Loud hum on 5f6a build

[ab764]

Hello,
This is my third scratch build and I am stumped.  I built a 5f6a circuit in a gutted crate 50 chassis and it is humming very loudly regardless of volume setting.  The hum goes away when I remove the PI tube or one of the power tubes.  The voltages all seem normal (PV 433, biased 30mA) and I have checked all grounds.  FWIW, the main filters are totem pole design and laid against the PT.  I am leaning toward a problem in the PI circuit: PI  DCvoltages are:  pin 1=284v, pin 2=29v, pin 3 & 8=47v, pin 6=264, pin 7=30v. 
Thanks

[Paul1453]

I would guess your hum issue originates right in this area.

Your heater wires cross almost everything right here.   

[eleventeen]

Measure AC (yes AC) volts at the junction of the 220Ks where the bias is fed to those grids. You have a clean bias supply? Eg; a functioning bias rectifier & filter with no (or very little) residual AC? You have good ground on the POSITIVE end of the bias filter cap and you haven't installed it backwards?


Can you measure AC (yes AC) on the power supply nodes without blowing up your meter? Should be very low AC volts on any of them, maybe 5 or so VAC on the highest one.



Removing EITHER 6L6 kills the hum? Do you have any spare 6L6 in case this is a H-K short?


Got a heater center tap, either tranny winding CT or a synthetic one with the two 100 ohm resistors?

[Paul1453]

If it were my amp, I might consider running the heater wires the long way around the PT from the lamp to the 1st output tube.

That would eliminate crossing your bias supply and other wires in that area.
Then I'd look to give the plate wires and anything else in that area as much clearance from the heater wires as I could.   

[eleventeen]

I don't fully accept your suspicion that your problem is in the PI section. That the hum goes away by pulling the PI tube means the hum originates in OR BEFORE that section. Pull earlier tubes before carving that conclusion in stone.


I don't especially think it's heater wires. And it's even less likely to be heater wires in the PI>output section because that section tends to hum canceling. I confess I am puzzled by the hum going away pulling ONE 6L6. Upon pulling one 6L6 does the amp play at all?

[Paul1453]

Are the heater wires on the PI tube on pins 9 and 4 or 5?
It is very hard to tell from the pic, but they don't look like it to me. 

[ab764]

Thanks for the quick responses. I have tried different tubes, the heater wiring is correct on the PI, and the bias capacitors are installed with + to ground.  I will take some ACV measurements and get back to you. I will also reroute the heater wires around the PT.  FWIW, I am using a new classic tone PT, a choke pulled from a BF Bandmaster, and the OT from the Crate 50.  BTW, when run through a current limiter, the bulb stays dim.

[ab764]

Also, I do get signal and no hum to the speaker with one output tube pulled(doesn't matter which tube).  There is a heater CT.        2.5VAC at the main filter, less than 1/2VAC at every other filter.  No ACVolts at junction of 220k.

[ab764]

Update; removed neg feedback from speaker jack; hum is gone, but now I have low distorted output with all tubes in.   I'm thinking maybe a bad OT? 

[sluckey]

Hi neighbor. I'm in Mobile.

Check the two resistors I circled. They should be 470Ω and 10K. Looks like you have used 47K and 100K? That will definitely make the output low and probably distorted.

[ab764]

Hey Sluckey,
Good to know you're close by in case this gets the better of me. I double checked the resistors, they are 10k and 470.

[EKDENTON]

I think i might see the problem. I see one center tap wire from the PT, that is the HT center tap correct? If the heater secondary coil doesnt have a CT then you would need to add the two 100ohm resisters to make the artificial tap. Unless I am not seeing it....the picture is a bit dark.

[ab764]

The heater does have a center tap from the PT. I think I have most of the problems solved.  The hum went away when I disconnected the negative feedback so I think the primary needs to be reversed. The low, distorted output was solved when i changed out the V2 socket ( a pin wasn't making contact). The amp now plays great except for an ugly distortion on some notes when the treble is turned up. I'm thinking this might be fixed when I swap the OT primary and hook the neg. feedback up again.

[sluckey]

The hum went away when I disconnected the negative feedback so I think the primary needs to be reversed.

May not be your case, but, hum associated with the NFB is often caused by not grounding the speaker jack. The OT secondary common lead must be grounded.

[ab764]

Success!  Reversed the OT primary and hooked up the NFB and the amp works great.  There is the slightest bit of distortion on a note every so often but I think I can cure this with some lead dress measures. 
Thanks so much for everyone's advice!