78 Champ rebuild needs detail on hum cure

[yelly]

I have a 78 Champ which has been almost totally rebuilt. I made a replacement for the old leaky fibre board and have renewed everything except the pots and transformers. It currently has the Torres tweed mod fitted. The tubes are are all NOS: Sylvania 12AX7, Brimar 6V6 GT, RCA 5Y3. I use it as set predominantly clean.

My problem is trying to eliminate as much of the typical single ended hum as I can. I am currently using a JJ Tesla filter cap - 40, 20, 20, 20 with the 40 on the first stage.

I found (and currently lost) a great thread with a picture showing a 1.5k 5 watt resistor wired in series with B+ and a spare 20 cap on an original Mallory cap (20,20, 20 20?) although I couldn't make our exactly where it connected. I didn't want to crash straight on and try it as the 40 I am use could stress the 5Y3 with extra caps involved?

What I want to know is exactly what I can do with my Champ.

[Bill Moore]

Yelly, I have an old Valco that I bought used over 40 years ago, and it had a noticeable hum. While in the Navy, I had some of the ET's look at it, and they replaced a few caps, and a couple of tubes, but it was still noisey. I rediscovered it a few years back, and got it up and running again, but still humming. It is a se 6V6 with ptp wiring on stand offs, I used a drumstick to move wires around, and found that a 1/4 inch either way would affect the level. It now is almost completely quiet!

[yelly]

Thanks Bill but unfortunately the hum hasn't responded to moving wires. I'm certain that it is the power supply causing it as it produces the same hum through the speaker even when the amp volume is set to zero.

It isn't unworkably loud but if I can reduce it I'd like to.

[Riffraff12571]

"It currently has the Torres tweed mod fitted."

Did you install the Tweed mod so you know the hum level before and after? I did the tweed mod to mine too a while back, Weber's. It ran SUPER HOT and there was a noticable boost in hum. It also sounded unreal. It definitely turned my polite little SFVC into a screamer. The down side is it ate 6V6 tubes often when pushed hard and on a few occations there were fireworks. I returned it to stock and haven't had a problem since even boosted with all of it's controls dimed.

[alnicopu]

Check all your grounding and across all your solder joints with a meter. Loosen and re-tightnen all your jacks and pots to the chassis. Remove the mods. Go through the whole circuit, take you all of 10 minutes. Try another guitar cable and plug it into a different circuit in your house. My Vibro champ is dead quiet with nothing plugged in and the volume on 10. I worked on a champ with similar symptoms. Turned out the ground wire on the newly installed 3 prong cord was hanging by 1 or 2 strands on a grainy blob of a cold solder joint.

Sometimes it becomes a process of eliminating things one at a time.

[yelly]

All the earth connections check out fine. When the amp is all the way up the hum I'm trying to minimise is no louder than when the amp is on zero.
If I was using the amp flat out the hum wouldn't even be noticed. It just that as I am using it quietly in the studio you can sometimes be bothered by it.
As to the tweed mod, it has been in there 10 years, so before and after is difficult to state if it made any difference to the hum.
I'm sure it is a filtering issue, it is just exactly what to do about it (if anything).

[celeste]

First thing you need to know to logically chase hum, is what frequency it is. Is it 50hz, or 100hz. Both are possible, and both are common, but they come from different causes and require different cures. If you are not sure, download a freeware signal generator and match the tone.

100hz would be rectified mains, so it would have to be from the B+. That means better filtering. You can not go any higher on your first cap, but you can on any node after that. Just try paralleling the existing 20uf with another 20uf and see if things improve.

50hz is mains, and comes from the heaters or noise radiated from the PT. Lets hope it is not the PT, and it would more likely be from the heaters anyway. I don't believe they had improved the heater scheme then, so from a noise perspective they only did one thing right, there is lots of room for easy improvement. But you first need to know what kind of noise you are hunting

[yelly]

This afternoon I removed the Torres tweed mod. Everything is now stock.
My filter cap is a jjTesla 40/20/20/20. I did have the 40 in the first stage (as did the amp from the factory). I currently have a 20 instead having read enough things to make me worry about being too close to a 5Y3 maximum.

I am now contemplating going back to the 40 on the first stage (which would be the same as another 20 in parallel).

BTW the hum is 50Hz as far as I can tell without a handy oscilloscope.

[muchxs]

Originally Posted by Riffraff12571

"It currently has the Torres tweed mod fitted."

Did you install the Tweed mod so you know the hum level before and after? I did the tweed mod to mine too a while back, Weber's. It ran SUPER HOT and there was a noticable boost in hum. It also sounded unreal. It definitely turned my polite little SFVC into a screamer. The down side is it ate 6V6 tubes often when pushed hard and on a few occations there were fireworks. I returned it to stock and haven't had a problem since even boosted with all of it's controls dimed.

The run of the mill "tweed mod" is like swatting flies with a 2"x4". Sure you'll get 'em but you end up with holes in your walls. The part of a BF / SF Champ that's not easily changed is the power supply. Supply voltage is too high. That makes the bias too hot. Pummel a hot 6V6 with too much signal and it goes, "whack!" It arcs. RCAs seem to be especially prone to arcing.

There are several necessary tweaks to make the "tweed mod" play nice. I've even figured out a way to have it "stock" then easily switchable to "tweed". No drilling, easily reversible. Both settings play nice.


Hum: "Typical SE hum", eh? I build SEs all the time. No hum.

I think part of the problem here is it's a Euro amp. 50hz. Probably the stock chassis ground filament setup with an extra yard of wire.

The 40uf first filter shoud be adequate even @ 50 hz. I get by with 20uf with a 60hz power supply. No hum!

I'd run twisted filament wires with artificial ground. Then your yard of filament wire won't broadcast hum.

[yelly]

I will order up some 100 ohm resistors and do the twisted heater wiring.

[dada]

Whenever I see the name "Torres" mentioned I cringe. There have been more amps in the greater SF bay area ruined by this guy than the rest of all the amp techs combined. Thank goodness he left the state.....

[backline]

Originally Posted by dada

Whenever I see the name "Torres" mentioned I cringe. There have been more amps in the greater SF bay area ruined by this guy than the rest of all the amp techs combined. Thank goodness he left the state.....

Did not realize he was gone. What is the story with that.?
I would have to count him second, after Ed Roman, as a guy who generates a lot of dislike.
Best

[yelly]

Well popular as Torres might be, he can't be blamed for my Champ's hum.

[dada]

Originally Posted by backline

Did not realize he was gone. What is the story with that.?
I would have to count him second, after Ed Roman, as a guy who generates a lot of dislike.
Best

I believe he is in the Pacific Northwest. Beware!