Hum in my 5F6-a

[vince76]

Hy guys.
I assembled a 5F6-A. It works but I can hear much hum (like mmmmmmmmmmmmmmmm), you can understand what I want to say, I think. The horrible hum is also present with volume set to 0 and without guitar connected.
At first I was thinking a problem with wrong ground wiring, so I re wired all grounds and made a star grounding, then I was thinking something was wrong with heater wires, I rewired, twisted better, but nothing.
Hum is still there...
I noticed two things that I would say to help someone to help me...
The hum become more intense as I put my hand near the second preamp tube (I was trying to move heater wire to see if make difference)
The Hum become less intense if I turn up the other volume control respect the channel I'm using to play, and there is a point, (3/4 of vol) you can even play guitar!!!!

I hope I was clear???
Can someone help me please? I don't know were to strat.

Thank you all, Vince.

[bnwitt]

Vince,
what layout did you use? Original Fender? Hoffman? Something Else? You say the hum changes when you put your hand around V2. Have you tried actually grabbing wires going to that tube inside your closed fist? If you find a wire that when grabbing it the hum is reduced, reflow the solder joint on both ends of the wire. I'd reflow all joints on that tube socket. I assume you've got 4 input jacks on the amp, does the hum change at all if you plug in a guitar in any of the four? I've attached a picture of one of my Hoffman 5F6-A chassis. Could you attach a picture of your amp please? One other possibility for the hum is one or more of your filter capacitors has a bad connection. Check those solder joints as well.
Barry

[vince76]

bnwitt,
thank you for the reply.
I'm actually out for some days, I will post the pics as soon as I will be back in Italy. I have a ceriatone but looking at your good job I think I have too much confusion in the chassis, I will shorten some cable an then will post again... and add some picture if I can hear noise again

thanks, vince

[tubeswell]

Also a simple quick test you can do is change the tubes. A bad triode can hum like that in one channel.

[vince76]

thanks a lot tubeswell
did you mean change the 3 tubes, swap the 12ax7 or change only that tube

vince

[tubeswell]

The way to see if its a bad tube is just get an extra tube and try it in each socket (one at a time) and see it that changes anything.

If the hum is there whether or not the vol is killed, its likely to be B+ ripple, (which better filtering would fix).

If you kill the vol and the hum goes away, it could be; bad grounding, bad soldering, bad lead dress, unwanted coupling into sensitive signal wires, or a bad component (like a bad plate resistor etc). You need to isolate each stage and eliminate the possible causes. The Jack Darr book is a good place to learn about troublehooting - easy to read and get your head around, and its free.

http://www.pacificrecone.com/JackDarrBook.html

[vince76]

Hey guys, thank you for the help.
Thank you tubeswell for the reading.
I rewired all, but the terrible mmmmmmmmmmm is still there.
I attach a picture and I go with tubeswell tip. I will find an extra 12ax7 and try to substitute in each pre socket.
If you have extra tips I'm happy to try because I really don't know what to do.

Thank you, vince

[djembutz]

I had same problem with you before. The hum is induction from PT to OT.Just reorinted OT and PT on different position. And the hum just gone.
That's all

[bnwitt]

Looking at the photo, it appears that all of the amp's grounds are going to a single point. The green wires coming from all different directions. A star ground. Is that the case?

edit:
Ok, I see it is a star ground and that it is a ceriatone kit. I would suggest you abandon the star ground concept. It is prone to hum.

At the location of the current ground terminal I would I would leave the PT's grounds and the bias pot ground along with the first ground on the circuit board coming from the two 10uf capacitors in the bias circuits. I would then take all other grounds (except the grounds from the filter capacitors) and chassis ground them as far to the right or preamp end of the chassis as possible. This will separate the preamp section from the power section. The fiter cap grounds can go by themselves on the chassis somewhere. You really need to shorten up some of those meandering wires too. All wires should take the shortest possible A to B run. An example is the PT HT red wires. Yoy have them running a round about way to the GZ34 socket. They should run straight there and not be intertwined with other wires. Your OT secondary wires to the speaker impedance switch need to be shortened. All wires going to tubes should be as short as possible too. Look at this attached picture of heater wiring done in the chassis hugging style as opposed to my flying wires. Either way is good if done right.

[Twist]

Is there a center tap for the filament winding? Be sure and not ground it with the DC grounds. I always attach my AC and filament grounds away from the DC grounds.

[vince76]

to bnwitt: thank you for this. I star grounded believing it was a grounding problem (before I had different grounding)... so I don't think it is my problem, maybe I have to shorten again the wire? I saw other amps. I don't think the wires are too much long, but I will try. I would say that killing the volume has no influence on the hum, but strange thing, if I put the vol near the max I can reduce the hum (I can't understand what it means). Only the eq section has an effect on the hum and it seems to me that that section does its work. Reading tubesweel' post I'm convincing that is not a grounding issue.

twist: thank you for your reply. the transformer I'm using has only one center tap between the 700V wires. should I ground at a differend point in your opinion?

I noticed one strange thing, maybe to someone can tell a useful information. One month ago I build a rangemaster clone, I know it was working good because I tried it on another amp... well It doesn't seem a rangemaster on 5f6-a I'm trying to debug, it doesn't drive (I can't get distorsion).

thank you all.

[bnwitt]

Originally Posted by vince76

to bnwitt: thank you for this. I star grounded believing it was a grounding problem (before I had different grounding)... so I don't think it is my problem, maybe I have to shorten again the wire? I saw other amps. I don't think the wires are too much long, but I will try. I would say that killing the volume has no influence on the hum, but strange thing, if I put the vol near the max I can reduce the hum (I can't understand what it means). Only the eq section has an effect on the hum and it seems to me that that section does its work. Reading tubesweel' post I'm convincing that is not a grounding issue.

twist: thank you for your reply. the transformer I'm using has only one center tap between the 700V wires. should I ground at a differend point in your opinion?

I noticed one strange thing, maybe to someone can tell a useful information. One month ago I build a rangemaster clone, I know it was working good because I tried it on another amp... well It doesn't seem a rangemaster on 5f6-a I'm trying to debug, it doesn't drive (I can't get distorsion).

thank you all.

If your transformer does not have a heater center tap then you need to take two 100 ohm 1/2watt resistors and put one from each leg of the heater circuit to ground. This will provide an artificial center tap for the heaters and will probably stop the hum or reduce it. I have attached a Pdf file of the layout. Also just be aware, I have never seen a star ground in any amp be anything but noisy.

[vince76]

thank you bnwitt.
I will try to artificially center tap as you say.
I will post.

vincenzo

[vince76]

It seem that it was depending by the fact that my transformer doesn't have center tap for the heaters.
I put the tho 100R resistors as bnwitt suggested and....
WoO it worked. now I still have a little bit of noise but the amp is playable.
Also I have the power tube with different voltages and so in my opinion non correctly biased but... this is a problem I can solve with more time.thank you all for the help.

vincenzo

[bnwitt]

Good for you Vince