Amp hum - help please!

Jcricket

Jcricket

not someday, but today
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Hey folks, POlease help. I have rebuilt an amp I got off the auction site. This was a national or a merril - not sure. It uses four 6v6, two 6211, and a 5r4 rectifier.

I have replaced all of the coupling caps, the multisection cap and almost all of the resistors. I have added a fuse and a three prong ground cord. When I rebuilt the unit I made multiple ground loops - everything is point to poi9nt wire grounded now - not just chasis ground.

When I started the project the hum was there. It is still there. It is barely audible. two things affect the hum, first being powered on or off. When powered on, it takes a few seconds until the tubes warm up and then the hum appears. When shut of the hum stops instantly. Second, the 6211's seem to vary the degree of the hum. Different tubes change the output levle of the hum(louder softer). Not significantly but some.

I have swapped tubes around and right and left inputs. I have also tried several different 6v6 tubes.

So any ideas? It seems to me to be either a power supply hum, or the 6211 tubes are a very noisy tube. Anyone know about the 6211?


Mark
 
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If you plan on using it with your high efficiency Fortes then I'd suggest you add a Pi filter in the power supply. All you'd have to do is add a choke and another cap. I doubt the national/merril was originally set up to have zero hum on 98db speakers.
 
Hey Scuzzer,
I am sure the hum would be bad on my forte's. It is bad on my Akai(my test bench speakers) SW155's. So you think this is a power supply hum then?
Sounds like I am going to have to have you over - show me how to do this choke thing.

Question, my amp has a 40x40x20x20 multi-cap. It only uses the 40x40. What if I soldered a jumper from each 40 to the 20 - thus creating a 60x60 cap - would this help?

Mark
 
You may be picking up the hum on one of the tube heater filaments or you may have a issue with the 5R4 rectifier.
 
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Heater -to- cathode path may be the issue, since it changes with the tube. Some things to look at:

1) A stage with an unbypasssed cathode resistor will pick up more hum.

2) 6211 is a computer tube, not necessarily the lowest noise. Try 12AU7 or 12AT7.

3) Heater circuit hum reduces more with each step below:

a) ground one side of a floating heater winding (only ONE place!)
b) ground center-tap of heater winding if present
c) hum pot across heater winding, wiper grounded.
d) heater circuit biased positive (20-30V) with capacitor for AC ground
e) center-tap or hum pot with positive bias.

The positive bias can come from a voltage divider from B+ or from the output tube cathodes.
 
Hey folks,

Thanks for the feedback thus far.
Avionic - I have many rectifiers - all of them produce the noise.

Hyperioin and Tom - I think you guys may be onto something here. I am going to take a close look, get out my books to be sure I know what I am doing, and I'll report back.

In my notes, I believe I had a ground coming from the heater. I thought I made a mistake in my notes so I did not put it back. I'll post a little more, a little later and before I do anything.

Mark
 
I made the assumption that you were talking about a "barely audible hum" while using it with the office Fortes that you mentioned in the cake pan thread. If it hums with lower efficiency speakers then you probably have an issue somewhere. Tom (as always) has great advice for heater related hum reduction. If I remember your tube list correctly you've got a few 12AT7's in that stash, this could be a good use for them.

I still think if you've got enough room under the hood you could add a choke and then use the two unused 20uf caps to form a pi filter to beef up the power supply. It won't cost much and I'm sure there's room under the cake pan hood.
 
Scuzzer,
I am going to try some of Tom's thoughts. But I would still love to put a choike in here too. BTW -the cake pan thread - this is that amp.
I am sure I have room, and I also have some 12at7 running now. That did not change it much - maybe a little but not much.

So when you coming over?:D

Putting in a choke is a bit over my head. I wouldn't know how to spec one out or where to begin. If you are available to help, I think I could make it worth your while.

Mark
 
Do you have enough room under the hood for 2 chokes? Then the cheapest way to get there would be to buy two of the chokes on the bottom of this page: http://www.radiodaze.com/RD-transformer01.htm

Otherwise you should look for something with about 200ma current capacity and as low of DC resistance as possible. You could top mount a Hammond 193H since it has end bells. Go to http://www.tubesandmore.com/ and type 193h in search or just browse the hammond choke page and see what they have. Putting a choke in will be one of the easiest things you've done to this amp.
 
So how many and what kind of tubes do you need for me to bribe you to come over and show me how this is done??? :thmbsp:

I'll pm you shortly.
Mark
 
three prong ground cord?
i have try puting 3 prong on my magnavox 9302 console amp and it hummmm 2 prong no hum.
 
tim614 said:
three prong ground cord?
i have try puting 3 prong on my magnavox 9302 console amp and it hummmm 2 prong no hum.
Click to expand...

Well, if the hum hadn't been there with the two prong then I would think that could be the issues, But it was there with the two pron as well as the three prong.

One thing for sure, it was mentioned that I may not have twisted my wires enough. Well, I didn't twist them at all. I just looked at my fishers(next to be rebuilt) and saw what he meant. I am going to get a little finer wire and put a good twist in the wires and redo that part. Maybe that will fix it.

Mark
 
To twist the wires chuck up about 4 ft of wire in a drill and put the other end in a vise or have someone hold them tightly. Pull the slack out of the wire and slowly spin the drill until you have 2-2.5 turns per inch. Route the heater wires as close to the chassis as possible and any signal wires that cross them should do it at a 90 degree angle (if possible)and with as much space between them as well. Also make sure that any other wires that carry AC voltage are treated the same way.
 
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Well, if the hum hadn't been there with the two prong then I would think that could be the issues, But it was there with the two pron as well as the three prong.

hum with Two prong before recap?
hum with Two prong after recap?
 
First off, how many places have you attached a ground physically to the chassis? You should only have one actual point where the ground leads connect to the chassis. If you grounded things all over the chassis, then connected all the ground spots, you will generally see a lot of hum.
The next issue is filament balance. If the filament winding on the power transformer has a center tap, connect it to your ground. If not, you need some way of properly balancing the filament circuit relative to ground. If all tubes have an AC filament, you can use a 25 ohm 3 watt wire wound pot and a couple 60 - 80 ohm 2 watt resistors. with these, you can make a balancing circuit for the filament circuit. You adjust the pot till you get the exact same voltage relative to ground on each hot leg of the 6.3 volt filament winding. Usually 3.15 volts a side. I see many vintage amps where they try all kinds of tricks to reduce hum. Connecting the center tap of the filament winding somewhere in the B+ circuit, adding resistors in series with the tube filaments. None of these old "Tricks" work. My self, when I build an amp, I only use AC filaments for the output tubes. I then use a full wave bridge rectifier and a 2,200 capacitor as a filter. I then use a balancing network consisting of the 25 ohm pot and two 80 ohm resistors. This works great and reduces hum to about nothing.
The improper balancing of the filament supply and screwing up grounding of the amp are the two biggest reasons amps hum after being rebuilt.
 
Scuzzer said:
... Route the heater wires as close to the chassis as possible and any signal wires that cross them should do it at a 90 degree angle (if possible)and with as much space between them as well.
Click to expand...

IIRC, my Dynaco and my Fender guitar amps all have the filament wiring twisted as you say, but looped from tube to tube up away from the chassis. Would there be a benefit to trying to squash the wiring down against the chassis?
 
That was a great thread analog addict and it goes to show that perfect heater wiring and routing is not necessary to eliminate hum. I was going to post a link to that thread for JCricket but I kinda forgot to at some point.
 
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