Another 120hz buzz post...I'm sorry. I'm really sorry

[jewishjay]

I got my Harvard 5f10 build completed and it plays and sounds great, very clean. About 5 hours of play time so far, and I'm loving it.  But there is a little 120hz buzz. I've inspected all the solder joints, chopsticked everything, wiggled wires around and not found the problem yet. When I turn the volume to max, it gets a lot quieter and if I pull out the phase inverter it goes silent. Any suggestions?

[sluckey]

Disconnect the NFB wire from the speaker jack. Make any difference?

[Latole]

120 hz buzz ;

60 hz buzz

It may not be the solution but in my opinion there would be some cleaning to do in the wires. The wires should not be parallel too close to each other as much as possible.
They must cross at 90 degrees.
Be very close to the chassis.
Be as short as possible

Failure to follow this rule will generate noise

[pdf64]

Such buzz may indicate that the heater circuit isn't balanced with respect to circuit common 0V. Maybe it's completely floating, I can't spot a winding CT wire or balancing resistors?

[Latole]

Such buzz may indicate that the heater circuit isn't balanced with respect to circuit common 0V. Maybe it's completely floating, I can't spot a winding CT wire or balancing resistors?

You are right.

There is no heater center tap. It's a must.
OP may build a artificial center tap with two 100 ohms resistors. Commonly see or 200 ohms

He may not follow original layout ?

[sluckey]

What's this?

[jewishjay]

What's this?

why, it's the heater circuit center tap!

[pdf64]

Haha, well spotted!
Has a bad tube been eliminated as the cause?

Might it be more of a hum than a buzz?
If so, it could be an 0V loop, due to all the -ve terminals of the HT caps being bussed together.
Rathrr the preamp node cap’s -ve terminal may be better taken to the input stage’s cathode bypass cap’s -ve terminal.

[sluckey]

Your chassis does not look like bare metal. It appears to have a coating or paint. If this is the case, you need to scratch through the coating down to bare metal at all your ground points, including jacks.

Did you disconnect the NFB yet? Any luck?

[DummyLoad]

see attached: it may not fix the buzz if the buzz is a faulty component/wiring error, but it will probably help.


--pete

[pdf64]

The tone controls and bias supply look different to a 5F10, what schematic have you used?

[jewishjay]

Disconnect the NFB wire from the speaker jack. Make any difference?

disconnecting NFB made the buzz slightly worse.
disconnecting the volume pot from the grid of v2 made it go silent, so the problem must be before that?

[jewishjay]

see attached: it may not fix the buzz if the buzz is a faulty component/wiring error, but it will probably help.


--pete

ok, possible ground loop. I can see that. Let me try it....

[jewishjay]

The tone controls and bias supply look different to a 5F10, what schematic have you used?

The tone stack is Vibro Champ (and a million other Fenders) and the bias is from Robrobs website bc the transformer doesnt have a 50v tap.

[DummyLoad]

see attached: it may not fix the buzz if the buzz is a faulty component/wiring error, but it will probably help.


--pete

ok, possible ground loop. I can see that. Let me try it....

not just a loop - plan isolates power amp supplies, bias supply, & heater CT grounds from preamp ground.

[jewishjay]

ok, i'll try that. but if everything is ultimately ground to the chassis....why would that matter?

[glass54]

Hi jewishjay,
Please forgive me, but as best I can tell, the Heater wire at back of lamp (circled) appears to be low on solder wetting. May just be the photo?
You might try to get a better photo for us or resolder and confirm.
I am also concerned that the V1 cathode, all pots, all power supply caps are earthed at one lug. I think Dummyload was eluding to this?
I find sharing main power supply earths with signal input earths creates problems.
Kind regards,
Mirek

[shooter]

why would that matter?

big signals produce dirty grounds, put them all together
small signal grounds are pretty clean, so they ground long way down the road, the chassis balances things out.
the way you have it, the chassis can't balance, so it hums

[pdf64]

ok, i'll try that. but if everything is ultimately ground to the chassis....why would that matter?

It would matter if the avoidance of excessive hum was a goal. Which your raising of the thread indicates it may be?
If so see http://www.valvewizard.co.uk/Grounding.html

[jewishjay]

FIXED. Thank you everybody. I don't understand exactly why, but I'm convinced now: you have to keep preamp earths separate from power amp earths. Thank you all for your patience. We don't learn if we don't ask.

[Latole]

FIXED. Thank you everybody. I don't understand exactly why, but I'm convinced now: you have to keep preamp earths separate from power amp earths. Thank you all for your patience. We don't learn if we don't ask.

Look where are ground in the Fender Harvard layout. Reply #4