That PT will still try to push two or three times that amount of current about 300 to 450ma through that series of resistances, including the 1/2 watt 100 ohm resistor... for quite a few seconds or more if the primary fuse doesn't open up.
Use 300ma as a long term current surge....
.3a^2*100ohms= 9 watts... I think the little 1/2 watt resistor fuse will burn open in a short amount of time.
Now that I think about it, I guess you could use a 1/4 watt resistor too.