5f1 build-reducing buzz/hum

[Triocd]

I was going to build a 5F1 for my first build, but I came across a great deal on one already built so I grabbed it. It sounds terrific but has a little more hum than I think is normal. The hum is actually buzz from 0-10 but switches to hum on 11 and 12. I've skimmed across some tricks to narrow down how to figure diagnose where the hum is coming from but I'm not fully sure about these tricks. The buzz definitely gets louder as I turn up the volume. Someone mentioned turning the amp on with various tubes removed to figure out if the hum is from the pre-amp or not. Does that make sense?

I also think the lead dress can greatly be improved. In the pictures the green wires (are these the grounds?) can benefit from better twisting. Will that help reduce the buss by twisting ground wires? Another area I see that can be improved are the wires running out of the 12ax7 tube. It looks like a birds nest. Most other builds have the wires running down from the solder tabs as to not cross each other. Should I bother re-wiring this tube so that the wires aren't crossing each other as much?

Also, when the volume is up there is a rattle somewhere in the chassis. I can't tell where its coming from but all the solder tabs in the 3 tube sockets are totally loose and can be moved around. On my other amps there are tight and don't move. Could these tabs possibly be rattling at volume? If so, should I superglue them or is there a way to tighten them?

Thanks for any help!

[DrGonz78]

Well to start the twisted green wires are the 6v heater filament supply and they are not ground wires. Twisting those better might help out a bit but I am not seeing a center-tapped connection for the heater supply. This transformer does not appear to have a green-yellow wire for a center tap. You can research and install a virtual ground, which is essentially two 100 resistor wired from each side of the filament supply to ground. Look at this example pic of what that looks like.

[Chuck H]

What Gonz sed ^^^^^^^^^^^^^^^^^ It was the first thing I looked for when I saw that you were using a twisted filament supply.

I also wanted to mention that it seems like a good, clean build but you sure didn't make things easier on yourself by making all your lead connections on the underside of the board

[Triocd]

What Gonz sed ^^^^^^^^^^^^^^^^^ It was the first thing I looked for when I saw that you were using a twisted filament supply.

I also wanted to mention that it seems like a good, clean build but you sure didn't make things easier on yourself by making all your lead connections on the underside of the board

Thanks for the feedback guys. I picked up a couple 100ohm resistors from work, but they are tiny 1/8watt resistors. Are these fine? In all the google images I've seen using this artificial center tap, they all have much larger looking resistors. Any advice?

Also, I didn't build this amp...I bought it off reverb in brand new shape for a wicked good price!

[Chuck H]

Don't use the 1/8W resistors. You're better off with 1W flame proof types here.

[Triocd]

Don't use the 1/8W resistors. You're better off with 1W flame proof types here.

Would radio shack have those or would I need to order online do you think?

[Chuck H]

Radio Shack is going through a change of ownership right now and most of their stores don't have the usual inventory. I was in one a few days ago and the clerk explained it to me. Their components area was a disorderly mess with many empty slots. So all I can say is, you can check and good luck.

[Triocd]

Radio Shack is going through a change of ownership right now and most of their stores don't have the usual inventory. I was in one a few days ago and the clerk explained it to me. Their components area was a disorderly mess with many empty slots. So all I can say is, you can check and good luck.

I twisted 4 400ohm 1/8 watt resistors together to replace each 100ohm resistor, soldered them in as described and the buzz is totally gone. There is a very minor hum with the volume up but I would say it's a pretty quiet amp. Thanks for the help!

[Chuck H]

I twisted 4 400ohm 1/8 watt resistors together to replace each 100ohm resistor...

That's a commendable demonstration of impatience worthy of my bench That should actually work fine. But do get a couple of 100 ohm, 1W flame proof resistors to replace them with, whenever. Not critical. 1/2 watt is ok for this application but I use 1W anyway. Flame proof is important though because if anything ever happens (and it can) that causes the resistors to burn up you'll have a lot less mess to clean up.

If what you have left for noise is actually hum, and not buzz, you can try referencing the two 100 ohm units to pin 8 on the power tube socket instead of ground. This places them on top of the cathode circuit where they will be elevated from ground by 20 something volts DC. This trumps the 6.3V of the filament circuit and isolates it better from potential interaction with the signal path. It could be especially helpful on your model since your preamp cathode circuits are not bypassed with a capacitor.

Once you get the flame proof resistors you can do this right on the power tube socket by soldering one resistor from pin 2 to pin 8 and the other from pin 7 to pin 8. Make sure to pull the two you have already installed if you do this.

[Justin Thomas]

I think a tiny bit of hum in a Champ when the volume on 12 would be normal? But if you find out how to kill it all, please post lots of gutshots and step by step instructions!

I personally don't like my amps so quiet I can't tell they're on... I forget to turn them off. :O
Justin

[mozz]

1/2 watt will get warm, they are dissipating .39 watts, best to double the wattage to be safe, so 1 watt as said before.

[Tom Phillips]

1/2 watt will get warm, they are dissipating .39 watts, best to double the wattage to be safe, so 1 watt as said before.

I get 0.198 W dissipation in each 100 Ohm resistor. Remember that there is 6.3V across a total of 200 Ohms.

[mozz]

Opps, you're right! 200ohms across the 6.3.

[Tom Phillips]

I think a tiny bit of hum in a Champ when the volume on 12 would be normal? But if you find out how to kill it all, please post lots of gutshots and step by step instructions!
I personally don't like my amps so quiet I can't tell they're on... I forget to turn them off. :O
Justin

The residual hum in the stock Champ is mostly due to the ripple on the B+ supply line. That hum component is present even when the volume in turned down. The fix is to add an additional stage of filtering before the B+ supply to the power amp stage. I'm sure that Google will find step by step instructions. There have been mentions of this on MEF too. The reason that better filtering is required is because the single ended power amp stage does not cancel the hum like a push pull stage does. Therefore, single ended power amps need better power supply filtering if you wish to achieve the same low hum level of a PP amp.

[Chuck H]

I'm doing a Champ right now. Twisted filament with false CT referenced to the power tube cathode and a Pi filter ahead of the plate supply. No hum. Well, no Champ hum anyway. The thing about the Pi filter is that you can make cap 1 a 22uf so that the rectifier tube is happy, follow with a 100 ohm resistor and then use a 47uf on the OT node. Business as usual from there. The extra filter should only drop about 7V with a 6V6 (I'm using an EL34 so just guessing). If it's your Champ, a friends Champ or a clients that wants less hum I can't see any reason not to do it. Hell, I hate wiring filament supplies, but a Champ only has two tubes on 6.3V. Even "I" don't sweat that.